Session F3: Crypto Pitfalls

CCS’17, October 30-November 3, 2017, Dallas, TX, USA

Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2
Mathy Vanhoef

Frank Piessens

imec-DistriNet, KU Leuven
Mathy.Vanhoef@cs.kuleuven.be

imec-DistriNet, KU LeuvenFrank.Piessens@cs.kuleuven.be

ABSTRACT

work, we present design flaws in the 4-way handshake, and in
related handshakes. Because we target these handshakes, both WPAand WPA2-certified products are affected by our attacks.
The 4-way handshake provides mutual authentication and session key agreement. Together with (AES)-CCMP, a data-confidentiality and integrity protocol, it forms the foundation of the 802.11i
amendment. Since its first introduction in 2003, under the name
WPA, this core part of the 802.11i amendment has remained freefrom attacks. Indeed, the only currently known weaknesses of
802.11i are in (WPA-)TKIP [57, 66]. This data-confidentiality protocol was designed as a short-term solution to the broken WEP
protocol. In other words, TKIP was never intended to be a longterm secure solution. Additionally, while several attacks against
protected Wi-Fi networks were discovered over the years, these did
not exploit flaws in 802.11i. Instead, attacks exploited flaws in Wi-Fi
Protected Setup (WPS) [73], flawed drivers [13, 20], flawed random
number generators [72], predictable pre-shared keys [45], insecure
enterprise authentication [21], and so on. That no major weakness
has been found in CCMP and the 4-way handshake, is not surprising. After all, both have been formally proven as secure [39, 42].
With this in mind, one might reasonably assume the design of the
4-way handshake is indeed secure.
In spite of its history and security proofs though, we show thatthe 4-way handshake is vulnerable to key reinstallation attacks.
Moreover, we discovered similar weaknesses in other Wi-Fi handshakes. That is, we also attack the PeerKey handshake, the group
key handshake, and the Fast BSS Transition (FT) handshake.
The idea behind our attacks is rather trivial in hindsight, and canbe summarized as follows. When a client joins a network, it executesthe 4-way handshake to negotiate a fresh session key. It will install
this key after receiving message 3 of the handshake. Once the key
is installed, it will be used to encrypt normal data frames using adata-confidentiality protocol. However, because messages may belost or dropped, the Access Point (AP) will retransmit message 3 if
it did not receive an appropriate response as acknowledgment. As
a result, the client may receive message 3 multiple times. Each time
it receives this message, it will reinstall the same session key, andthereby reset the incremental transmit packet number (nonce) andreceive replay counter used by the data-confidentiality protocol.
We show that an attacker can force these nonce resets by collecting
and replaying retransmissions of message 3. By forcing nonce reusein this manner, the data-confidentiality protocol can be attacked,
e.g., packets can be replayed, decrypted, and/or forged. The same
technique is used to attack the group key, PeerKey, and fast BSS
transition handshake.
When the 4-way or fast BSS transition handshake is attacked,
the precise impact depends on the data-confidentiality protocol
being used. If CCMP is used, arbitrary packets can be decrypted.
In turn, this can be used to decrypt TCP SYN packets, and hijack
TCP connections. For example, an adversary can inject malicious

We introduce the key reinstallation attack. This attack abuses design
or implementation flaws in cryptographic protocols to reinstall an
already-in-use key. This resets the key’s associated parameters suchas transmit nonces and receive replay counters. Several types ofcryptographic Wi-Fi handshakes are affected by the attack.
All protected Wi-Fi networks use the 4-way handshake to generate a fresh session key. So far, this 14-year-old handshake hasremained free from attacks, and is even proven secure. However,
we show that the 4-way handshake is vulnerable to a key reinstallation attack. Here, the adversary tricks a victim into reinstalling an
already-in-use key. This is achieved by manipulating and replaying
handshake messages. When reinstalling the key, associated parameters such as the incremental transmit packet number (nonce) andreceive packet number (replay counter) are reset to their initialvalue. Our key reinstallation attack also breaks the PeerKey, groupkey, and Fast BSS Transition (FT) handshake. The impact depends
on the handshake being attacked, and the data-confidentiality protocol in use. Simplified, against AES-CCMP an adversary can replay
and decrypt (but not forge) packets. This makes it possible to hijack
TCP streams and inject malicious data into them. Against WPATKIP and GCMP the impact is catastrophic: packets can be replayed,
decrypted, and forged. Because GCMP uses the same authentication
key in both communication directions, it is especially affected.
Finally, we confirmed our findings in practice, and found thatevery Wi-Fi device is vulnerable to some variant of our attacks.
Notably, our attack is exceptionally devastating against Android 6.0:
it forces the client into using a predictable all-zero encryption key.

KEYWORDS
security protocols; network security; attacks; key reinstallation;
WPA2; nonce reuse; handshake; packet number; initialization vector

1

INTRODUCTION

All protected Wi-Fi networks are secured using some version of
Wi-Fi Protected Access (WPA/2). Moreover, nowadays even public
hotspots are able to use authenticated encryption thanks to the
Hotspot 2.0 program [7]. All these technologies rely on the 4-way
handshake defined in the 802.11i amendment of 802.11 [4]. In thisPermission to make digital or hard copies of all or part of this work for personal orclassroom use is granted without fee provided that copies are not made or distributedfor profit or commercial advantage and that copies bear this notice and the full citationon the first page. Copyrights for components of this work owned by others than theauthor(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or
republish, to post on servers or to redistribute to lists, requires prior specific permission
and/or a fee. Request permissions from permissions@acm.org.
CCS’17, October 30–November 3, 2017, Dallas, TX, USA.
© 2017 Copyright held by the owner/author(s). Publication rights licensed to Association for Computing Machinery.
ACM ISBN 978-1-4503-4946-8/17/10. . . $15.00
https://doi.org/10.1145/3133956.3134027

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Session F3: Crypto Pitfalls

CCS’17, October 30-November 3, 2017, Dallas, TX, USA

CCS’17, October 30–November 3, 2017, Dallas, TX, USA.

Mathy Vanhoef and Frank Piessens

2.1

content into unencrypted HTTP connections. If TKIP or GCMP isused, an adversary can both decrypt and inject arbitrary packets.
Although GCMP is a relatively new addition to Wi-Fi, it is expected
to be adopted at a high rate in the next few years [58]. Finally,
when the group key handshake is attacked, an adversary can replay
group-addressed frames, i.e., broadcast and multicast frames.
Our attack is especially devastating against version 2.4 and 2.5 of
wpa_supplicant, a Wi-Fi client commonly used on Linux. Here, theclient will install an all-zero encryption key instead of reinstalling
the real key. This vulnerability appears to be caused by a remark
in the 802.11 standard that suggests to clear parts of the session
key from memory once it has been installed [1, §12.7.6.6]. Because
Android uses a modified wpa_supplicant, Android 6.0 and Android
Wear 2.0 also contain this vulnerability. As a result, currently 31.2%
of Android devices are vulnerable to this exceptionally devastating
variant of our attack [33].
Interestingly, our attacks do not violate the security properties
proven in formal analysis of the 4-way and group key handshake.
In particular, these proofs state that the negotiated session key
remains private, and that the identity of both the client and Access
Point (AP) is confirmed [39]. Our attacks do not leak the session
key. Additionally, although normal data frames can be forged if
TKIP or GCMP is used, an attacker cannot forge EAPOL messages
and hence cannot impersonate the client or AP during (subsequent)handshakes. Instead, the problem is that the proofs do not model
key installation. Put differently, their models do not state when anegotiated key should be installed. In practice, this means the same
key can be installed multiple times, thereby resetting nonces andreplay counters used by the data-confidentiality protocol.
To summarize, our main contributions are:2.2

Authentication and Association

When a client wants to connect to a Wi-Fi network, it starts by(mutually) authenticating and associating with the AP. In Figure 2
this is illustrated in the association stage of the handshake. However,when first connecting to a network, no actual authentication takes
places at this stage. Instead, Open System authentication is used,which allows any client to authenticate. Actual authentication will
be performed during the 4-way handshake. Real authentication is
only done at this stage when roaming between two APs of the same
network using the fast BSS transition handshake (see Section 3).
After (open) authentication, the client associates with the network. This is done by sending an association request to the AP.
This message contains the pairwise and group cipher suites theclient wishes to use. The AP replies with an association response,
informing the client whether the association was successful or not.

