A Look Back at “Security Problems in the TCP/IP Protocol Suite”
Steven M. Bellovin
AT&T Labs—Research
bellovin@acm.org
Abstract
About fifteen years ago, I wrote a paper on security problems in the TCP/IP protocol suite, In particular, I focused on
protocol-level issues, rather than implementation flaws. It
is instructive to look back at that paper, to see where my focus and my predictions were accurate, where I was wrong,
and where dangers have yet to happen. This is a reprint of
the original paper, with added commentary.

1. Introduction
The paper “Security Problems in the
TCP/IP Protocol Suite” was originally published in Computer Communication Review, Vol. 19, No. 2, in April, 1989. It was a
protocol-level analysis; I intentionally did notconsider implementation or operational issues. I felt—and still feel—that that was theright approach. Bugs come and go, and everyone’s operational environment is different.
But it’s very hard to fix protocol-level problems, especially if you want to maintain compatibility with the installed base.
This paper is a retrospective on my original work. New commentary is shown indented, in a sans serif font. The originaltext is otherwise unchanged, except for possible errors introduced when converting itfrom troff to LATEX. I’ve left the references intact, too, even if there are better versions
today. The reference numbers and pagination are, of course, different; the section numbers remain the same, except for
a new “Conclusions” section. As a general rule, the commentary follows the section
it’s discussing.
It helps to understand where this paper
came from. When I started work at Bell Labs
Murray Hill in 1982, I assumed ownership of

1 21 of the first three pieces of Ethernet cable in all of AT&T, then a giant monopoly telephone company. My lab had one cable, another lab had a second, and a “backbone”
linked the two labs. That backbone grew, as
other labs connected to it. Eventually, we
scrounged funds to set up links to other Bell
Labs locations; we called the resuling network the “Bell Labs Internet” or the “R&D Internet”, the neologism “Intranet” not having
been invented.
Dedicated routers were rare then; we generally stuck a second Ethernet board into a
VAX or a Sun and used it to do the routing.
This mean that the routing software (we used
Berkeley’s routed) was accessible to systemadministrators. And when things broke, we
often discovered that it was a routing problem: someone had misconfigured their machine. Once, we found that someone had
plugged a new workstation into the Murray
Hill backbone, rather than into his department’s network; worse yet, the (proprietary)
address assignment software on his machine
didn’t see any (proprietary) address assignment servers on that network, so it allocated.1—the gatewayy router—to itself. These twosituations worried me; it was clear that anything that could happen by accident could bedone deliberately, possibly with serious consequences.
Several other things focused my attention on security even more. One was RobertMorris’ discovery of sequence number
guessing attacks; these are discussed extensively below. Another was the “Shadow
Hawk” incident—a teenager broke into various AT&T computers [2]. He was detected
when he tried to use uucp to grab password files from various Research machines;
a number of us had installed detectors for exactly that sort of activity, and noticed the un-usual behavior. At that, we were lucky—mostof the connectivity within AT&T was via the
proprietary Datakit network, which he didn’tknow how to exploit.
By the time of the Internet worm of 1988,
this paper was already in substantially its current form. The result was an analysis (to thebest of my ability; I was relatively new to security at the time) of protocol-level problems
in TCP/IP.
The original paper was criticized in [54].
Some of the criticisms were valid; some, in
my opinion, were not. At the time, I chose
not to publish a detailed rebuttal; I will not
do so here. I have, where appropriate, noted
where my analysis was especially incorrect. I
did and do feel that my conclusions were substantially correct.
The TCP/IP protocol suite [41, 21] which is very widelyused today, was developed under the sponsorship of the Department of Defense. Despite that, there are a number of serious security flaws inherent in the protocols. Some of theseflaws exist because hosts rely on IP source address for authentication; the Berkeley “r-utilities” [22] are a notable example. Others exist because network control mechanisms,
and in particular routing protocols, have minimal or nonexistent authentication.
When describing such attacks, our basic assumption isthat the attacker has more or less complete control oversome machine connected to the Internet. This may be due
to flaws in that machine’s own protection mechanisms, or it
may be because that machine is a microcomputer, and inherently unprotected. Indeed, the attacker may even be a rogue
system administrator.

1.1. Exclusions
We are not concerned with flaws in particular implementations of the protocols, such as those used by the Internet“worm” [95, 90, 38] Rather, we discuss generic problems
with the protocols themselves. As will be seen, careful implementation techniques can alleviate or prevent some ofthese problems. Some of the protocols we discuss are derived from Berkeley’s version of the UNIX system; others
are generic Internet protocols.
We are also not concerned with classic network attacks,
such as physical eavesdropping, or altered or injected messages. We discuss such problems only in so far as they are
facilitated or possible because of protocol problems.
For the most part, there is no discussion here ofvendor-specific protocols. We do discuss some problems with Berkeley’s protocols, since these have be-come de facto standards for many vendors, and not just forUNIX systems.
One of the criticisms in [54]) was that I
had lumped Berkeley-specific protocols together with standardized protocols describedin RFCs. It’s quite clear from the preceeding paragraph that I understood the difference. However, the use of address-based
authentication—a major flaw that I criticize throughout the paper—was peculiar to
Berkeley’s software; I did not make that distinction clear. It is indeed a bad way to do authentication, but it was not blessed by any
official standard.

2. TCP Sequence Number Prediction
One of the more fascinating security holes was first described by Morris [70]. Briefly, he used TCP sequence number prediction to construct a TCP packet sequence without
ever receiving any responses from the server. This allowed
him to spoof a trusted host on a local network.
The normal TCP connection establishment sequence involves a 3-way handshake. The client selects and transmits
an initial sequence number ISNC , the server acknowledges
it and sends its own sequence number ISNS , and the client
acknowledges that. Following those three messages, datatransmission may take place. The exchange may be shown
schematically as follows:

and/or

C→S:
S→C:
C→S:
C→S:

SYN(ISNC )SYN(ISNS ), ACK(ISNC )ACK(ISNS )
data

S→C:

data

That is, for a conversation to take place, C must first hear
ISNS , a more or less random number.
Suppose, though, that there was a way for an intruder X
to predict ISNS . In that case, it could send the following sequence to impersonate trusted host T :
X→S:
S→T :
X→S:
X→S:

SYN(ISNX ), SRC = T
SYN(ISNS ), ACK(ISNX )ACK(ISNS ), SRC = T
ACK(ISNS ), SRC = T, nasty − data

Even though the message S → T does not go to X, X
was able to know its contents, and hence could send data.
If X were to perform this attack on a connection that allows command execution (i.e., the Berkeley rsh server), malicious commands could be executed.

How, then, to predict the random ISN? In Berkeley systems, the initial sequence number variable is incremented
by a constant amount once per second, and by half thatamount each time a connection is initiated. Thus, if one initiates a legitimate connection and observes the ISNS used,
one can calculate, with a high degree of confidence, ISN0S
used on the next connection attempt.
Morris points out that the reply message
S → T : SYN(ISNS ), ACK(ISNX )does not in fact vanish down a black hole; rather, the real
host T will receive it and attempt to reset the connection.
This is not a serious obstacle. Morris found that by impersonating a server port on T , and by flooding that portwith apparent connection requests, he could generate queue
overflows that would make it likely that the S → T message would be lost. Alternatively, one could wait until T
was down for routine maintenance or a reboot.
I mischaracterized Morris’ paper on thispoint. While flooding can work—without explicitly stating it, I anticipated the denialof service attacks that started occurring in
1996—Morris in fact exploited an implementation error in the Berkeley kernel toaccomplish his goal with many fewer packets. That flaw (described in [10] as well as
in Morris’ paper) received very little attention at the time, and was not fixed until many
years later.
For that matter, sequence number attacks
received little attention outside of my paper, until Kevin Mitnick reimplemented Morris’ idea and used it to attack Tsutomu Shimomura [93]. Shimomura then proceeded to
track down Mitnick.
A variant on this TCP sequence number attack, not described by Morris, exploits the netstat [86] service. In this
attack, the intruder impersonates a host that is down. If netstat is available on the target host, it may supply the necessary sequence number information on another port; thiseliminates all need to guess.1
The Berkeley implementation of netstat was
dangerous, but not for the reasons that I gave
here. It did list the open ports on the machine,
as well as all current connections; both items
are very valuable to would-be attackers. Indeed, discovering the former is a major pieceof functionality of many attack tools. Fortunately, even in 1989 netstat was not available
1

The netstat protocol is obsolete, but is still present on some Internet
hosts. Security concerns were not behind its elimination.

