This POODLE Bites: Exploiting The
SSL 3.0 Fallback
Security Advisory
Bodo Möller, Thai Duong, Krzysztof Kotowicz
GoogleSeptember 2014
{bmoeller, thaidn, koto}@google.com

Introduction
SSL 3.0 [RFC6101] is an obsolete and insecure protocol. While for most practical
purposes it has been replaced by its successors TLS 1.0 [RFC2246], TLS 1.1 [RFC4346],and TLS 1.2 [RFC5246], many TLS implementations remain backwards­compatible with
SSL 3.0 to interoperate with legacy systems in the interest of a smooth user experience.
The protocol handshake provides for authenticated version negotiation, so normally thelatest protocol version common to the client and the server will be used.
However, even if a client and server both support a version of TLS, the security level
offered by SSL 3.0 is still relevant since many clients implement a protocol downgrade
dance to work around server­side interoperability bugs. In this Security Advisory, we
discuss how attackers can exploit the downgrade dance and break the cryptographic
security of SSL 3.0. Our POODLE attack (Padding Oracle On Downgraded LegacyEncryption) will allow them, for example, to steal “secure” HTTP cookies (or other bearer
tokens such as HTTP Authorization header contents).
We then give recommendations for both clients and servers on how to counter the attack:
if disabling SSL 3.0 entirely is not acceptable out of interoperability concerns, TLS
implementations should make use of TLS_FALLBACK_SCSV.
CVE­2014­3566 has been allocated for this protocol vulnerability.

The POODLE Attack
To work with legacy servers, many TLS clients implement a downgrade dance: in a firsthandshake attempt, offer the highest protocol version supported by the client; if thishandshake fails, retry (possibly repeatedly) with earlier protocol versions. Unlike proper
protocol version negotiation (if the client offers TLS 1.2, the server may respond with, say,
TLS 1.0), this downgrade can also be triggered by network glitches, or by active attackers.
So if an attacker that controls the network between the client and the server interferes withany attempted handshake offering TLS 1.0 or later, such clients will readily confinethemselves to SSL 3.0.
Encryption in SSL 3.0 uses either the RC4 stream cipher, or a block cipher in CBC mode.
RC4 is well known to have biases [RC4­biases], meaning that if the same secret (such asa password or HTTP cookie) is sent over many connections and thus encrypted with many
RC4 streams, more and more information about it will leak. We show here how to put
together an effective attack against CBC encryption as used by SSL 3.0, again assumingthat the attacker can modify network transmissions between the client and the server.
Unlike with the BEAST [BEAST] and Lucky 13 [Lucky­13] attacks, there is no reasonable
workaround. This leaves us with no secure SSL 3.0 cipher suites at all: to achieve secureencryption, SSL 3.0 must be avoided entirely.
The most severe problem of CBC encryption in SSL 3.0 is that its block cipher padding is
not deterministic, and not covered by the MAC (Message Authentication Code): thus, theintegrity of padding cannot be fully verified when decrypting. Padding by 1 to L bytes(where L is the block size in bytes) is used to obtain an integral number of blocks before
performing blockwise CBC (cipher­block chaining) encryption. The weakness is the easiest
to exploit if there’s an entire block of padding, which (before encryption) consists of L­1arbitrary bytes followed by a single byte of value L­1. To process an incoming ciphertext
record C1 ... Cn also given an initialization vector C0 (where each Ci is one block), therecipient first determines P1 ... Pn as Pi = DK(Ci) ⊕ Ci­1 (where DK denotes block­cipher
decryption using per­connection key K), then checks and removes the padding at the end,
and finally checks and removes a MAC. Now observe that if there’s a full block of padding
and an attacker replaces Cn by any earlier ciphertext block Ci from the same encrypted
stream, the ciphertext will still be accepted if DK(Ci) ⊕ Cn­1 happens to have L­1 as its finalbyte, but will in all likelihood be rejected otherwise, giving rise to a padding oracle attack[tls­cbc].
In the web setting, this SSL 3.0 weakness can be exploited by a man­in­the middle
attacker to decrypt “secure” HTTP cookies, using techniques from the BEAST attack[BEAST]. To launch the POODLE attack (Padding Oracle On Downgraded Legacy
Encryption), run a JavaScript agent on evil.com (or on http://example.com) to get thevictim’s browser to send cookie­bearing HTTPS requests to https://example.com, andintercept and modify the SSL records sent by the browser in such a way that there’s anon­negligible chance that example.com will accept the modified record. If the modified
record is accepted, the attacker can decrypt one byte of the cookies.
Assume that each block C has 16 bytes, C[0] ... C[15]. (Eight­byte blocks can be handledsimilarly.) Also assume, for now, that the size of the cookies is known. (Later we will show
how to start the attack if it isn’t.) The MAC size in SSL 3.0 CBC cipher suites is typically 20
bytes, so below the CBC layer, an encrypted POST request will look as follows:
POST /path Cookie: name=value...\r\n\r\nbody ‖ 20­byte MAC ‖ padding
The attacker controls both the request path and the request body, and thus can induce
requests such that the following two conditions hold:● The padding fills an entire block (encrypted into Cn).
● The cookies’ first as­of­yet unknown byte appears as the final byte in an earlier
block (encrypted into Ci).
The attacker then replaces Cn by Ci and forwards this modified SSL record to the server.