• We introduce key reinstallation attacks. Here, an attacker
forces the reinstallation of an already-in-use key, thereby
resetting any associated nonces and/or replay counters.
• We show that the 4-way handshake, PeerKey handshake,group key handshake, and fast BSS transition handshake arevulnerable to key reinstallation attacks.
• We devise attack techniques to carry out our attacks in practice. This demonstrates that all implementations are vulnerable to some variant of our attack.
• We evaluate the practical impact of nonce reuse for all dataconfidentiality protocols of 802.11.

2.3

The 4-way Handshake

The 4-way handshake provides mutual authentication based on ashared secret called the Pairwise Master Key (PMK), and negotiatesa fresh session key called the Pairwise Transient Key (PTK). During
this handshake, the client is called the supplicant, and the AP iscalled the authenticator (we use these terms as synonyms). The
PMK is derived from a pre-shared password in a personal network,
and is negotiated using an 802.1x authentication stage in an enterprise network (see Figure 2). The PTK is derived from the PMK,
Authenticator Nonce (ANonce), Supplicant Nonce (SNonce), andthe MAC addresses of both the supplicant and authenticator. Once
generated, the PTK is split into a Key Confirmation Key (KCK), Key
Encryption Key (KEK), and Temporal Key (TK). The KCK and KEK
are used to protect handshake messages, while the TK is used toprotect normal data frames with a data-confidentiality protocol. If
WPA2 is used, the 4-way handshake also transports the current
Group Temporal Key (GTK) to the supplicant.

The remainder of this paper is structured as follows. Section 2
introduces relevant aspects of the 802.11 standard. Our key reinstallation attack is illustrated against the 4-way and PeerKey handshakein Section 3, against the group key handshake in Section 4, andagainst the fast BSS transition handshake in Section 5. In Section 6
we asses the impact of our attacks, present countermeasures, explain where proofs failed, and discuss lessons learned. Finally, we
present related work in Section 7 and conclude in Section 8.

2

The 802.11i Amendment

After researchers showed that Wired Equivalent Privacy (WEP) was
fundamentally broken [30, 65], the IEEE offered a more robust solution in the 802.11i amendment of 802.11. This amendment defines
the 4-way handshake (see Section 2.3), and two data-confidentiality
and integrity protocols called (WPA-)TKIP and (AES-)CCMP (seeSection 2.4). While the 802.11i amendment was under development,
the Wi-Fi Alliance already began certifying devices based on draft
version D3.0 of 802.11i. This certification program was called Wi-Fi
Protected Access (WPA). Once the final version D9.0 of 802.11i was
ratified, the WPA2 certification was created based on this officiallyratified version. Because both WPA and WPA2 are based on 802.11i,
they are almost identical on a technical level. The main differenceis that WPA2 mandates support for the more secure CCMP, andoptionally allows TKIP, while the reverse is true for WPA.
Required functionality of both WPA and WPA2, and used by all
protected Wi-Fi networks, is the 4-way handshake. Even enterprisenetworks rely on the 4-way handshake. Hence, all protected Wi-Fi
networks are affected by our attacks.
The 4-way handshake, group key handshake, and CCMP protocol, have formally been analyzed and proven to be secure [39, 42].

BACKGROUND

In this section we introduce the 802.11i amendment, the various
messages and handshakes used when connecting to a Wi-Fi network, and the data-confidentiality and integrity protocols of 802.11.

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CCS’17, October 30-November 3, 2017, Dallas, TX, USA

Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2

CCS’17, October 30–November 3, 2017, Dallas, TX, USA.

replay counter nonce
82 bytes

RSC

MIC

Key Data

variableencrypted

Supplicant (client)association stage

headerFigure 1: Simplified layout of an EAPOL frame.
Every message in the 4-way handshake is defined using EAPOL
frames. We will briefly discuss the layout and most important fieldsof these frames (see Figure 1). First, the header defines which message in the handshake a particular EAPOL frame represents. We will
use the notation message n and MsgN to refer to the n-th messageof the 4-way handshake. The replay counter field is used to detectreplayed frames. The authenticator always increments the replay
counter after transmitting a frame. When the supplicant replies
to an EAPOL frame of the authenticator, it uses the same replay
counter as the one in the EAPOL frame it is responding to. The
nonce field transports the random nones that the supplicant andauthenticator generate to derive a fresh session key. Next, in case
the EAPOL frame transports a group key, the Receive Sequence
Counter (RSC) contains the starting packet number of this key. The
group key itself is stored in the Key Data field, which is encryptedusing the KEK. Finally, the authenticity of the frame is protectedusing the KCK with a Message Integrity Check (MIC).
Figure 2 illustrates the messages that are exchanged during the
4-way handshake. In it, we use the following notation:
MsgN(r, Nonce; GTK)
It represents message N of the 4-way handshake, having a replay
counter of r , and with the given nonce (if present). All parameters
after the semicolon are stored in the key data field, and hence are
encrypted using the KEK (recall Figure 1).
The authenticator initiates the 4-way handshake by sending message 1. It contains the ANonce, and is the only EAPOL message thatis not protected by a MIC. On reception of this message, the supplicant generates the SNonce and derives the PTK (i.e., the sessionkey). The supplicant then sends the SNonce to the authenticator inmessage 2. Once the authenticator learns the SNonce, it also derives
the PTK, and sends the group key (GTK) to the supplicant. Finally,
to finalize the handshake, the supplicant replies with message 4 andafter that installs the PTK and GTK. After receiving this message,
the authenticator also installs the PTK (the GTK is installed whenthe AP is started). To summarize, the first two messages are used totransport nonces, and the last two messages are used to transport
the group key and to protect against downgrade attacks.
Note that in an existing connection, the PTK can be refreshedby initiating a new 4-way handshake. During this rekey, all 4-way
handshake messages are encrypted by the data-confidentiality protocol using the current PTK (we rely on this in Section 3.4).

2.4

Authenticator (AP)

Authentication Request
Authentication Response(Re)Association Request(Re)Association Response
optional 802.1x authentication

4-way handshakeMsg1(r, ANonce)
Derive PTK

Msg2(r, SNonce)
Msg3(r+1; GTK)

Derive PTK

Msg4(r+1)
Install PTK & GTK

Install PTK

group key handshakeencrypted data frames can now be exchanged
Refresh GTK
Encxptk { Group1(r+2; GTK) }y
Encptk { Group2(r+2) }Install GTK

Install GTK

Figure 2: Messages exchanged when a supplicant (client) connects with an authenticator (AP), performs the 4-way handshake, and periodically executes the group key handshake.

amendment also adds support for short-range communications in
the 60 GHz band, which requires a fast cipher such as GCM [3].
Right now, 802.11ad is being rolled out under the name WirelessGigabit (WiGig), and is expected to be adopted at a high rate overthe next few years [58]. Finally, the 802.11ac amendment further
extends GCMP by adding support for 256-bit keys [2].
When TKIP is used, the Temporal Key (TK) part of the session
key (PTK) is further split into a 128-bit encryption key, and two
64-bit Message Integrity Check (MIC) keys. The first MIC key isused for AP-to-client communication, while the second key is used
for the reverse direction. RC4 is used for encryption, with a uniqueper-packet key that is a mix of the 128-bit encryption key, the sender
MAC address, and an incremental 48-bit nonce. This nonce is incremented after transmitting a frame, used as a replay counter by thereceiver, and initialized to 1 when installing the TK [1, §12.5.2.6].
Message authenticity is provided by the Michael algorithm. Unfortunately, Michael is trivial to invert: given plaintext data and its
MIC value, one can efficiently recover the MIC key [66, 69].
The CCMP protocol is based on the AES cipher operating in
CCM mode (counter mode with CBC-MAC). It is an AuthenticatedEncryption with Associated Data (AEAD) algorithm, and secure aslong as no Initialization Vector (IV) is repeated under a particularConfidentiality and Integrity Protocols

The 802.11i amendment defines two data-confidentiality protocols.
The first is called the Temporal Key Integrity Protocol (TKIP). However, nowadays TKIP is deprecated due to security concerns [74].
The second protocol is commonly called (AES-)CCMP, and is currently the most widely-used data-confidentiality protocol [69]. In
2012, the 802.11ad amendment added a new data-confidentiality
protocol called the Galios/Counter Mode Protocol (GCMP) [3]. This

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CCS’17, October 30-November 3, 2017, Dallas, TX, USA

CCS’17, October 30–November 3, 2017, Dallas, TX, USA.