by default on any 4.2BSD or 4.3BSD systems.
There were still TOPS-20 systems on the net
at that time; those systems had a vulnerable netstat, a fact I refrained from mentioning
for fear of pointing attackers at targets. Actual output is shown in Figure 1.
There are several salient points here. The
first, of course, which I stressed at the time,is that address-based authentication is very
vulnerable to attack. I will return to this point
later. A second point is a threat I mention
later, but not in this context: if you know thesequence numbers of an active session, you
can hijack it. This attack was implemented a
few years later by Joncheray [53].
A more important point (and this is one
made in [54]) is that the r-utilities are implicitly relying on TCP sequence numbers—andhence on TCP session correctness—for security properties. However, TCP was neverdesigned to be a secure protocol, nor were
there ever any guarantees about the properties of the sequence number. The underlying issue is this: what properties of a layer
are “exported” to a higher layer? Assuming
too much at any higher later is a mistake; it
can lead to correctness failurs as well as tosecurity failures. For that matter, it is necessary to inquire even more closely, even of sequence numbers in a security protocol: what
properties are they guaranteed to have? Arethey simply packet sequence numbers, or
can they be used as, say, the initialization
vector for counter mode encryption [35]?
Was there a security problem? Yes, there
certainly was, as demonstrated graphicallya few years later in the Mitnick vs. Shimomura incident. But the architectural flaw was
the assumption that TCP sequence numbers had security properties which they did
not. (Ironically, I have Heard that analyses ofthe security properties of sequence numbers
were, in fact, done in the classified world—
and they concluded that such attacks were
not feasible. . . )The sequence number attack story isn’t
over. In 2004, Watson observed that TCP
reset packets were honored if the RST bit
was set on a packet whose initial sequencenumber was anywhere within the receive window (see US-CERT Technical Cyber Security
Alert TA04-111A). On modern systems, thereceive window is ofen 32K bytes or more,

JCN
0,-1
6,6
6,5
0,21
0,2

STATE
-3-.EST.OOPA--FIN.FIN.--P---FIN.FIN.--P----3-.EST.O-PAV--3-.EST.O-PAV--

LPORT
15
15
79
23
23

FPORT
2934
0
0
4119
1792

FGN-HOSTATT.ARPA
0,0,0,0 0
0,0,0,0 026,1,0,16
192,33,33,115

R-SEQUENCE
333888001
002928942175
739613342

S-SEQUENCE
760807425
00701235845
660542923

SENDW
4096
319
4096

Figure 1. Output from a TOPS-20 netstat command. Note the “send” and ”‘receive” sequence numbers. The first line in the status display is the session I used to retrieve the data.
which means that it takes less than 217 trials to generate such a packet via a blind attack. That sounds like a lot of packets, and it’s
only a denial of service attack, but for longlived sessions (and in particular for BGP [84]sessions between routers), it’s quite a feasible attack. Furthermore, tearing down a single BGP session has wide-spread effects on
the global Internet routing tables.

Defenses
Obviously, the key to this attack is the relatively coarse
rate of change of the initial sequence number variable on
Berkeley systems. The TCP specification requires that thisvariable be incremented approximately 250,000 times per
second; Berkeley is using a much slower rate. However, thecritical factor is the granularity, not the average rate. Thechange from an increment of 128 per second in 4.2BSD to125,000 per second in 4.3BSD is meaningless, even though
the latter is within a factor of two of the specified rate.
Let us consider whether a counter that operated at a true
250,000 hz rate would help. For simplicity’s sake, we will
ignore the problem of other connections occurring, and only
consider the fixed rate of change of this counter.
To learn a current sequence number, one must send a
SYN packet, and receive a response, as follows:
X→S:
S→X:

SYN(ISNX )SYN(ISNS ), ACK(ISNX )(1)

The first spoof packet, which triggers generation of the next
sequence number, can immediately follow the server’s response to the probe packet:
X → S : SYN(ISNX ), SRC = T(2)The sequence number ISNS used in the response
S → T : SYN(ISNS ), ACK(ISNX )is uniquely determined by the time between the originationof message (1) and the receipt at the server of message (2).
But this number is precisely the round-trip time between X

and S. Thus, if the spoofer can accurately measure (and predict) that time, even a 4 µ-second clock will not defeat this
attack.
How accurately can the trip time be measured? If we assume that stability is good, we can probably bound it within
10 milliseconds or so. Clearly, the Internet does not exhibit
such stability over the long-term [64], but it is often goodenough over the short term.2 There is thus an uncertainty
of 2500 in the possible value for ISNS . If each trial takes
5 seconds, to allow time to re-measure the round-trip time,
an intruder would have a reasonable likelihood of succeeding in 7500 seconds, and a near-certainty within a day. Morepredictable (i.e., higher quality) networks, or more accurate
measurements, would improve the odds even further in the
intruder’s favor. Clearly, simply following the letter of the
TCP specification is not good enough.
We have thus far tacitly assumed that no processing takes
places on the target host. In fact, some processing does takeplace when a new request comes in; the amount of variability in this processing is critical. On a 6 MIPS machine, one
tick—4 µ-seconds—is about 25 instructions. There is thus
considerable sensitivity to the exact instruction path followed. High-priority interrupts, or a slightly different TCB
allocation sequence, will have a comparatively large effecton the actual value of the next sequence number. This randomizing effect is of considerable advantage to the target. It
should be noted, though, that faster machines are more vulnerable to this attack, since the variability of the instruction
path will take less real time, and hence affect the increment
less. And of course, CPU speeds are increasing rapidly.
This suggests another solution to sequence number attacks: randomizing the increment. Care must be taken to use
sufficient bits; if, say, only the low-order 8 bits were picked
randomly, and the granularity of the increment was coarse,
the intruder’s work factor is only multiplied by 256. A combination of a fine-granularity increment and a small random
number generator, or just a 32-bit generator, is better. Note,though, that many pseudo-random number generators are2

At the moment, the Internet may not have such stability even over the
short-term, especially on long-haul connections. It is not comfortingto know that the security of a network relies on its low quality of service.

easily invertible [13]. In fact, given that most such generators work via feedback of their output, the enemy could simply compute the next “random” number to be picked. Somehybrid techniques have promise—using a 32-bit generator,
for example, but only emitting 16 bits of it—but brute-force
attacks could succeed at determining the seed. One would
need at least 16 bits of random data in each increment, andperhaps more, to defeat probes from the network, but thatmight leave too few bits to guard against a search for theseed. More research or simulations are needed to determine
the proper parameters.
Rather than go to such lengths, it is simpler to use a cryptographic algorithm (or device) for ISNS generation. The
Data Encryption Standard [73] in electronic codebook mode[74] is an attractive choice as the ISNS source, with a simple counter as input. Alternatively, DES could be used in
output feedback mode without an additional counter. Eitherway, great care must be taken to select the key used. The
time-of-day at boot time is not adequate; sufficiently good
information about reboot times is often available to an intruder, thereby permitting a brute-force attack. If, however,
the reboot time is encrypted with a per-host secret key, thegenerator cannot be cracked with any reasonable effort.
Performance of the initial sequence number generator is
not a problem. New sequence numbers are needed only once
per connection, and even a software implementation of DES
will suffice. Encryption times of 2.3 milliseconds on a 1
MIPS processor have been reported [12].
An additional defense involves good logging and alerting mechanisms. Measurements of the round-trip time—essential for attacking RFC-compliant hosts—would
most likely be carried out using ICMP Ping messages;
a “transponder” function could log excessive ping requests. Other, perhaps more applicable, timing measurement techniques would involve attempted TCP connections; these connections are conspicuously short-lived, andmay not even complete SYN processing. Similarly, spoofing an active host will eventually generate unusual types of
RST packets; these should not occur often, and should be
logged.
After many years of thinking about it, I finally came up with a a solution to classicalsequence number attacks. The scheme, described in RFC 1948 [10], used a cryptographic hash function to create a separate
sequence number space for each “connection”, a connection being defined per RFC
791 [81] as the unique 4-tuple <localhost, localport, remotehost, remoteport>. This
scheme has not been adopted as widely
as I would like; my claim here that extra
CPU load during TCP connection establishment was irrelevant was rendered obsoleteby the advent of very large Web servers. Indeed, maximum TCP connection rate is avital metric when assessing modern systems.
Instead, many implementations use random ISNs or (especially) random increments.
This has obvious negative effects on the correctness of TCP in the presence of duplicate packets, a property that is guaranteed tohigher layers. (Also see the appendix of [52].)
Worse yet, Newsham pointed out that by thecentral limit theorem, the sum of a sequenceof random increments will have a normal distribution, which implies that the actual rangeof the ISNs is quite small. (see CERT Advisory CA-2001-09).
There are hybrid schemes that don’t fall tothese attacks, but the underlying message isthe same as it was in 1989: don’t rely on TCP
sequence numbers for security.
Also worth noting is the suggestion that intrusion detection system can play a role: they
can alert you to an attack that for some reason you can’t ward off.