Usually, the server will reject this record, and the attacker will simply try again with a new
request. Occasionally (on average, once in 256 requests), the server will accept themodified record, and the attacker will conclude that DK(Ci)[15] ⊕ Cn­1[15] = 15, and thusthat Pi[15] = 15 ⊕ Cn­1[15] ⊕ Ci­1[15]. This reveals the cookies’ first previously unknown
byte. The attacker proceeds to the next byte by changing the sizes of request path andbody simultaneously such that the request size stays the same but the position of theheaders is shifted1, continuing until it has decrypted as much of the cookies as desired.
The expected overall effort is 256 SSL 3.0 requests per byte.
As the padding hides the exact size of the payload, the cookies’ size is not immediately
apparent, but inducing requests GET /, GET /A, GET /AA, ... allows the attacker toobserve at which point the block boundary gets crossed: after at most 16 such requests,
this will reveal the padding size, and thus the size of the cookies.

Recommendations
The attack described above requires an SSL 3.0 connection to be established, sodisabling the SSL 3.0 protocol in the client or in the server (or both) will completely avoid it.
If either side supports only SSL 3.0, then all hope is gone, and a serious update required
to avoid insecure encryption. If SSL 3.0 is neither disabled nor the only possible protocolversion, then the attack is possible if the client uses a downgrade dance forinteroperability.
Disabling SSL 3.0 entirely right away may not be practical if it is needed occasionally towork with legacy systems. Also, similar protocol version downgrades are still a concern
with newer protocol versions (although not nearly as severe as with SSL 3.0). The
TLS_FALLBACK_SCSV mechanism from [draft­ietf­tls­downgrade­scsv­00] addresses thebroader issue across protocol versions versions, and we consider it crucial especially forsystems that maintain SSL 3.0 compatibility. The following recommendations summarize
how TLS_FALLBACK_SCSV works.
TLS clients that use a downgrade dance to improve interoperability should include thevalue 0x56, 0x00 (TLS_FALLBACK_SCSV) in ClientHello.cipher_suites in any fallback
handshakes. This value serves as a signal allowing updated servers to reject theconnection in case of a downgrade attack. Clients should always fall back to the next
lower version (if starting at TLS 1.2, try TLS 1.1 next, then TLS 1.0, then SSL 3.0) becauseskipping a protocol version forgoes its better security. (With TLS_FALLBACK_SCSV,
skipping a version also could entirely prevent a successful handshake if it happens to be
the version that should be used with the server in question.)
In TLS servers, whenever an incoming connection includes 0x56, 0x00
(TLS_FALLBACK_SCSV) in ClientHello.cipher_suites, compare ClientHello.client_version
to the highest protocol version supported by the server. If the server supports a version
higher than the one indicated by the client, reject the connection with a fatal alert(preferably, inappropriate_fallback(86) from [draft­ietf­tls­downgrade­scsv­00]).

1

Experimenting with a proof of concept for the POODLE attack, we found that some browsers sendrequest header and request body in separate SSL records. In this case, only the path size needs to bechanged when proceeding to the next target byte.

This use of TLS_FALLBACK_SCSV will ensure that SSL 3.0 is used only when a legacy
implementation is involved: attackers can no longer force a protocol downgrade. (Attacksremain possible if both parties allow SSL 3.0 but one of them is not updated to support
TLS_FALLBACK_SCSV, provided that the client implements a downgrade dance down toSSL 3.0.)

References[BEAST] T. Duong, J. Rizzo: “Here Come The ⊕ Ninjas”, 2011.
[draft­ietf­tls­downgrade­scsv­00] B. Möller, A. Langley: “TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade Attacks”, Internet­Draft
draft­ietf­tls­downgrade­scsv­00, 2014.
[Lucky­13] N.J. AlFardan, K.G. Paterson: “Lucky Thirteen: Breaking the TLS and DTLS
Record Protocols”, IEEE Symposium on Security and Privacy, 2013.
[RC4­biases] N.J. AlFardan, D.J. Bernstein, K.G. Paterson, B. Poettering, J.C.N. Schuldt:“On the Security of RC4 in TLS and WPA”, USENIX Security Symposium, 2013.
[RFC2246] T. Dierks, C. Allen: “The TLS Protocol Version 1.0”, RFC 2246, 1998.
[RFC4346] T. Dierks, E. Rescorla: “The Transport Layer Security (TLS) Protocol Version1.1”, RFC 4346, 2006.
[RFC5246] T. Dierks, E. Rescorla: “The Transport Layer Security (TLS) Protocol Version1.2”, RFC 5246, 2008.
[RFC6101] A. Freier, P. Karlton, P. Kocher: “The Secure Sockets Layer (SSL) Protocol
Version 3.0”, RFC 6101, 1996 (published as Historic RFC in 2011).
[tls­cbc] B. Möller: “Security of CBC Ciphersuites in SSL/TLS: Problems andCountermeasures”, http://www.openssl.org/~bodo/tls­cbc.txt, 2004.