Mathy Vanhoef and Frank Piessens

key1 . In CCMP, the IV is the concatenation of the sender MAC
address, a 48-bit nonce, and some additional flags derived fromthe transmitted frame. The nonce is also used as a replay counterby the receiver, incremented by one before sending each frame,
and initialized to 0 when installing the TK [1, §12.5.3.4.4]. Thisis supposed to assure that IVs do not repeat. Additionally, thisconstruction allows the TK to be used directly as the key for bothcommunication directions.
The GCMP protocol is based on AES-GCM, meaning it uses
counter mode for encryption, with the resulting ciphertext being
authenticated using the GHASH function [28]. Similar to CCMP, it
is an AEAD cipher, and secure as long as no IV is repeated under aparticular key. In GCMP, the IV is the concatenation of the sender
MAC address and a 48-bit nonce. The nonce is also used as a replay
counter by the receiver, incremented by one before sending eachframe, and initialized to 0 when installing the TK [1, §12.5.5.4.4].
This normally assures each IV is only used once. As with CCMP,
the TK is used directly as the key for both communication directions. If a nonce is ever repeated, it is possible to reconstruct theauthentication key used by the GHASH function [43].
To denote that a frame is encrypted and authenticated using adata-confidentiality protocol, we use the following notation:

Enter 4-way handshake stage
PTK-INITPMK = shared master secretMsg1 Received
PTK-START
Calculate SNonce
TPTK = CalcPTK(PMK, ANonce, SNonce)
Send Msg2(SNonce)
Msg1 Received

Msg3 Received && MIC-Verified &&!ReplayedMsg

PTK-NEGOTIATINGPTK = TPTK
Send Msg4()
unconditional
PTK-DONE
MLME-SETKEYS.request(PTK)
MLME-SETKEYS.request(GTK)802.1X::portValid = TRUE

Encnk {·}

Msg3 Received &&MIC-Verified &&!ReplayedMsg

Figure 3: Supplicant 4-way handshake state machine as defined in the 802.11 standard [1, Fig. 13-17]. Keys are installed
for usage by calling the MLME-SETKEYS.request primitive.

Here n denotes the nonce being used (and thus also the replay
counter). The parameter k denotes the key, which is the PTK (session key) for unicast traffic. For group-addressed traffic, i.e., broadcast and multicast frames, this is the GTK (group key). Finally, thetwo notations

the frame using the group key, and broadcasts it to all clients. Thisassures all clients within the range of the AP receive the frame.

Data(payload)
GroupData(payload)3

are used to represent an ordinary unicast or group-addressed data
frame, respectively, with the given payload.

2.5

Msg1 Received

ATTACKING THE 4-WAY HANDSHAKEIn this section we show that the state machine behind the 4-way
handshake is vulnerable to a key reinstallation attack. We thendemonstrate how to execute this attack in real-life environments.

The Group Key Handshake

The authenticator periodically refreshes the group key, and distributes this new group key to all clients using the group key handshake. This handshake was proven to be secure in [39], and is
illustrated in the last stage of Figure 2. The authenticator initiates
the handshake by sending a group message 1 to all clients. The
supplicant acknowledges the receipt of the new group key by replying with group message 2. Depending on the implementation, theauthenticator installs the GTK either after sending group message 1,
or after receiving a reply from all connected clients (see Section 4).
Finally, group message 1 also contains the current receive replay
counter of the group key in the RSC field (see Figure 1).
Both messages in the group key handshake are defined using
EAPOL frames, and are represented using Group1 and Group2 inFigure 2. Note that group message 1 stores the new group key in
the Key Data field, and hence is encrypted using the KEK (recall
Figure 1). Since at this point a PTK is installed, the complete EAPOL
frame is also protected using a data-confidentiality protocol.
Finally, if a client transmits a broadcast or multicast frame, she
first sends it as a unicast frame to the AP. The AP then encrypts

3.1

Supplicant State Machine

The 802.11i amendment does not contain a formal state machine describing how the supplicant must implement the 4-way handshake.
Instead, it only provides pseudo-code that describes how, but notwhen, certain handshake messages should be processed [4, §8.5.6].2
Fortunately, 802.11r slightly extends the 4-way handshake, and does
provide a detailed state machine of the supplicant [1, Fig. 13-17].
Figure 2 contains a simplified description of this state machine.
When first connecting to a network and starting the 4-way handshake, the supplicant transitions to the PTK-INIT state (see Figure 3).
Here, it initializes the Pairwise Master Key (PMK). When receiving
message 1, it transitions to the PTK-START stage. This may happen when connecting to a network for the first time, or when thesession key is being refreshed after a previous (completed) 4-way
handshake. When entering PTK-START, the supplicant generates arandom SNonce, calculates the Temporary PTK (TPTK), and sends
its SNonce to the authenticator using message 2. The authenticator
will reply with message 3, which is accepted by the supplicant if
2 Strangely, this pseudo-code is only present in the original 802.11i amendment. Later revisions of the 802.11 standard, which are supposed to combine all existing amendments
into one updated document, no longer contain this pseudo-code.

1 Note that we deviate from official 802.11 terminology, where what we call the nonce

is called the packet number, and what we call the IV is called the nonce.

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Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2

CCS’17, October 30–November 3, 2017, Dallas, TX, USA.

3.2

Re. Msg3

Pt. EAPOL

Quick Pt.

Quick Ct.

4-way

Group

Table 1: Behaviour of clients: 2nd column shows whether retransmission of message 3 are accepted, 3rd whether plaintext EAPOL messages are accepted if a PTK is configured,
4th whether it accepts plaintext EAPOL messages if sent immediately after the first message 3, and 5th whether it is affected by the attack of Section 3.4. The last two columns denote if the client is vulnerable to a key reinstallation attack
against the 4-way or group key handshake, respectively.

the MIC and replay counter are valid. If so, it moves to the PTKNEGOTIATING state, where it marks the TPTK as valid by assigningit to the PTK variable, and sends message 4 to the authenticator.
Then it immediately transitions to the PTK-DONE state, where the
PTK and GTK are installed for usage by the data-confidentiality
protocol using the MLME-SETKEYS.request primitive. Finally, it
opens the 802.1x port such that the supplicant can receive and send
normal data frames. Note that the state machine explicitly takes
into account retransmissions of either message 1 or 3, which occur
if the authenticator did not receive message 2 or 4, respectively.
These retransmissions use an incremented EAPOL replay counter.
We confirmed that the state machine in 802.11r matches the original state machine, that was “defined” in 802.11i using textual descriptions scattered throughout the amendment. Most importantly,
we verified two properties which we abuse in our key reinstallation
attack. First, 802.11i states that the AP retransmits message 1 or 3 if
it did not receive a reply [4, §8.5.3.5]. Therefore, the client must handle retransmissions of message 1 or 3, matching the state machineof 802.11r. Additionally, 802.11i states that the client should install
the PTK after processing and replying to message 3 [4, §8.5.3.3].
This again matches the state machine given in 802.11r.

OS X 10.9.5

✓

✗

✗

✓

✓

✓

macOS Sierra 10.12

✓

✗

✗

✓

✓

✓

iOS 10.3.1 c

✗

N/A

N/A

N/A

✗

✓

wpa_supplicant v2.3

✓

✓

✓

✓

✓

✓

wpa_supplicant v2.4-5

✓

✓

✓

✓a

✓a ✓

wpa_supplicant v2.6

✓

✓

✓

✓b

✓b ✓

✓a

✓a ✓

Implementation

The Key Reinstallation Attack

Our key reinstallation attack is now easy to spot: because the supplicant still accepts retransmissions of message 3, even when it isin the PTK-DONE state, we can force a reinstallation of the PTK.
More precisely, we first establish a man-in-the-middle (MitM) position between the supplicant and authenticator. We use this MitM
position to trigger retransmissions of message 3 by preventing
message 4 from arriving at the authenticator. As a result, it will
retransmit message 3, which causes the supplicant to reinstall an
already-in-use PTK. In turn, this resets the nonce being used by thedata-confidentiality protocol. Depending on which protocol is used,
this allows an adversary to replay, decrypt, and/or forge packets.
In Section 6.1 we will explore in detail what the practical impacts
of nonce reuse are for each data-confidentiality protocol.
In practice, some complications arise when executing the attack.
First, not all Wi-Fi clients properly implement the state machine. In
particular, Windows and iOS do not accept retransmissions of message 3 (see Table 1 column 2). This violates the 802.11 standard. As
a result, these implementations are not vulnerable to our key reinstallation attack against the 4-way handshake. Unfortunately, froma defenders perspective, both iOS and Windows are still vulnerable to our attack against the group key handshake (see Section 4).
Additionally, because both OSes support 802.11r, it is still possible
to indirectly attack them by performing a key reinstallation attack
against the AP during an FT handshake (see Section 5).
A second minor obstacle is that we must obtain a MitM position between the client and AP. This is not possible by setting up
a rouge AP with a different MAC address, and then forwarding
packets between the real AP and client. Recall from Section 2.3
that the session key is based on the MAC addresses of the clientand AP, meaning both would derive a different key, causing thehandshake and attack to fail. Instead, we employ a channel-based
MitM attack [70], where the AP is cloned on a different channel