3. The Joy of Routing
As noted at the beginng, routing problems
were one of the initial motivations for thiswork. For a fair number of years, though, I
said that “the only attack I discussed in thispaper that hasn’t been seen in the wild isrouting attacks”. That’s no longer the case;the bad guys have caught up.
Abuse of the routing mechanisms and protocols is probably the simplest protocol-based attack available. There area variety of ways to do this, depending on the exact routing
protocols used. Some of these attacks succeed only if the remote host does source address-based authentication; others
can be used for more powerful attacks.
A number of the attacks described below can also be
used to accomplish denial of service by confusing the routing tables on a host or gateway. The details are straightforward corollaries of the penetration mechanisms, and will
not be described further.

3.1. Source Routing
If available, the easiest mechanism to abuse is IP source
routing. Assume that the target host uses the reverse of thesource route provided in a TCP open request for return traffic. Such behavior is utterly reasonable; if the originator of
the connection wishes to specify a particular path for someuse it to view paths from different vantage points. Internally, such packets may
or may not be blocked; the question is irrelevant for many organizations, since therise of Microsoft Windows and the relative demise of UNIX-style remote login has
rendered the attack somewhat less interesting.

reason—say, because the automatic route is dead—replies
may not reach the originator if a different path is followed.
The attacker can then pick any IP source address desired, including that of a trusted machine on the target’s local network. Any facilities available to such machines become available to the attacker.
Again, I’m focusing here on address-based
authentication. But I move on to more subtle routing attacks.

Defenses
It is rather hard to defend against this sort of attack. The
best idea would be for the gateways into the local net to reject external packets that claim to be from the local net. Thisis less practical than it might seem since some Ethernet3
network adapters receive their own transmissions, and thisfeature is relied upon by some higher-level protocols. Furthermore, this solution fails completely if an organization
has two trusted networks connected via a multi-organization
backbone. Other users on the backbone may not be trustable
to the same extent that local users are presumed to be, or
perhaps their vulnerability to outside attack is higher. Arguably, such topologies should be avoided in any event.
Note that I’m alluding here to what are now
called “firewalls”. I’m not sure why I didn’t usethat word in this paper; I was using it in email
several years earlier.
A simpler method might be to reject pre-authorized connections if source routing information was present. This
presumes that there are few legitimate reasons for using
this IP option, especially for relatively normal operations.
A variation on this defense would be to analyze the source
route and accept it if only trusted gateways were listed; thatway, the final gateway could be counted on to deliver the
packet only to the true destination host. The complexity ofthis idea is probably not worthwhile.
Newer versions of the r -utilities do, in fact, reject source-routed connections. But there’s
a more subtle risk: though they reject theconnection attempt, an ACK packet is returned; this conveys the information needed
to launch a sequence number attack.
The most common configuration today is to reject source-routed packets atborder routers, whether or not they fillother firewall-related roles. Source routing is permitted on the backbone; ISPs
3

Ethernet is a registered trademark of Xerox Corporation.

Some protocols (i.e., Berkeley’s rlogin and rsh) permit
ordinary users to extend trust to remote host/user combinations. In that case, individual users, rather than an entire
system, may be targeted by source routing attacks.4 Suspicious gateways [69] will not help here, as the host being
spoofed may not be within the security domain protectedby the gateways.
Note the warning here against putting
too much trust in firewalls: they don’t defend against insider attacks.

3.2. Routing Information Protocol Attacks
The Routing Information Protocol [49], (RIP) is used
to propagate routing information on local networks, especially broadcast media. Typically, the information received
is unchecked. This allows an intruder to send bogus routing information to a target host, and to each of the gateways along the way, to impersonate a particular host. The
most likely attack of this sort would be to claim a route to aparticular unused host, rather than to a network; this would
cause all packets destined for that host to be sent to the intruder’s machine. (Diverting packets for an entire network
might be too noticeable; impersonating an idle work-stationis comparatively risk-free.) Once this is done, protocols that
rely on address-based authentication are effectively compromised.
This attack can yield more subtle, and more serious, benefits to the attacker as well. Assume that the attacker claimsa route to an active host or workstation instead. All packets for that host will be routed to the intruder’s machine forinspection and possible alteration. They are then resent, using IP source address routing, to the intended destination.
An outsider may thus capture passwords and other sensitive data. This mode of attack is unique in that it affects
outbound calls as well; thus, a user calling out from the targeted host can be tricked into divulging a password. Most of
the earlier attacks discussed are used to forge a source address; this one is focused on the destination address.
4

Permitting ordinary users to extend trust is probably wrong in any
event, regardless of abuse of the protocols. But such concerns are beyond the scope of this paper.

This and [77] are the earliest mentions ofrouting attacks in the literature. The attacks described here—abusing the routing protocols for eavesdropping and/or
packet modification—remain a very serious threat. Indeed, a National ResearchCouncil study [89] identified routing attacks as one of the two major threats
to the Internet. While there are proposals to solve this problem (see, for example,[71, 56, 55]), nothing has been implemented; all of the proposed solutions havetheir drawbacks. Defense against routing attacks must still be considered a research
problem.
Routing attacks have happened frequently
by accident. In the most famous case, known
as the “AS 7007” incident, an ISP started advertising that it had the best routes to most of
the Internet. Even after they powered down
their router, it took more than four hours forthe global routing tables to stabilize.
As suggested here, more subtle routing problems are harder to diagnose. AT&T’s
dial-up Internet service was knocked off the
air for many hours when another ISP started
advertising a route to a small, internal network. There are many other such incidents
as well.
Are malicious routing attacks happening?
Yes, they are, and the culprits are a very
low life form: the spammers. In some cases,
they’re hijacking a route, injecting spam, andthen withdrawing the route. The attack is hardto trace, because by the time someone notices it the source addresses of the email are(again) either non-existent or innocent.

Defenses
A RIP attack is somewhat easier to defend against than
the source-routing attacks, though some defenses are similar. A paranoid gateway—one that filters packets based on
source or destination address—will block any form of hostspoofing (including TCP sequence number attacks), since
the offending packets can never make it through. But there
are other ways to deal with RIP problems.
Filtering out packets with bogus source addresses would help against many forms of attack. Too few ISPs do it, even though it is arecommended practice [42].
One defense is for RIP to be more skeptical about theroutes it accepts. In most environments, there is no goodreason to accept new routes to your own local networks.
A router that makes this check can easily detect intrusion attempts. Unfortunately, some implementations rely on
hearing their own broadcasts to retain their knowledge ofdirectly-attached networks. The idea, presumably, is thatthey can use other networks to route around local outages.
While fault-tolerance is in general a good idea, the actual
utility of this technique is low in many environments compared with the risks.
It would be useful to be able to authenticate RIP packets;in the absence of inexpensive public-key signature schemes,
this is difficult for a broadcast protocol. Even if it were done,
its utility is limited; a receiver can only authenticate the immediate sender, which in turn may have been deceived by
gateways further upstream.
This paragraph summarizes the essential difficulty in defending against routing attacks:
the problem can originate with non-local machines. That is, even if your routing link toyour neighbors is authenticated, they may bedeceived rather than dishonest.
More and more sites are starting to protect their routing protocols against direct attacks. The most commonly used mechanism
is described in [50], caveats on key selection are given in [59]. Another mechanismis the so-called TTL Security Hack [45]: if apacket is supposed to originate on-link, send
it with a TTL of 255, and verify that on receipt.
Any off-link packets will have passed throughat least one router which would have decremented the TTL.
Even if the local routers don’t implement defense mechanisms, RIP attacks carry another risk: the bogus routing
entries are visible over a wide area. Any router (as opposed
to host) that receives such data will rebroadcast it; a suspicious administrator almost anywhere on the local collection of networks could notice the anomaly. Good log generation would help, but it is hard to distinguish a genuine
intrusion from the routing instability that can accompany agateway crash.
[104] analyzes how stable routes are to major name servers. The answer is encouraging: such routes change very infrequently..