Android 6.0.1

✓

✗

✓

OpenBSD 6.1 (rum)

✓

✗

✗

✗

✗

✓

OpenBSD 6.1 (iwn)

✓

✗

✗

✓

✓

✓

Windows 7 c

✗

N/A

N/A

N/A

✗

✓

Windows 10 c

✗

N/A

N/A

N/A

✗

✓

MediaTek

✓

✓

✓

✓

✓

✓

a Due to a bug, an all-zero TK will be installed, see Section 6.3.
b Only the group key is reinstalled in the 4-way handshake.
c Certain tests are irrelevant (not applicable) because the im-

plementation does not accept retransmissions of message 3.

with the same MAC address as the targeted AP. This assures theclient and AP derive the same session key.
The third obstacle is that certain implementations only accept
frames protected using the data-confidentiality protocol once a PTK
has been installed (see Table 1 column 3). This is problematic for our
attack, because the authenticator will retransmit message 3 without
encryption. This means the retransmitted message will be ignoredby the supplicant. Although this would seem to foil our attack, wefound a technique to bypass this problem (see Section 3.4).
In the next two Sections, we will describe in detail how to execute our key reinstallation attack against the 4-way handshake
under various conditions. More precisely, we first explain our attack
when the client (victim) accepts plaintext retransmissions of message 3 (see Table 1 column 3). Then we demonstrate the attack whenthe victim only accepts encrypted retransmissions of message 3 (seeTable 1 column 4). Table 1 column 6 summarizes which devices arevulnerable to some variant of the key reinstallation attack against
the 4-way handshake. We remark that the behaviour of a device
depends both on the operating system, and the wireless NIC being
used. For example, although Linux accepts plaintext retransmissions of message 3, the Wi-Fi NICs used in several Android devices

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Mathy Vanhoef and Frank Piessens
Supplicant (victim)

reject them. However, Android phones with a different wireless
NIC may in fact accept plaintext retransmissions of message 3.

3.3

Plaintext Retransmission of message 3

Adversary (MitM)

Authenticator

Msg1(r, ANonce)

Msg1(r, ANonce)

Msg2(r, SNonce)

Msg2(r, SNonce)

Msg3(r+1; GTK)

Msg3(r+1; GTK)○1

If the victim still accepts plaintext retransmissions of message 3 after
installing the session key, our key reinstallation attack is straightforward. First, the adversary uses a channel-based MitM attack
so she can manipulate handshake messages [70]. Then she blocks
message 4 from arriving at the authenticator. This is illustrated instage 1 of Figure 4. Immediately after sending message 4, the victim
will install the PTK and GTK. At this point the victim also opens
the 802.1x port, and starts transmitting normal data frames (recall
Section 2.3). Notice that the first data frame uses a nonce valueof 1 in the data-confidentiality protocol. Then, in the third stageof the attack, the authenticator retransmits message 3 because it
did not receive message 4. The adversary forwards the retransmitted message 3 to the victim, causing it to reinstall the PTK and
GTK. As a result, it resets the nonce and replay counter used by thedata-confidentiality protocol. Note that the adversary cannot replay
an old message 3, because its EAPOL replay counter is no longer
fresh. We ignore stage 4 of the attack for now. Finally, when thevictim transmits its next data frame, the data-confidentiality protocol reuses nonces. Note that an adversary can wait an arbitraryamount of time before forward the retransmitted message 3 to thevictim. Therefore, we can control the amount of nonces that will
be reused. Moreover, an adversary can always perform the attack
again by deauthenticating the client, after which it will reconnect
with the network and execute a new 4-way handshake.
Figure 4 also shows that our key reinstallation attack occurs
spontaneously if message 4 is lost due to background noise. Put
differently, clients that accept plaintext retransmissions of message 3, may already be reusing nonces without an adversary evenbeing present. Inspired by this observation, an adversary could also
selectively jam message 4 [70], resulting in a stealthy attack that is
indistinguishable from random background interference.
We now return to stage 4 of the attack. The goal of this stage
is to complete the handshake at the authenticator side. This is nottrivial because the victim already installed the PTK, meaning its last
message 4 is encrypted.3 And since the authenticator did not yet
install the PTK, it will normally reject this encrypted message 4.4
However, a careful inspection of the 802.11 standard reveals thatthe authenticator may accept any replay counter that was used in
the 4-way handshake, not only the latest one [1, §12.7.6.5]:

Msg4(r+1)
Install PTK & GTK○
2

Enc1ptk { Data(. . . ) }Msg3(r+2; GTK)○3

Msg3(r+2; GTK)

Enc2ptk { Msg4(r+2) }Reinstall PTK & GTK
Enc2ptk { Msg4(r+2) }Msg4(r+1)○
4

Install PTK ?
next transmitted frame(s) will reuse nonces○
5

Enc1ptk { Data(. . . ) }Enc1ptk { Data(. . . ) }Figure 4: Key reinstallation attack against the 4-way handshake, when the supplicant (victim) still accepts plaintext
retransmissions of message 3 if a PTK is installed.
counter r + 1 in Figure 4. As a result, these AP will install the PTK,
and will start sending encrypted unicast data frames to the client.
Although Figure 4 only illustrates nonce reuse in frames sent by
the client, our attack also enables us to replay frames. First, after
the client reinstalls the GTK in stage 3 of the attack, broadcast andmulticast frames that the AP sent after retransmitting message 3
can be replayed. This is because replay counters are also reset whenreinstalling a key. Second, if we can make the AP install the PTK,
we can also replay unicast frames sent from the AP to the client.
We confirmed that the attack shown in Figure 4 works against
MediaTek’s implementation of the Wi-Fi client, and against certain
versions of wpa_supplicant (see Section 6.3). How we attacked
other implementations is explained in the next section.

“On reception of message 4, the Authenticator verifies
that the Key Replay Counter field value is one that it
used on this 4-way handshake.”

3.4

In practice, we found that several APs indeed accept an older replay
counter. More precisely, some APs accept replay counters that wereused in a message to the client, but were not yet used in a reply
from the client (see column 2 in Table 2 on page 8). These APs
will accept the older unencrypted message 4, which has the replay

Encrypted Retransmission of message 3

We now describe how we can attack clients that, once they installed
the PTK, only accept encrypted retransmissions of message 3. To
accomplish this, we exploit an inherent race condition between theentity executing the 4-way handshake, and the entity implementing
the data-confidentiality protocol.
As a warm-up, we first attack Android’s implementation of the
supplicant. Here we found that Android accepts plaintext retransmissions of message 3 when they are sent immediately after theoriginal message 3 (see column 4 of Table 1). Figure 5 shows why

3 The 802.11 standard says that a retransmitted message 4 must be sent in plaintext in

the initial 4-way handshake [1, §12.7.6.5], but nearly all clients send it using encryption.
4 Similar to fixes in [63, 64], a non-standard implementation may already install the

PTK for reception-only after sending message 3. We found no AP doing this though.

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Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2

CCS’17, October 30–November 3, 2017, Dallas, TX, USA.