3.3. Exterior Gateway Protocol
The Exterior Gateway Protocol (EGP) [65] is intended
for communications between the core gateways and socalled exterior gateways. An exterior gateway, after going through a neighbor acquisition protocol, is periodicallypolled by the core; it responds with information about thenetworks it serves. These networks must all be part of its
autonomous system. Similarly, the gateway periodically requests routing information from the core gateway. Data is
not normally sent except in response to a poll; furthermore,since each poll carries a sequence number that must be
echoed by the response, it is rather difficult for an intruder
to inject a false route update. Exterior gateways are allowed
to send exactly one spontaneous update between any two
polls; this, too, must carry the sequence number of the lastpoll received. It is thus comparatively difficult to interfere
in an on-going EGP conversation.
One possible attack would be to impersonate a second
exterior gateway for the same autonomous system. This
may not succeed, as the core gateways could be equippedwith a list of legitimate gateways to each autonomous system. Such checks are not currently done, however. Even if
they were, they could be authenticated only by source IP address.
A more powerful attack would be to claim reachability
for some network where the real gateway is down. Thatis, if gateway G normally handles traffic for network N ,and G is down, gateway G0 could advertise a route to that
network. This would allow password capture by assorted
mechanisms. The main defense against this attack is topological (and quite restrictive): exterior gateways must be on
the same network as the core; thus, the intruder would needto subvert not just any host, but an existing gateway or hostthat is directly on the main net.
A sequence number attack, similar to those used against
TCP, might be attempted; the difficulty here is in predicting what numbers the core gateway is using. In TCP, one
can establish arbitrary connections to probe for information; in EGP, only a few hosts may speak to the core. (Moreaccurately, the core could only speak to a few particularhosts, though as noted such checks are not currently implemented.) It may thus be hard to get the raw data needed
for such an attack.
EGP was reasonably secure because of thevery restricted topologies it could deal with: asingle core that talked with a variety of stub
networks. It couldn’t possibly work for today’s
Internet, which is why it’s been replaced by
BGP [84]. But loosening the restrictions has
had negative effects as well: BGP is easier toattack, since it’s conveying more complex information.
I asserted in the original paper that it waspossible to filter addresses announced inrouting packets. That wasn’t done in 1989;
today, however, major ISPs do filter advertisements from stub networks they talk to.
But not all ISPs do that, and it’s not possi-

ble to do much filtering when talking to peers
or transit ISPs.

3.4. The Internet Control Message Protocol
The Internet Control Message Protocol (ICMP) [79] isthe basic network management tool of the TCP/IP protocol
suite. It would seem to carry a rich potential for abuse. Surprisingly, ICMP attacks are rather difficult; still, there areoften holes that may be exploited.
The first, and most obvious target, is the ICMP Redirect
message; it is used by gateways to advise hosts of betterroutes. As such it can often be abused in the same way that
RIP can be. The complication is that a Redirect message
must be tied to a particular, existing connection; it cannot
be used to make an unsolicited change to the host’s routing
tables. Furthermore, Redirects are only applicable within alimited topology; they may be sent only from the first gateway along the path to the originating host. A later gateway
may not advise that host, nor may it use ICMP Redirect tocontrol other gateways.
Suppose, though, that an intruder has penetrated a secondary gateway available to a target host, but not the primary one. (It may suffice to penetrate an ordinary host on
the target’s local network, and have it claim to be a gateway.) Assume further that the intruder wishes to set up afalse route to trusted host T through that compromised secondary gateway. The following sequence may then be followed. Send a false TCP open packet to the target host,
claiming to be from T . The target will respond with its own
open packet, routing it through the secure primary gateway.
While this is in transit, a false Redirect may be sent, claiming to be from the primary gateway, and referring to the bogus connection. This packet will appear to be a legitimate
control message; hence the routing change it contains will
be accepted. If the target host makes this change to its global
routing tables, rather than just to the per-connection cachedroute, the intruder may proceed with spoofing host T .
Some hosts do not perform enough validity checks on
ICMP Redirect messages; in such cases, the impact of this
attack becomes similar to RIP-based attacks.
ICMP may also be used for targeted denial of service
attacks. Several of its messages, such as Destination Unreachable and Time to Live Exceeded, may be used to reset
existing connections. If the intruder knows the local and remote port numbers of a TCP connection, an ICMP packet
aimed at that connection may be forged.5 Such information
is sometimes available through the netstat service.
A more global denial of service attack can be launchedby sending a fraudulent Subnet Mask Reply message. Some
5

In fact, such programs are available today; they are used as administrative tools to reset hung TCP connections.

which gives them a much better idea of topology; hosts, by contrast, should not listen to
routing traffic. (This contradicts advice I gavein 1989.) That said, there is a current trend
towards hosts that know more about the network topology.
The paper mentions a number of denialof service attacks that could be launched bysending spurious ICMP error packets. Those
became reasonably common in the early andmid-1990s. But as systems started complying with the advice in RFC 1122 [1], that trenddied down; [1] mandates that most ICMP errors be treated as advisory messages, rather
than fatal errors.

hosts will accept any such message, whether they have senta query or not; a false one could effectively block all communications with the target host.

Defenses
Most ICMP attacks are easy to defend against with justa modicum of paranoia. If a host is careful about checking
that a message really does refer to a particular connection,
most such attacks will not succeed. In the case of TCP, thisincludes verifying that the ICMP packet contains a plausible sequence number in the returned-packet portion. Thesechecks are less applicable to UDP, though.
A defense against Redirect attacks merits additional attention, since such attacks can be more serious. Probably,
the best option is to restrict route changes to the specified
connection; the global routing table should not be modified
in response to ICMP Redirect messages.6
Finally, it is worth considering whether ICMP Redirects
are even useful in today’s environment. They are only usable on local networks with more than one gateway to theoutside world. But it is comparatively easy to maintain complete and correct local routing information. Redirect messages would be most useful from the core gateways to local exterior gateways, as that would allow such local gateways to have less than complete knowledge of the Internet;
this use is disallowed, however.
Subnet Mask attacks can be blocked if the Reply packet
is honored only at the appropriate time. In general, a hostwants to see such a message only at boot time, and only
if it had issued a query; a stale reply, or an unsolicited reply, should be rejected out of hand. There is little defense
against a forged reply to a genuine Subnet Mask query, as ahost that has sent such a query typically has few resources
with which to validate the response. If the genuine response
is not blocked by the intruder, though, the target will receive
multiple replies; a check to ensure that all replies agree
would guard against administrative errors as well.
ICMP attacks against routing have neverbeen very real. Anyone with the ability launchsuch an attack can use ARP-spoofing much
more easily; that, in fact, has been done.
Early RFCs suggested that routers could
also listen to ICMP Redirect messages; alater document [44] permits routers to ignore
such packets if directed to them. Routers,
almost by definition, run routing protocols,
6

This has other benefits as well, especially in environments where
ICMP-initiated route changes are not timed out. The author has seensituations where RIP instability following a gateway crash has led toerroneous ICMP Redirect messages. These had the effect of permanently corrupting the routing tables on other hosts.

4. The “Authentication” Server
As an alternative to address-based authentication, some
implementations use the Authentication Server [96] A
server that wishes to know the identity of its client may
contact the client host’s Authentication Server.7 and ask it
for information about the user owning a particular connection. This method is inherently more secure than simple
address-based authentication, as it uses a second TCP connection not under control of the attacker. It thus candefeat sequence number attacks and source routing attacks. There are certain risks, however.
The first, and most obvious, is that not all hosts are competent to run authentication servers. If the client host is notsecure, it does not matter who the user is claimed to be; theanswer cannot be trusted. Second, the authentication message itself can be compromised by routing table attacks. If
RIP has been used to alter the target’s idea of how to reachsome host, the authentication query will rely on the same altered routing data. Finally, if the target host is down, a variant on the TCP sequence number attack may be used; after
the server sends out a TCP open request to the presumed authentication server, the attacker can complete the open sequence and send a false reply. If the target runs a netstat
server, this is even easier; as noted, netstat will often supply the necessary sequence numbers with no need to guess.
A less-obvious risk is that a fake authentication server
can always reply “no”. This constitutes a denial of service
attack.

Defenses
A server that wishes to rely on another host’s idea of auser should use a more secure means of validation, such as
7

The Internet Activities Board does not currently recommend the Authentication Server for implementation [14]. However, the decision
was not made because of security problems [80].

the Needham-Schroeder algorithm [75, 28, 76]. TCP by itself is inadequate.
The original paper strongly suggested
that the authentication server was a bad
idea. Unfortunately, it has been modernized [98] and is still used today. Fortunately, its primary use is for auditing (especially of email), rather than authentication; even so, the weaknesses outlined
here (as well as in the Security Considerations section of [98] remain. Indeed,
I personally run a readily-available implementation that replies to all queries
with the message “ident-is-a-completelypointless-protocol-that-offers-no-security-ortraceability-at-all-so-take-this-and-log-it!”