Supplicant (victim)
Main CPU

Wireless NIC

Msg1(r, ANonce)○
1

Supplicant (vitcim)

Msg2(r, SNonce)

Adversary (MitM)

Main CPU

Adversary (MitM)

Wireless NIC

initial 4-way or FT handshakeMsg1(r, ANonce)○1

Msg2(r, SNonce)

Install PTK & GTK

Msg3(r+1; GTK)○
2

pairwise rekey in progress

Msg3(r+2; GTK)

Encxptk { Msg3(r+1; GTK) }Msg3(r+1; GTK)

+1 { Msg3(r+2; GTK) }Encxptk○2

Msg3(r+2; GTK)

Msg3(r+1; GTK)
Msg4(r+1)○3

Msg3(r+2; GTK)

Msg4(r+1)
Install-keys command

Msg4(r+1)

Install PTK & GTK
Msg4(r+2)○
4○
3

Install-keys command

Enc1ptk { Msg4(r+2) }Install PTK ′ & GTK

Install-keys command
Msg4(r+2)

Reinstall PTK & GTK○
4

Data(. . . )Enc1ptk′ { Msg4(r+2) }Install-keys commandnext transmitted frame will reuse nonce 1○
5yEncptk { Msg4(r+1) }Reinstall PTK ′ & GTK

Enc1ptk { Data(. . . ) }next transmitted frame will reuse nonce 1○
5

Figure 5: Key reinstallation attack against the 4-way handshake, when the victim accepts a plaintext message 3 retransmission if sent instantly after the first one. We assume
encryption and decryption is offloaded to the wireless NIC.

Data(. . . )Enc1ptk′ { Data(. . . ) }Figure 6: Key reinstallation attack against the 4-way handshake, when the victim only accepts encrypted message 3
retransmissions once a PTK is installed. We assume encryption and decryption is offloaded to the wireless NIC.

this happens, and how it can be exploited. Note that the AP is not
drawn in this figure: its actions are clear from context. In our attack,we first let the client and AP exchange Message 1 and 2. However,
we do not forward the first message 3 to the client. Instead we wait
until the AP retransmits a second message 3. In stage two of theattack, we send both message 3’s instantly after one another to theclient. The wireless NIC, which implements the data-confidentiality
protocol, does not have a PTK installed, and hence forwards bothmessages to the packet receive queue of the main CPU. The main
CPU, which implements the 4-way handshake, replies to the firstmessage 3 and commands the wireless NIC to install the PTK. In
stage 4 of the attack, the main CPU of the client grabs the second
message 3 from its receive queue. Although it notices the frame
was not encrypted, Android and Linux allow unencrypted EAPOL
frames as an exception, and therefore the main CPU will process
the retransmitted message 3. Because the NIC has just installed the
PTK, the reply will be encrypted with a nonce value of 1. After this,it commands the wireless NIC to reinstall the PTK. By doing this,the NIC resets the nonce and replay counter associated to the PTK,
meaning the next transmitted data frame will reuse nonce 1.

We now show how to attack OpenBSD, OS X, and macOS (seeTable 1 column 5). These devices only accept encrypted retransmissions of message 3. Similar to the Android attack, we abuse race
conditions between the wireless NIC and main CPU. However, we
now target a 4-way handshake execution that refreshes (rekeys) the
PTK. Recall from Section 2.3 that all messages transmitted during a
rekey undergo encryption by the data-confidentiality protocol.
Figure 6 illustrates the details of the attack. Note that the AP is
not draw in this figure: its actions are clear from context. Again theadversary uses a channel-based MitM position [70]. She then lets
the victim and adversary execute the initial 4-way handshake, andwaits until a second 4-way handshake is initiated to refresh the PTK.
Even though she only sees encrypted frames, messages in the 4-way
handshake can be detected by their unique length and destination.
At this point, the attack is analogous to the Android case. That is,in stage 2 of the attack, the adversary does not instantly forward
the first message 3. Instead, she waits until the AP retransmits
message 3, and then forwards both messages right after one anotherto the victim (see Figure 6 stage 2). The wireless NIC will decrypt1319

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CCS’17, October 30–November 3, 2017, Dallas, TX, USA.

Mathy Vanhoef and Frank Piessens
Table 2: Behaviour of Access Points. The 2nd column showswhether it accepts replay counters it used in a message to theclient, but did not yet receive in a reply, or if it only accepts
the latest used counter. Column 3 shows whether the GTK
is installed immediately after sending group message 1, or if
this is delayed until all clients replied with group message 2.

both messages using the current PTK, and forwards them to thepacket receive queue of the main CPU. In the third stage of theattack, the main CPU of the victim processes the first message 3,
replies to it, and commands the NIC to install the new PTK. In
the fourth stage, the main CPU picks the second message 3 fromthe receive queue. Since a PTK is installed, OpenBSD, OS X, andmacOS (here called the main CPU) will mandate that the message
was encrypted. However, they do not check under which key themessage was encrypted. As a result, even though the message wasdecrypted under the old PTK, the main CPU will process it. The
message 4 sent as a reply is now encrypted under the new PTK
using a nonce value of 1. After this, the main CPU commands the
NIC to reinstall the PTK, thereby resetting the nonce and replay
counters. Finally, the next data frame that the victim transmits
will again be encrypted using the new PTK with a nonce of 1. We
confirmed this attack against OpenBSD 6.1, OS X 10.9.5, and macOS
Sierra 10.12.
OpenBSD is only vulnerable if encryption is offloaded to the wireless NIC. For example, the iwn driver and associated devices support
hardware encryption, and therefore are vulnerable. However, therum driver performs software encryption in the same entity as the
4-way handshake, and is not vulnerable (see Table 1 column 5).
This attack technique requires us to wait until a rekey of thesession key occurs. Several APs do this every hour [66], some examples being [24, 26]. In practice, clients can also request a rekey bysending an EAPOL frame to the AP with the Request and Pairwise
bits set. Coincidently, Broadcom routers do not verify the authenticity (MIC) of this frame, meaning an adversary can force Broadcom
APs into starting a rekey handshake. All combined, we can assume
a rekey will eventually occur, meaning an adversary can carry out
the key reinstallation attack.

3.5

Implementation

Replay CheckGTK Install Time802.11 standard

not yet received

delayedBroadcom
Hostapd
OpenBSDMediaTek
Aironet (Cisco)
Aerohive
Ubiquiti
macOS Sierra 10.12
Windows 7
Windows 10

latest only
not yet received
latest only
latest only
latest only
not yet received
not yet received
latest only alatest only
latest only

immediate
delayeddelayedimmediate
immediate
delayeddelayedimmediate
The group key isnever refresheda Retransmitted handshake messages do not use a new re-play counter, so at all times there is only one allowed value.

not find other devices that support PeerKey. As a result, the impactof our attack against the PeerKey handshake is rather low.

4

BREAKING THE GROUP KEY HANDSHAKEIn this section we apply our key reinstallation technique against thegroup key handshake. We show all Wi-Fi clients are vulnerable toit, enabling an adversary to replay broadcast and multicast frames.

Attacking the PeerKey Handshake

The PeerKey handshake is related to the 4-way handshake, andused when two clients want to communicate with each other directly in a secure manner. It consists of two phases [1, §12.7.8]. In
the first phase, a Station-To-Station Link (STSL) Master Key (SMK)
handshake is performed. It negotiates a shared master secret between both clients. In the second phase, a fresh session key isderived from this master key using the STSL Transient Key (STK)handshake. Although this protocol does not appear to be widely
supported [49], it forms a good test case to gauge how applicableour key reinstallation technique is.
Unsurprisingly, the SKM handshake is not affected by our key
reinstallation attack. After all, the master key negotiated in thishandshake is not used by a data-confidentiality protocol, meaning
there are no nonces or replay counters to reset. However, the STK
handshake is based on the 4-way handshake, and it does install a
key for use by a data-confidentiality protocol. As a result, it canbe attacked in precisely the same manner as the 4-way handshake.
The resulting attack was tested against wpa_supplicant. To carryout the test, we modified another wpa_supplicant instance to senda second (retransmitted) message 3. This confirmed that an unmodified wpa_supplicant will reinstall the STK key when receiving a
retransmitted message 3 of the STK handshake. However, we did

4.1

Details of the Group Key Handshake

Networks periodically fresh the group key, to assure that only recently authorized clients posses this key. In the most defensive case,the group key is renewed whenever a client leaves the network. The
new group key is distributed using a group key handshake, and thishandshake has been formally proven as secure in [39]. As shownin Figure 2, the handshake is initiated by the AP when it sends agroup message 1 to all clients. The AP retransmits this message if
it did not receive an appropriate reply. Note that the EAPOL replay
counter of these retransmitted messages is always incremented
by one. In our attack, the goal is to collect a retransmitted groupmessage 1, block it from arriving at the client, and forward it to
the client at a later point in the time. This will trick to client into
reinitializing the replay counter of the installed group key.
The first prerequisite of our attack, is that clients will reinitialize
the replay counter when installing an already-in-use group key.
Since clients also use the MLME-SETKEYS.request primitive to
install the group key, this should be the case. We confirmed that inpractice all Wi-Fi clients indeed reinitialize the replay counter ofan already-in-use group key (see Table 1 column 7). Therefore, all
Wi-Fi clients are vulnerable to our subsequent attacks.

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Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2

CCS’17, October 30–November 3, 2017, Dallas, TX, USA.
Supplicant (victim)

The second prerequisite is that we must be able to collect a groupmessage 1 that the client (still) accepts, and that contains a group
key that is already in use by the AP. How to achieve this depends
on when the AP starts using the new group key. In particular, theAP may start using the new group key immediately after sending
the first group message 1, or it may delay the installation of thegroup key until all clients replied using group message 2. Table 2,column 3, summarizes this behaviour for several APs. Note that
according to the standard, the new group key should be installed
after all stations replied with a group message 2, i.e., the GTK
should be installed in a delayed fashion [1, Fig. 12-53]. When the
AP immediately installs the group key, our key reinstallation attack
is straightforward. However, if the AP installs the group key in adelayed fashion, our attack becomes more intricate. We will discussboth these cases in more detail in Section 4.2 and 4.3, respectively.
Recall from Section 2.3 that only the AP will transmit real broadcast and multicast frames (i.e., group frames) which are encryptedusing the group key. Since our key reinstallation attack targets theclient, this means we cannot force nonce reusing during encryption.
However, the client resets the replay counter when reinstalling thegroup key, which can be abused to replay frames towards clients.
Most APs refresh the group key every hour. Some networks evenrefresh this key whenever a client leaves the network. Additionally,
clients can trigger a group key handshake by sending an EAPOL
frame having the flags Request and Group [1, §12.7.7.1]. Again,
Broadcom routers do not verify the authenticity of this message,
meaning an attacker can forge it to trigger a group key update. All
combined, we can assume most networks will eventually execute agroup key update, which we can subsequently attack.

Authenticatorinitial 4-way or FT handshake
Refresh GTK
Encxptk { Group1(r; GTK) }Encxptk { Group1(r; GTK) }○1
Install GTK

Install GTKyEncptk { Group2(r) }+1 { Group1(r+1; GTK) }Encxptk

○
2
○
3

○
4

Enc1gtk { GroupData(. . . ) }Enc1gtk { GroupData(. . . ) }+1 { Group1(r+1; GTK) }Encxptk

Reinstall GTK○
5

Enc1gtk { GroupData(. . . ) }Figure 7: Key reinstallation attack against the group key
handshake, when the authenticator (AP) immediately installs the GTK after sending a Group Message 1 to all clients.

4.3
4.2

Adversary (MitM)

Attacking Delayed Key Installation

Attacking the group key handshake when the AP installs the GTK
in a delayed fashion is more tedious. Note that the previous attack
would fail because the broadcast frame transmitted in stage 3 ofFigure 7 would still be encrypted under the old group key. Indeed,at this point the AP did not yet receive group message 2 from theclient, meaning it is still using the old group key. This is problematic
because group message 1 (re)installs the new group key, and hencecannot be abused to reset the replay counter of the old group key.
One way to deal with this problem is illustrated in Figure 8. The
first two stages of this attack are similar to the previous one. That is,
the AP generates a new group key, transports it to the victim, andthe adversary blocks group message 2 from arriving at the AP. This
makes the AP retransmit group message 1 using an incremented
EAPOL replay counter of r + 1. In stage 3 of the attack, however,
we forward the older group message 2 with replay counter value r
to the AP. Interestingly, the AP should accept this message eventhough it does not use the latest replay counter value [1, §12.7.7.3]:

Attacking Immediate Key Installation

Figure 7 illustrates our key reinstallation attack when the AP immediately installs the group key after sending group message 1 to allclients. Notice that the group key handshakes messages themselves
are encrypted using the data-confidentiality algorithm under thecurrent PTK. On receipt of group message 1, the client installs the
new GTK, and replies with group message 2. The adversary blocks
this message from arriving at the AP. Hence, the AP will retransmit
a new group message 1 in stage 2 of the attack. We now wait until
a broadcast data frame is transmitted, and forward it to the victim.
After this, we forward the retransmitted group message 1 fromstage 2 to the victim. As a result, the victim will reinstall the GTK,
and will thereby reinitialize its associated replay counter. This allows us to replay the broadcast data frame (see stage 5). The client
accepts this frame because its replay counter was reinitialized.
It is essential that the broadcast frame we replay is sent before theretransmission of group message 1. This is because group message 1
contains the group key’s current value of the replay counter (recall
Section 2.5). Therefore, if it is sent after the broadcast frame, itwould contain the updated replay counter and therefore cannot beabused to reinitialize the replay counter of the victim.
We confirmed this attack in practice for APs that immediately
install the group key after sending group message 1 (see Table 2 column 3). Based on our experiments, all Wi-Fi clients are vulnerable
to this attack when connected to an AP behaving in this manner.

On reception of [group] message 2, the AP verifies
that the Key Replay Counter field value matches one
it has used in the group key handshake.
The standard does not require that the replay counter matches thelatest one that the AP used. Instead, it must match one that wasused in the group key handshake, that is, one used in any of the(re)transmitted group message 1’s. In practice we discovered thatseveral implementations indeed accept this older not-yet-received
replay counter (see Table 2 column 2). As a result, the AP installs the1321

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CCS’17, October 30–November 3, 2017, Dallas, TX, USA.
Supplicant (victim)

Adversary (MitM)

Mathy Vanhoef and Frank Piessens
this required a handshake that includes a new 802.1x and 4-way
handshake (recall Figure 2). However, because the FT handshake
relies on master keys derived during a previous connection with
the network, a new 802.1x handshake is not required. Additionally,
it embeds the 4-way handshake stage in the authentication andreassociation frames.
A normal FT handshake is shown in stage 1 of Figure 9. Observethat unlike the 4-way handshake, the FT handshake is initiatedby the supplicant. The first two messages are an Authentication
Request (AuthReq), and an Authentication Response (AuthResp).
They are functionality equivalent to Message 1 and 2 of the 4-way
handshake, respectively, and carry randomly generated nonces thatwill be used to derive a fresh session key. After this, the client
sends a Reassociation Request (ReassoReq), and the AP replies
with a Reassociaton Response (ReassoResp). They are similar infunctionality to Message 3 and 4 of the 4-way handshake, finalize
the FT handshake, and transport the GTK to the client.
Only the two reassociation messages are authenticated using a
MIC (see Figure 9). Additionally, none of the messages in the FT
handshake contain a replay counter. Instead, the FT handshake relies on the random SNonce and ANonce to provide replay protection
between different invocations of the handshake [1, §13.5.2].
According to the standard, the PTK must be installed after theauthentication response is sent or received [1, §13.9]. This is illustrated by the gray boxes in stage 1 of Figure 9. Additionally, the802.1x logical port is only opened after sending or receiving the reassociation request. This assures that, even though the PTK is alreadyinstalled while the handshake is still in progress, the AP and client
only transmit and accept data frames once the handshake completed. Combined, this implies that the FT handshake, as definedin the 802.11r amendment, is not vulnerable to a key reinstallation
attack. However, through experiments and code inspections, wefound that most implementations actually install the PTK, as well as
the GTK, after sending or receiving the reassociation response. Thisbehaviour is illustrated by the black boxes in stage 1 of Figure 9. As
a result, in practice most implementations of the FT handshake arevulnerable to a key reinstallation attack.