5. Here be Dragons
Some protocols, while not inherently flawed, are nevertheless susceptible to abuse. A wise implementor would dowell to take these problems into account when providing theservice.

5.1. The “Finger” Service
Many systems implement a finger service [48]. This
server will display useful information about users, such as
their full names, phone numbers, office numbers, etc. Unfortunately, such data provides useful grist for the mill ofa password cracker [46]. By running such a service, a system administrator is giving away this data.
It is debatable whether or not this is an architectural problem or an implementation problem. The RFC never says precisely what
information shoudl be returned, though thesamples do show full names and a few phone
numbers. The precise question is generallymoot today for external attacks—firewalls will
generally block the finger protocol—but modern Web servers often release the same sortof information. Search engines yield evenmore data. Is password-guessing still useful to attackers? Beyond question, yes; I’ve
seen new implementations within the last few
months.

5.2. Electronic Mail
Electronic mail is probably the most valuable service on
the Internet. Nevertheless, it is quite vulnerable to misuse.

As normally implemented [24, 82], the mail server provides no authentication mechanisms. This leaves the door
wide open to faked messages. RFC 822 does support an Encrypted header line, but this is not widely used. (However,see RFC 1040 [60] for a discussion of a proposed new encryption standard for electronic mail.)
Authenticating and encrypting email have become far more important today than in 1989.
There is still no widely-deployed method ofauthenticating email; one is likely to be deployed in the near future, though arguably
for the wrong reason. Spammers and “phishers” use fake email addresses; there are anumber of proposals on the table to somehow authenticate the source of the email.
But they won’t work. The problem is that authentication proves an identity ; it says nothing of whether or not that party is authorized to send you email. Most people are willing to accept email from anyone; spammers
can and do claim to be sending email fromsites like asdfghij.com, and they’re right: thatis their email address. What does authentication prove?
It may slow down the phishers slightly,
but only slightly. True, if email were authenticated they could no longer claim to be YourRealBank.com, but they could claim tobe E-YourRealBank.com, YourRealBankOnilne.com, www-YourRealBank.com, etc.
(The very first phishing attempt I know ofclaimed to be from paypa1.com—and it was.
Of course, lots of people read that as paypal.com.)
Fake email has been used for other sinister purposes, such as stock market fraud:
the perpetrator sent a message to an investor’s newswire saying that wonderfulthings were happening to some company. Naturally, its stock went up—and hesold at the peak. Digitally-signed press releases would prevent that sort of thing—if therecipients checked the signatures, the certificates, etc., against some known-goodvalues.
Encrypting email is also useful in some situations, and there are a number of choices
available [39, 5]. The practical problem is thatthe endpoints aren’t secure.
5.2.1. The Post Office Protocol The The Post Office Protocol (POP) [15] allows a remote user to retrieve mail stored
on a central server machine. Authentication is by means ofa single command containing both the user name and thepassword. However, combining the two on a single command mandates the use of conventional passwords. And
such passwords are becoming less popular; they are too vulnerable to wire-tappers, intentional or accidental disclosure,etc.
As an alternative, many sites are adopting “one-time
passwords”.8 With one-time passwords, the host and somedevice available to the user share a cryptographic key. The
host issues a random challenge; both sides encrypt this number, and the user transmits it back to the host. Since the challenge is random, the reply is unique to that session, thereby
defeating eavesdroppers. And since the user does not know
the key—it is irretrievably stored in the device—the password cannot be given away without depriving the user of
the ability to log in.
The newest version of POP [87] has split the user nameand password into two commands, which is useful. However, it also defines an optional mechanism for preauthenticated connections, typically using Berkeley’s mechanisms.
Commendably, the security risks of this variant are mentioned explicitly in the document.
POP3 [72] has gained in importance; it’s theprincple mechanism people use to retrieve
email from servers. Simple passwords arestill the most common authentication mechanism; while a variant that uses SSL encryption [85] is available, most people don’t use it.
Another mail retrieval protocol, IMAP4 [23],has similar security properties: encryption isavailable but largely unused.
5.2.2. PCMAIL The PCMAIL protocol [58] uses authentication mechanisms similar to those in POP2. In
one major respect, PCMAIL is more dangerous: it supports a password-change command. This request requiresthat both the old and new passwords be transmitted unencrypted.
This protocol is no longer used.

5.3. The Domain Name System
The Domain Name System (DNS) [67, 68] provides fora distributed database mapping host names to IP addresses.
An intruder who interferes with the proper operation of the
DNS can mount a variety of attacks, including denial of service and password collection. There are a number of vulnerabilities.
In some resolver implementations, it is possible to mounta sequence number attack against a particular user. When
8

One-time passwords were apparently first used for military IFF (Identification Friend or Foe) systems [29].

the target user attempts to connect to a remote machine, anattacker can generate a domain server response to the target’s query. This requires knowing both the UDP port usedby the client’s resolver and the DNS sequence number used
for the query. The latter is often quite easy to obtain, though,since some resolvers always start their sequence numberswith 0. And the former may be obtainable via netstat orsome analogous host command.
A combined attack on the domain system and the routing mechanisms can be catastrophic. The intruder can intercept virtually all requests to translate names to IP addresses,
and supply the address of a subverted machine instead; thiswould allow the intruder to spy on all traffic, and build anice collection of passwords if desired.
For this reason, domain servers are high-value targets; asufficiently determined attacker might find it useful to takeover a server by other means, including subverting the machine one is on, or even physically interfering with its linkto the Internet. There is no network defense against the former, which suggests that domain servers should only run on
highly secure machines; the latter issue may be addressedby using authentication techniques on domain server responses.
The DNS, even when functioning correctly, can be used
for some types of spying. The normal mode of operationof the DNS is to make specific queries, and receive specific responses. However, a zone transfer (AXFR) request
exists that can be used to download an entire section of thedatabase; by applying this recursively, a complete map of
the name space can be produced. Such a database represents
a potential security risk; if, for example, an intruder knowsthat a particular brand of host or operating system has a particular vulnerability, that database can be consulted to findall such targets. Other uses for such a database include espionage; the number and type of machines in a particular organization, for example, can give away valuable data about
the size of the organization, and hence the resources committed to a particular project.
Fortunately, the domain system includes an error codefor “refused”; an administrative prohibition against suchzone transfers is explicitly recognized as a legitimate reason for refusal. This code should be employed for zonetransfer requests from any host not known to be a legitimate secondary server. Unfortunately, there is no authentication mechanism provided in the AXFR request; source
address authentication is the best that can be done.
Recently, a compatible authentication extension to the
DNS has been devised at M.I.T. The Hesiod name server[36] uses Kerberos [99] tickets to authenticate queries andresponses. The additional information section of the query
carries an encrypted ticket, which includes a session key;
this key, known only to Hesiod and the client, is used tocompute a cryptographic checksum of the both the querydoes not protect the data, and the responding name server might itself have been
deceived. The right solution is DNSsec[37], which provides for digitally-signed resource records. DNSsec deployment has
been very slow, partially because it wasso hard to get many of the design details right; see, for example, [47, 4, 105],

and the response. These checksums are also sent in the additional information field.
The DNS remains a crucial weak spot in theInternet [89]. Other attacks have been foundin the intervening years.
I was told of the first shortly after this paper
was published. Though I had railed against
address-based authentication, in fact the r utilities do name-based authentication: theylook up the hostname corresponding to theoriginator’s IP address, and use it to makean authentication decision. But given the way
hostname lookups work with the DNS, theowner of the IP address block involved controls what names are returned. Thus, if I own
192.0.2.0/24, I can create a PTR record for,say, 192.0.2.1; this record would identify thehost as YourTrustedHost.com. There are no
violations of the DNS or r -utility protocols involved here, nor any tricky address spoofing.
Instead, the attacker simply needs to lie.
The lesson is clear: when building security
systems, understand exactly what elements
are being trusted. In this case, not only wastrust residing in the DNS (itself a problem),trust was residing in a piece of the DNS controlled by the enemy. The fix (against this particular attack) was simple: use the returned
hostname and look up its IP address, andverify that it matches the address used in theconnection. Assuming that you control the
relevant portion of the DNS tree, this foils theattack. Of course, and as noted above, users
are able to extend trust, possibly to a partof the DNS tree not controlled by someone
trustworthy.
Even without ill-advised trust, you’re relying on the rest of the DNS. Cache contamination attacks [9] can create false entries.
For that matter, the technical and administrative procedures used to update zones suchas .com can be subverted; that’s happened,
too.
There are defenses against some of the
DNS attacks. Filtering [20] can prevent certain kinds of cache contamination attacks.
Many popular implementations have been
hardened against sequence number attacks[102]. A comprehensive discussion of DNSrelated threats can be found in [8].
I was wrong to laud Hesiod as a solution to DNS security problems. Hesiod protects the transmission and response; it