Authenticatorinitial 4-way or FT handshake
Refresh GTK
Encxptk { Group1(r; GTK) }Encxptk { Group1(r; GTK) }○1
Install GTKyEncptk { Group2(r) }+1 { Group1(r+1; GTK) }Encxptk○2yEncptk { Group2(r) }○3Install GTK
○
4

○
5

Enc1gtk { GroupData(. . . ) }Enc1gtk { GroupData(. . . ) }+1 { Group1(r+1; GTK) }Encxptk

Reinstall GTK○
6

Enc1gtk { GroupData(. . . ) }Figure 8: Key reinstallation against the group key handshake, when the AP installs the GTK after accepting replies
with a non-yet-received replay counter from all clients.

new group key. From this point on, the attack proceeds in a similar
fashion as the previous one. That is, we wait until a broadcast frame
is transmitted, perform the group key reinstallation in stage 5 of
the attack, and then replay the broadcast frame in stage 6.
Again it is essential that the broadcast frame we want to replay
is sent before the retransmission of group message 1. Otherwise it
includes the updated replay counter of the group key.
We tested this attack against APs that install the GTK in a delayed
fashion, and that accept replay counters it has used in a message tothe client, but did not yet receive in a reply (recall Table 2 column 2).
Note that we already know that all Wi-Fi clients reset the replay
counter when reinstalling a GTK, and hence are all vulnerable.
Finally, an OpenBSD AP is not vulnerable because it installs the
GTK in a delayed fashion, and only accepts the latest replay counter.

55.2

Since the AP installs the PTK in response to a reassociation request,
our goal will be to replay this frame. We remark that, in practice,APs must accept retransmissions of reassociation requests. This isbecause the reassociation response of the AP may be lost due tobackground noise, making the client send a new request.
Figure 9 shows the resulting key reinstallation attack against theFT handshake. Note that we do not require a man-in-the-middle
position. Instead, being able to eavesdrop and inject frames is sufficient. In the first stage of the attack, we let the client and AP executea normal FT handshake. We then wait until the AP has transmitted
one or more encrypted data frames. At this point, we replay thereassociation request to the AP. Because it does not contain a replay counter, and has a valid MIC, the AP will accept and process
the replayed frame. As a result, the AP will reinstall the PTK in
stage 3 of the attack, thereby resetting the associated nonce andreplay counter. Finally, the next data frame sent by the AP will beencrypted using an already used nonce. Similar to our previous key

ATTACKING THE 802.11R FT HANDSHAKEIn this section we introduce the Fast BSS Transition (FT) handshake,and show that implementations of it are also affected by our key
reinstallation attack.

5.1

A Key Reinstallation Attack against the AP

The Fast BSS Transition (FT) Handshake

Amendment 802.11r added the Fast Basic Service Set (BSS) Transition (FT) handshake to 802.11 [5]. Its goal is to reduce the roaming
time when a client moves from one AP, to another one of the same
protected network (i.e. of the same Basic Service Set). Traditionally,1322

Session F3: Crypto Pitfalls

CCS’17, October 30-November 3, 2017, Dallas, TX, USA

Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2

CCS’17, October 30–November 3, 2017, Dallas, TX, USA.

Supplicant (client)

Adversary

Authenticator (victim)

802.11r. Then, if no other AP of this network is within range ofthe client, we clone a real AP of this network next to the client
using a wormhole attack [41]. This makes the client think anotherAP of the targeted network is nearby. Finally, we send a BSS Transition Management Request to the client. This frame is used forload balancing [1, 11.24.7] and commands the client to roam toanother AP. It is an unauthenticated management frame, and hence
can be forged by an adversary. Consequently, the client accepts thisframe, and roams to the (wormholed) AP using an FT handshake.
We tested this against clients supporting 802.11r. This confirmedthat wpa_supplicant, iOS [8], and Windows 10 [52] accept thetransition request, and roam to another AP using an FT handshake.

AuthReq(SNonce)
AuthResp(ANonce, SNonce)
Install PTK ?
○1

Install PTK ?

ReassoReq(ANonce, SNonce, MIC)
ReassoResp(ANonce, SNonce, MIC; GTK)
Install PTK & GTK○
2Install PTK
Enc1ptk { Data(. . . ) }6

In this section we evaluate the impact of nonce reuse for the dataconfidentiality protocols of 802.11, present example attack scenarios, discuss implementation specific vulnerabilities, explain why
security proofs missed our attacks, and present countermeasures.

ReassoResp(A/Snonce, MIC; GTK)○
3

EVALUATION AND DISCUSSION

ReassoResp(ANonce, SNonce, MIC; GTK)
Reinstall PTK

6.1

next transmitted frame(s) will reuse nonces○4

Enc1ptk { Data(. . . ) }Figure 9: Key reinstallation attack against the Fast BSS Transition (FT) handshake. Note that a MitM position is not required, only the ability to eavesdrop and replay frames.

reinstallation attacks, this also enables an attacker to replay old
data frames sent by the client to the AP. We remark that our attack
is particularly devastating against the FT handshake because its
messages do not contain replay counters. This enables an adversary to replay the reassociation request continuously, each time
resetting both the nonce and replay counter used by the AP.
We tested this attack against all our three APs supporting 802.11r.
The first is the open source hostapd implementation, the second isMediaTek’s implementation for home routers running on a Linksys
RE7000, and the third is a professional Aerohive AP. All three werevulnerable to the above key reinstallation attack.
Note that if the reassociation response is lost due to backgroundnoise, the client will retransmit the reassociation request spontaneously, causing the AP to reinstall the key. That is, without anadversary being present, APs may already be reusing nonces.
Note that messages in the FT handshake never undergo (additional) protection using a data-confidentiality protocol. In particular,
Management Frame Protection (MFP) does not protect authentication and reassociation frames [1, §12.6.19]. Hence, key reinstallation
attacks against the FT handshake are trivial even if MFP is enabled.

5.3

Impact of Nonce Reuse in 802.11

The precise impact of nonce reuse caused by our attacks depends onthe data-confidentiality protocol being used. Recall that this can beeither TKIP, CCMP, or GCMP. All three protocol use a stream cipher
to encrypt frames. Therefore, reuse of a nonce always implies reuseof the keystream. This can be used to decrypt packets. We remarkthat in our attack the replay counter of the victim is also reseted.
Therefore, all three protocols are also vulnerable to replay attacks.
When TKIP is used, we can also recover the MIC key as follows.
First, we abuse nonce reuse to decrypt a full TKIP packet, including
its MIC field. Then we attack the weak Michael algorithm: given
the plaintext frame and its decrypted MIC value, we can recover
the MIC key [66]. Because TKIP uses a different MIC key for each
communication direction (recall Section 2.4), this allows us to forge
frames in one specific direction. The origin of this direction is thedevice targeted by the key reinstallation attack. Table 3 summarizes
this under the rows mentioning TKIP.
When CCMP is used, practical attacks are restricted to replay anddecryption of packets. Although there is some work that discusses
message forging attacks when nonces are repeated, the attacks aretheoretic and cannot be used to forge arbitrary messages [31].
When GCMP is used, the impact is catastrophic. First, it is possible to replay and decrypt packets. Additionally, it is possible torecover the authentication key [43], which in GCMP is used to protect both communication directions (recall Section 2.4). Therefore,
unlike with TKIP, an adversary can forge packets in both directions.
Given that GCMP is expected to be adopted at a high rate in the next
few years under the WiGig name [58], this is a worrying situation.
In general an adversary can always replay, decrypt, or forge packets in a specific communication direction. The concrete direction
depends on the handshake being attacked. For example, because
the 4-way handshake attacks the client, it can be used to: (1) replay
unicast and broadcast/multicast frames towards the client; (2) decrypt frames sent by the client to the AP; and (3) forge frames fromthe client to the AP. However, against the FT handshake we attack
the AP instead of the client, meaning we can replay, decrypt, and/or

Abusing BSS Transition Requests

An FT handshake is only performed when a station roams from one
AP to another. This limits when an attack can take place. However,
we can force a victim to perform an FT handshake as follows. First,
assume a client is connected to an AP of a network that supports

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Mathy Vanhoef and Frank Piessens

Table 3: Impact of our key reinstallation attack against the
4-way, FT, and group key handshake, in function of the dataconfidentiality protocol used. Each cell shows in which direction frames can be replayed, decrypted, or forged.
Replay c
4-way impact
TKIP
CCMP
GCMP
FT impact
TKIP
CCMP
GCMP
Group impact
any

Decrypt a

Forge

AP → client
AP → client
AP → client

client→ AP
client→ AP
client→ AP

client→ AP bclient→ AP
client→ AP
client→ AP

AP → client
AP → client
AP → client

send commands to devices [40]. We conjecture that our key reinstallation attack allows us to replay these commands. All combined,
these examples illustrate that the impact of replaying broadcast or
multicast frames should not be underestimated.