5.4. The File Transfer Protocol
The File Transfer Protocol (FTP) [83] itself is notflawed. However, a few aspects of the implementation merit some care.
5.4.1. FTP Authentication FTP relies on a login andpassword combination for authentication. As noted, simple passwords are increasingly seen as inadequate; more
and more sites are adopting one-time passwords. Nothing in the FTP specification precludes such an authentication method. It is vital, however, that the “331” response toa USER subcommand be displayed to the user; this message would presumably contain the challenge. An FTP implementation that concealed this response could not be usedin this mode; if such implementations are (or become) common, it may be necessary to use a new reply code to
indicate that the user must see the content of the challenge.
5.4.2. Anonymous FTP A second problem area is“anonymous FTP”. While not required by the FTP specification, anonymous FTP is a treasured part of the oral
tradition of the Internet. Nevertheless, it should be implemented with care.
One part of the problem is the implementation techniquechosen. Some implementations of FTP require creation ofa partial replica of the directory tree; care must be taken to
ensure that these files are not subject to compromise. Norshould they contain any sensitive information, such as encrypted passwords.
The second problem is that anonymous FTP is truly
anonymous; there is no record of who has requested what
information. Mail-based servers will provide that data; theyalso provide useful techniques for load-limiting,9 background transfers, etc.
FTP is hard to secure. It’s reasonably
straight-forward to encrypt the control channel; protecting the data channels is harder,
because they’re dynamic. Worse yet, a malicious FTP client can use the data channels
9

Recently, a host was temporarily rendered unusable by massive numbers of FTP requests for a popular technical report. If this were deliberate, it would be considered a successful denial of service attack.

to cause an innocent FTP server to attack athird host, in what’s known as a bounce attack. A security analysis of FTP (including
details of the bounce attack) can be foundin [7]; [51] describes cryptogprahic protection for FTP.

5.5. Simple Network Management Protocol
The Simple Network Management Protocol (SNMP)[17] has recently been defined to aid in network management. Clearly, access to such a resource must be heavily
protected. The RFC states this, but also allows for a null authentication service; this is a bad idea. Even a “read-only”
mode is dangerous; it may expose the target host to netstat-type attacks if the particular Management Information Base (MIB) [62] used includes sequence numbers. (The current standardized version does not; however,the MIB is explicitly declared to be extensible.)
SNMP authentication, as originally defined, boils down to a simple plaintext password known as the community string. All theusual problems with plaintext passwords apply, including eavesdropping and guessability. SNMPv3 [18] defines a User-based
Security Model [92] with cryptographic authentication. In addition, new MIBs arecarefully scrutinized for security-sensitive elements: one proposal that would have put
TCP sequence numbers into the MIB wascaught.

5.6. Remote Booting
Two sets of protocols are used today to boot disklessworkstations and gateways, Reverse ARP (RARP) [43] with
the Trivial File Transfer Protocol (TFTP) [94] and BOOTP[25] with TFTP. A system being booted is a tempting target; if one can subvert the boot process, a new kernel withaltered protection mechanisms can be substituted. RARPbased booting is riskier because it relies on Ethernet-likenetworks, with all the vulnerabilities adhering thereto. One
can achieve a modest improvement in security by ensuring that the booting machine uses a random number for its
UDP source port; otherwise, an attacker can impersonate
the server and send false DATA packets.
BOOTP adds an additional layer of security by including a 4-byte random transaction id. This prevents an attacker from generating false replies to a workstation known
to be rebooting. It is vital that these numbers indeed be random; this can be difficult in a system that is freshly powered up, and hence with little or no unpredictable state. Careshould be taken when booting through gateways; the more

networks traversed, the greater the opportunity for impersonation.
The greatest measure of protection is that normally, theattacker has only a single chance; a system being booted
does not stay in that state. If, however, communications between the client and the standard server may be interrupted,
larger-scale attacks may be mounted.
A newer boot-time protocol, DHCP [34] iseven more important. It provides hosts withIP addresses, DNS servers, default router,and more. Furthermore, DHCP queries canhappen with some frequency, if the leasetime of the address is short; this gives anattacker many more opportunities to do mischief.
There is a DHCP authentication option [3],but it is little-used. One reason is that anyone
who can mount a DHCP attack can launcha local network attack just as easily; merely
protecting DHCP does little to protect theclient.

6. Trivial Attacks
A few attacks are almost too trivial to mention; nevertheless, completeness demands that they at least be noted.

6.1. Vulnerability of the Local Network
Some local-area networks, notably the Ethernet networks, are extremely vulnerable to eavesdropping andhost-spoofing. If such networks are used, physical access must be strictly controlled. It is also unwise to trust
any hosts on such networks if any machine on the network is accessible to untrusted personnel, unless authentication servers are used.
If the local network uses the Address Resolution Protocol (ARP) [78] more subtle forms of host-spoofing are possible. In particular, it becomes trivial to intercept, modify,
and forward packets, rather than just taking over the host’srole or simply spying on all traffic.
It is possible to launch denial of service attacks by triggering broadcast storms. There are a variety of ways to do
this; it is quite easy if most or all of the hosts on the network
are acting as gateways. The attacker can broadcast a packet
destined for a non-existent IP address. Each host, upon receiving it, will attempt to forward it to the proper destination. This alone will represent a significant amount of traffic, as each host will generate a broadcast ARP query forthe destination. The attacker can follow up by broadcasting an ARP reply claiming that the broadcast Ethernet address is the proper way to reach that destination. Each suspectible host will then not only resend the bogus packet, itwill also receive many more copies of it from the other suspectible hosts on the network.
ARP attacks are easy to launch and hard to
spot. End-to-end encryption can prevent theworst problems, but they remain a potent vehicle for denial of service attacks. This is being addressed for IPv6 with the SEND enhancements to IPv6 Neighbor Discovery, itsversion of ARP.
There are two modern venues where
ARP attacks are particularly nasty. One ison wireless networks, especially in public hotspots. Another is using ARP to defeat
the presumed security properties of Ethernet switches. Some administrators assume
that using a switch prevents classical eavesdropping; it does, but ARP can be used toredirect the traffic despite that.
Broadcast storms have been launchedmaliciously, but in a more dangerous fashion. The attacker sends an ICMP Echo Request packet to the broadcast address ofsome local network; all hosts on that network send a reply to the originator of the
ICMP message. Of course, the source IP address in that packet isn’t that of the attacker; instead, it’s the address of the victim,
who gets bombarded with enough Echo Reply messages to clog the link. Smurf attacks(see CERT Advisory CA-1998-01) have become relatively rare since the default routerconfigurations were changed to disable directed broadcasts [91].

6.2. The Trivial File Transfer Protocol
TFTP [94] permits file transfers without any attempt at
authentication. Thus, any publicly-readable file in the entire universe is accessible. It is the responsibility of the implementor and/or the system administrator to make that universe as small as possible.

6.3. Reserved Ports
Berkeley-derived TCPs and UDPs have the notion of a“privileged port”. That is, port numbers lower than 1024
may only be allocated to privileged processes. This restriction is used as part of the authentication mechanism. However, neither the TCP nor the UDP specifications containany such concept, nor is such a concept even meaningful ona single-user computer. Administrators should never rely on
the Berkeley authentication schemes when talking to suchmachines.

Privileged ports are a bad idea, but they
could have been even worse. When FTP
bounce attacks are launched, the source
port is 21, in the privileged range. In otherwords, bounce attacks could have beenused to attack rlogin and rsh servers. Fortunately, out of a nagging sense of unease, those servers only accepted connections coming from ports in the range
512–1023.

7. Comprehensive Defenses
Thus far, we have described defenses against a variety of
individual attacks. Several techniques are broad-spectrum
defenses; they may be employed to guard against not only
these attacks, but many others as well.

7.1. Authentication
Many of the intrusions described above succeed only because the target host uses the IP source address for authentication, and assumes it to be genuine. Unfortunately, there
are sufficiently many ways to spoof this address that suchtechniques are all but worthless. Put another way, source
address authentication is the equivalent of a file cabinet secured with an S100 lock; it may reduce the temptation level
for more-or-less honest passers-by, but will do little or nothing to deter anyone even slightly serious about gaining entry.
Some form of cryptographic authentication is needed.
There are several possible approaches. Perhaps the bestknown is the Needham-Schroeder algorithm [75, 28, 76].
It relies on each host sharing a key with an authentication server; a host wishing to establish a connection obtains
a session key from the authentication server and passes a
sealed version along to the destination. At the conclusionof the dialog, each side is convinced of the identity of theother. Versions of the algorithm exist for both private-key
and public-key [30] cryptosystems.
How do these schemes fit together with TCP/IP? One answer is obvious: with them, preauthenticated connections
can be implemented safely; without them, they are quiterisky. A second answer is that the DNS provides an ideal
base for authentication systems, as it already incorporates
the necessary name structure, redundancy, etc. To be sure,key distribution responses must be authenticated and/or encrypted; as noted, the former seems to be necessary in any
event.
In some environments, care must be taken to use the session key to encrypt the entire conversation; if this is notdone, an attacker can take over a connection via the mechanisms described earlier.

Doing cryptography properly is a lot harderthan I made it seem. Indeed, the NeedhamSchroeder scheme—the oldest cryptographic protocol in the open literature—wasfound in 1996 to be vulnerable to a new flaw
[61]. That variant, expressed in modern notation, is only three lines long. . .

7.2. Encryption
Suitable encryption can defend against most of the attacks outlined above. But encryption devices are expensive,
often slow, hard to administer, and uncommon in the civilian sector. There are different ways to apply encryption;
each has its strengths and weaknesses. A comprehensive
treatment of encryption is beyond the scope of this paper;interested readers should consult Voydock and Kent [103]or Davies and Price [26]Link-level encryption—encrypting each packet as itleaves the host computer—is an excellent method of guarding against disclosure of information. It also works wellagainst physical intrusions; an attacker who tapped in toan Ethernet cable, for example, would not be able to inject spurious packets. Similarly, an intruder who cut theline to a name server would not be able to impersonate it. The number of entities that share a given key determines the security of the network; typically, a key
distribution center will allocate keys to each pair of communicating hosts.
Link-level encryption has some weaknesses, however.
Broadcast packets are difficult to secure; in the absence offast public-key cryptosystems, the ability to decode an encrypted broadcast implies the ability to send such a broadcast, impersonating any host on the network. Furthermore,
link-level encryption, by definition, is not end-to-end; security of a conversation across gateways implies trust in
the gateways and assurance that the full concatenated internet is similarly protected. (This latter constraint may be enforced administratively, as is done in the military sector.) If
such constraints are not met, tactics such as source-routing
attacks or RIP-spoofing may be employed. Paranoid gateways can be deployed at the entrance to security domains;these might, for example, block incoming RIP packets orsource-routed packets.
As pointed out in [54], I had Ethernet link encryptors in mind. Link encryptors on pointto-point links are quite different; they protect
traffic down to the bit level, sometimes hiding even the HDLC framing.
Many portions of the DARPA Internet employ forms oflink encryption. All Defense Data Network (DDN) IMPto-IMP trunks use DES encryption, even for non-classified

traffic; classified lines use more secure cryptosystems [27].
These, however, are point-to-point lines, which are comparatively easy to protect.
A multi-point link encryption device for TCP/IP is theBlacker Front End (BFE) [40]. The BFE looks to the host
like an X.25 DDN interface, and sits between the host andthe actual DDN line. When it receives a call request packet
specifying a new destination, it contacts an Access Control
Center (ACC) for permission, and a Key Distribution Center (KDC) for cryptographic keys. If the local host is denied permission to talk to the remote host, an appropriate
diagnostic code is returned. A special “Emergency Mode”
is available for communications to a restricted set of destinations at times when the link to the KDC or ACC is notworking.
The permission-checking can, to some extent, protect
against the DNS attacks described earlier. Even if a hosthas been mislead about the proper IP address for a particular destination, the BFE will ensure that a totally unauthorized host does not receive sensitive data. That is, assume
that a host wishes to send Top Secret data to some host foo.
A DNS attack might mislead the host into connecting topenetrated host 4.0.0.4, rather than 1.0.0.1. If 4.0.0.4 is notcleared for Top Secret material, or is not allowed communications with the local host, the connection attempt will
fail. To be sure, a denial of service attack has taken place;
this, in the military world, is far less serious than information loss.
The BFE also translates the original (“Red”) IP address
to an encrypted (“Black”) address, using a translation table supplied by the ACC. This is done to foil traffic analysis techniques, the bane of all multi-point link encryption
schemes.
I got a lot wrong here (see [54] for somedetails), partly because of my lack of experience with crypto at the time and partly
because of the sketchy information publicly
available on Blacker. With the benefit of hindsight (and a lot more experience with cryptography), I’d call the BFE a network-layer
encryptor for non-broadcast multiple accessnetworks, rather than a multi-point link-levelencryptor. But Blacker was obsolescent evenas I wrote the original paper; SP3—the ancestor of IPsec [57] —was being defined aspart of the Secure Data Network System. Regardless, the BFE (and SP3 and IPsec) all
create virtual private networks, with their own
address spaces; while there is some protection against traffic analysis, these technologies do not prevent an adversary from noticing which protected networks are talking towhich other protected networks.

End-to-end encryption, above the TCP level, may be
used to secure any conversation, regardless of the number ofhops or the quality of the links. This is probably appropriate
for centralized network management applications, or otherpoint-to-point transfers. Key distribution and management
is a greater problem, since there are more pairs of correspondents involved. Furthermore, since encryption and decryption are done before initiation or after termination of
the TCP processing, host-level software must arrange forthe translation; this implies extra overhead for each such
conversation.10
End-to-end encryption is vulnerable to denial of service
attacks, since fraudulently-injected packets can pass the
TCP checksum tests and make it to the application. A combination of end-to-end encryption and link-level encryption
can be employed to guard against this. An intriguing alternative would be to encrypt the data portion of the TCP segment, but not the header; the TCP checksum would be calculated on the cleartext, and hence would detect spurious
packets. Unfortunately, such a change would be incompatible with other implementations of TCP, and could not bedone transparently at application level.
I wrote “link-level encryption” here; it shouldhave been “network-level”. But there’s a more
subtle problem: the TCP checksum is not
cryptographically strong; it’s pretty easy tolaunch a variety of attacks against such a
weak integrity protection mechanism. See,for example, [11].
Regardless of the method used, a major benefit of encrypted communications is the implied authentication they
provide. If one assumes that the key distribution center issecure, and the key distribution protocols are adequate, thevery ability to communicate carries with it a strong assurance that one can trust the source host’s IP address for identification.
This implied authentication can be especially importantin high-threat situations. A routing attack can be used to“take over” an existing connection; the intruder can effectively cut the connection at the subverted machine, send
dangerous commands to the far end, and all the while translate sequence numbers on packets passed through so as todisguise the intrusion.
Today’s network-level encryptors would foil
this; they protect the TCP (and sometimesIP) headers as well as the payload. I suspectthat the BFE would as well.
10 We are assuming that TCP is handled by the host, and not by a frontend processor.

It should be noted, of course, that any of these encryption schemes provide privacy. Often that is the primary goalof such systems.

7.3. Trusted Systems
Given that TCP/IP is a Defense Department protocol
suite, it is worth asking to what extent the Orange Book
[31] and Red Book [33] criteria would protect a host fromthe attacks described above. That is, suppose that a target
host (and the gateways!) were rated B1 or higher. Could
these attacks succeed? The answer is a complex one, anddepends on the assumptions we are willing to make. In general, hosts and routers rated at B2 or higher are immune tothe attacks described here, while C2-level systems are susceptible. B1-level systems are vulnerable to some of these
attacks, but not all.
In order to understand how TCP/IP is used in secure environments, a brief tutorial on the military security model isnecessary. All objects in the computer system, such as files
or network channels, and all data exported from them, musthave a label indicating the sensitivity of the informationin them. This label includes hierarchical components (i.e.,
Confidential, Secret, and Top Secret) and non-hierarchicalcomponents. Subjects —i.e., processes within the computer
system—are similarly labeled. A subject may read an object if its label has a higher or equal hierarchical level and if
all of the object’s non-hierarchical components are includedin the subject’s label. In other words, the process must havesufficient clearance for the information in a file. Similarly,
a subject may write to an object if the object has a higher
or equal level and the object’s non-hierarchical components
include all of those in the subject’s level. That is, the sensitivity level of the file must be at least as high as that ofthe process. If it were not, a program with a high clearance
could write classified data to a file that is readable by a process with a low security clearance.
A corollary to this is that for read/write access to any file,its security label must exactly match that of the process. The
same applies to any form of bidirectional interprocess communication (i.e., a TCP virtual circuit): both ends must haveidentical labels.
We can now see how to apply this model to the TCP/IP
protocol suite. When a process creates a TCP connection,that connection is given the process’s label. This label is encoded in the IP security option. The remote TCP must ensure that the label on received packets matches that of thereceiving process. Servers awaiting connections may be eligible to run at multiple levels; when the connection is instantiated, however, the process must be forced to the levelof the connection request packet.
IP also makes use of the security option [97]. A packet
may not be sent over a link with a lower clearance level.

If a link is rated for Secret traffic, it may carry Unclassified or Confidential traffic, but it may not carry Top Secretdata. Thus, the security option constrains routing decisions.
The security level of a link depends on its inherent characteristics, the strength of any encryption algorithms used, thesecurity levels of the hosts on that network, and even thelocation of the facility. For example, an Ethernet cable located in a submarine is much more secure than if the samecable were running through a dormitory room in a university.
Several points follow from these constraints. First, TCPlevel attacks can only achieve penetration at the level of theattacker. That is, an attacker at the Unclassified level could
only achieve Unclassified privileges on the target system,
regardless of which network attack was used.11 Incoming
packets with an invalid security marking would be rejectedby the gateways.
Attacks based on any form of source-address authentication should be rejected as well. The Orange Book requiresthat systems provide secure means of identification and authentication; as we have shown, simple reliance on the IP
address is not adequate. As of the B1 level, authentication
information must be protected by cryptographic checksums
when transmitted from machine to machine.12
The authentication server is still problematic; it can be
spoofed by a sequence number attack, especially if netstat
is available. This sort of attack could easily be combinedwith source routing for full interactive access. Again, cryptographic checksums would add significant strength.
B1-level systems are not automatically immune fromrouting attacks; RIP-spoofing could corrupt their routing tables just as easily. As seen, that would allow an intruder tocapture passwords, perhaps even some used on other trusted
systems. To be sure, the initial penetration is still restrictedby the security labelling, but that may not block future logins captured by these means.
Routing attacks can also be used for denial of service.
Specifically, if the route to a secure destination is changed
to require use of an insecure link, the two hosts will not beable to communicate. This change would probably be detected rather quickly, though, since the gateway that noticed
the misrouted packet would flag it as a security problem.
At the B2 level, secure transmission of routing control
information is required. Similar requirements apply to othernetwork control information, such as ICMP packets.
Several attacks we have described rely on data derived from “information servers”, such as netstat and fin11 We are assuming, of course, that the penetrated system does not havebugs of its own that would allow further access.
12 More precisely, user identification information must be protected to an
equal extent with data sensitivity labels. Under certain circumstances,
described in the Red Book, cryptographic checks may be omitted. In
general, though, they are required.

ger. While these, if carefully done, may not represent a
direct penetration threat in the civilian sense, they are often seen to represent a covert channel that may be used toleak information. Thus, many B-division systems do not implement such servers.
In a practical sense, some of the technical features wehave described may not apply in the military world. Administrative rules [32] tend to prohibit risky sorts of interconnections; uncleared personnel are not likely to have evenindirect access to systems containing Top Secret data. Suchrules are, most likely, an accurate commentary on anyone’s
ability to validate any computer system of non-trivial size.
This is an odd section for this paper, in that it
attempts a very brief look at a completely different technology architecture: Orange Bookstyle secure systems. Worse yet, the Orange Book prescribes results, not methods;
as such, it is difficult to find a precise match
between the attacks I described and abstract
mechanisms.
In any event, the question is moot today.
The Orange Book and its multi-hued kin havebeen replaced by the Common Criteria [19],an international effort at defining secure systems. In most ways, the Common Criteria iseven more abstract than its DoD predecessor; one could not make meaningful statements without at least specifying what protection profile one was interested in. (Apartfrom that, one can make a strong argumentthat that entire approach to system security
is flawed [89], but such a discussion is out of
scope for this paper.)Two technical points are worth noting. First, routing attacks could be mitigated by maintenance of separate routing tables (by multi-level secure routers)for different security classifications. Second, exactly what forms of authentication
are acceptable in any situation would depend critically on detailed knowledge of exactly what sorts of hosts were connected towhat sorts of network. In other words, eavesdropping may or may not be a concern.

8. Conclusions
Several points are immediately obvious from this analysis. The first, surely, is that in general, relying on the IP
source address for authentication is extremely dangerous.13
13 There are some exceptions to this rule. If the entire network, and all of
its components (hosts, gateways, cables, etc.) are physically protected,

Fortunately, the Internet community is starting to accept
this on more than an intellectual level. The Berkeley manuals [22] have always stated that the authentication protocol was very weak, but it is only recently that serious attempts (i.e., Kerberos [99] and SunOS 4.0’s DES authentication mode [101]) have been made to correct the problem.
Kerberos and SunOS 4.0 have their weaknesses, but bothare far better than their predecessor. More recently, an extension to the Network Time Protocol (NTP) [66] has been
proposed that includes a cryptographic checksum [63].
A second broad class of problems is sequence number
attacks. If a protocol depends on sequence numbers—andmost do—it is vital that they be chosen unpredictably. It is
worth considerable effort to ensure that these numbers are
not knowable even to other users on the same system.
We may generalize this by by stating that hosts should
not give away knowledge gratuitously. A finger server, forexample, would be much safer if it only supplied information about a known user, rather than supplying information about everyone logged on. Even then, some censorship might be appropriate; a refusal to supply the last login
date and other sensitive information would be appropriate
if the account was not used recently. (Never-used accounts
often have simple default passwords. Infrequently-used accounts are often set up less carefully by the owner.) We havealso seen how netstat may be abused; indeed, the combination of netstat with the authentication server is the single
strongest attack using the standardized Internet protocols.
Finally, network control mechanisms are dangerous, andmust be carefully guarded. Static routes are not feasible in alarge-scale network, but intelligent use of default routes andverifiable point-to-point routing protocols (i.e., EGP) are farless vulnerable than broadcast-based routing.

9. Acknowledgments
Dave Presotto, Bob Gilligan, Gene Tsudik, and especially Deborah Estrin made a number of useful suggestions
and corrections to a draft of this paper.

10. Retrospective Conclusions
The Internet of 1989 was a much
simpler—and much friendlier—place than it
is today. Most of the protocols I looked atwere comparatively simple client-server protocols; today’s multi-party protocols—SIP[88], Diameter [16], various peer-to-peer protocols, etc.—are much harder to analyze.
and if all of the operating systems are sufficiently secure, there would
seem to be little risk.

Often, the crucial question is not authentication but authorization: how do you know if acertain party is permitted to perform a certain action?
The overall trend has been good. The Internet Engineering Task Force (IETF) will
not standardize protocols where the only
mandatory-to-implement form of authentication is plaintext passwords; address-based
authentication is acceptable only in very restricted circumstances. There are standardized and/or widely deployed cryptographic
protocols for remote login, sending and receiving email, Web browsing, etc. As discussed earlier, outing is the major exception;
operationally, it is still not securable.
Most of the security problems we encounter on the Internet today are due tobuggy code (including, of course, buggy
code in cryptographic modules). To some extent, of course, that’s because there areso many bugs; why launch a difficult attack when there’s so much low-hanging fruit
available?
Password-guessing is still a common attack; indeed, a new wave of passwordguessing has recently been observed
against ssh [106], a cryptographicallyprotected replacement for the r -utilities.
There is no need to belabor the earlier
conclusions that predictable sequence numbers and address-based authentication arebad. This is now well-accepted; for example,
the specification for a new transport protocol,
SCTP [100], mandates strong random number generation [6] for its analogous fields. But
how does one design a secure protocol?
One answer is to look at data flow of
the protocol. On what elements does authentication depend? Can an attacker tamper
with or mimic those elements? What are theguarantees, especially the security guarantees, of each element? Naturally, the powersof the attacker must be taken into account.
Seen from this perspective, the flaws in ther -utilities are clear.
One more point should be mentioned: it
is often the availability of auxiliary data thatmakes attacks possible. An attacker who
can create just one TCP connection cannot guess the correct sequence number.
This is often a fruitful mechanism for blocking (or at least detecting) attacks.

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