6.3

client↔ AP b
AP → clientAP ↔ client b

AP → client ca With this ability, we can hijack TCP connections to/from anInternet endpoint and inject data into them.
b With this ability, we can use the AP as a gateway to inject packets

towards any device connected to the network.
c This denotes in which direction we can replay unicast and group-

addressed frames. For the group key handshake, only groupaddressed frames can be replayed.
forge packets in the reverse directions. Table 3 in the Appendix
summarizes this, taking into account the handshake being attacked.
Finally, in various cases we can forge messages from the client
towards the AP (see Table 3). Interestingly, the AP is generally not
the final destination of a frame, and instead will forward the frame
to its real destination. This means we can forge packets towards
any device connected to the network. Depending on the AP, it iseven possible to send a frame that is reflected back to the client.

6.2

Example Attack Scenarios

Our key reinstallation attack against the 4-way handshake uncovered special behavior in wpa_supplicant. First, version 2.3 andlower are vulnerable to our attacks without unexpected side-effects.
However, we found that version 2.4 and 2.5 install an all-zero encryption key (TK) when receiving a retransmitted message 3. Thisvulnerability appears to be caused by a remark in the 802.11 standard that indirectly suggests to clear the TK from memory once it
has been installed [1, §12.7.6.6]. Version 2.6 fixed this bug by only
installing the TK when receiving message 3 for the first time [50].
However, when patching this bug, only a benign scenario was considered where message 3 got retransmitted because message 4 waslost due to background noise. They did not consider that an active
attacker can abuse this bug to force the installation of an all-zero
key. As a result, the patch was not treated as security critical, andwas not backported to older versions. Independent of this bug, allversions of wpa_supplicant reinstall the group key when receiving a retransmitted message 3, and are also vulnerable to the group
key attack of Section 4.
Because Android internally uses a slightly modified version of
wpa_supplicant, it is also affected by these attacks. In particular, we inspected the official source code repository of Android’s
wpa_supplicant [32, 34], and found that all Android 6.0 releases
contain the all-zero encryption key vulnerability. Android Wear 2.0
also is vulnerable to this attack. Though third party manufacturers
might use a different wpa_supplicant version in their Android
builds, this is a strong indication that most Android 6.0 releases
are vulnerable. In other words, 31.2% of Android smartphones arelikely vulnerable to the all-zero encryption key vulnerability [33].
Finally, we also empirically confirmed that Chromium is vulnerable
to the all-zero encryption key vulnerability [68].

6.4

Among other things, our key reinstallation attacks allow an adversary to decrypt a TCP packet, learn the sequence number, andhijack the TCP stream to inject arbitrary data [37]. This enables
one of the most common attacks over Wi-Fi networks: injecting
malicious data into an unencrypted HTTP connection.
The ability to replay broadcast and multicast frames, i.e., group
frames, is also a clear security violation. To illustrate how this could
impact real systems, consider the Network Time Protocol (NTP)
operating in broadcast mode. In this mode, the client first goes
through an initialization process, and then synchronizes its clockby listening to authenticated broadcast NTP packets [53]. Malhotra and Goldberg have shown that if these broadcast frames are
replayed, victims get stuck at a particular time forever [48]. Using
our group key attack, we can replay these frames even if they aresent over a protected Wi-Fi network. Note that manipulating thetime in this manner undermines the security of, for example, TLS
certificates [44, 54, 61], DNSSEC [47], Kerberos authentication [47],and bitcoin [25]. Another example is the xAP and xPL home automation protocol. These generally use broadcast UDP packets to

All-Zero Encryption Key Vulnerability

Limitations of the Security Proofs

Interestingly, our attacks do not violate the security properties
proven in formal analysis of the 4-way and group key handshake.
First, He et al. proved that the 4-way handshake provides key
secrecy and session authentication [39]. Key secrecy states that
only the authenticator and supplicant will posses the PTK. Since
we do not recover the PTK, this properly still holds. Session authentication was proven using the standard notion of matching
conversations [39]. Intuitively, this says a protocol is secure if the
only way that an adversary can get a party to complete the protocol is by faithfully relaying messages [12]. Our attacks, including
the channel-based MitM position we employ, do not violate thisproperty: we can only make endpoints complete the handshake byforwarding (retransmitted) messages.
Second, He et al. proved key ordering and key secrecy for thegroup key handshake [39]. Key ordering assures that supplicants
do not install an old GTK. This remains true in our attack, since wereinstall the current group key. Additionally, we do not learn thegroup key, hence key secrecy is also not violated by our attacks.

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Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2

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However, the proofs do not model key installation. Put differently,
they do not state when the key is installed for use by the dataconfidentiality protocol. In practice, this means the same key canbe installed multiple times, thereby resetting associated nonces
and/or replay counters used by the data-confidentiality protocol.

6.5

did we realize the formal model did not match reality, and that keys
may be reinstalled. In this regard, formal proofs may in fact be counterproductive: once a protocol is formally verified, the community
may become less interested in auditing actual implementations.
Interestingly, the observation that a model may be wrong, andtherefore does not accurately reflect reality, also applies to the proofof our own countermeasure. Put differently, it is not because wemodeled the countermeasure in NuSMV, that all implementations
are now suddenly secure. In reality, our formal state machine may
not accurately reflect certain implementations, patches of vendors
may be flawed, or a vendor may be affected by as-of-yet unknown
variants of the attack. As a result, it is critical to keep auditing andtesting actual implementations.
Another lesson is that the data-confidentiality protocol should
provide some protection against nonce reuse. For example, with
GCMP the authentication key can be recovered in case of nonce
recuse, while this is not so for CCMP. More generally, a nonce
misuse-resistant encryption scheme should be used, examples being
AES-SIV, GCM-SIV, or HS1-SIV [16]. These reduce the impact ofnonce reuse, and hence also the impact of key reinstallation attacks.

Countermeasures

Key reinstallation attacks can be mitigated at two layers. First, theentity implementing the data-confidentiality protocol should check
whether an already-in-use key is being installed. If so, it should
not reset associated nonces and replay counters. This prevents our
attacks, at least if an adversary cannot temporarily trick an implementation into installing a different (old) key before reinstallingthe current one. In particular, when using this countermeasure it isessential that the replay counter of received group key handshakemessages only increases. Otherwise, an adversary can use an oldgroup message 1 to make a victim temporarily install an old (different) key, to subsequently reinstall the current group key using amore recent group message 1.
A second solution is to assure that a particular key is only installed once into the entity implementing the data-confidentiality
protocol during a handshake execution. For example, the generated
session key in a 4-way handshake should only be installed once.
When the client receives a retransmitted message 3, it should reply,
but not reinstall the session key. This can be accomplished by adding
a boolean variable to the state machine of Figure 3. It is initialized tofalse, and set to true when generating a fresh PTK in PTK-START. If
the boolean is true when entering PTK-DONE, the PTK is installed
and the boolean is set to false. If the boolean is false when entering
PTK-DONE, installation of the PTK is skipped. Note that this isprecisely what version 2.6 and higher of wpa_supplicant is doing.
Proving the correctness of the above countermeasure is straightforward: we modeled the modified state machine in NuSMV [23],and used this model to prove that two key installations are always
separated by the generation of a fresh PTK. This implies the same
key is never installed twice. Note that key secrecy and session
authentication was already proven in other works [39].
We are currently notifying vendors about the vulnerabilities we
discovered, such that they can implement these countermeasures.
A full list of vendors that are known to be affected by some variantof our attacks will be made available at [22].

6.6

7

RELATED WORKIn this section we explore the history of key reinstallation attacks,
and give an overview of other Wi-Fi and protocol security works.

7.1

Key Reinstallation Attacks

We are not aware of prior work on key reinstallation attacks. This
lack of prior work is likely one of the reasons why the cryptographic
Wi-Fi handshakes we investigated were still vulnerable to these
attacks. For example, only now did we discover that the 14-yearold 4-way handshake is vulnerable to key reinstallation attacks.
Moreover, this flaw is not just present in implementations, but inthe protocol specification (standard) itself.
One somewhat related scenario that also leads to nonce reuse
are power failures. Here, after a power failure, the key is restoredfrom non-violate memory on boot, but the nonce will be reset toits initial value. Suggested solutions to this problem are given by
Zenner [76]. However, unlike key reinstallation attacks, triggering
power failures cannot be done remotely over a network. Instead,this requires physical access to the device being attacked. Moreover,power failures do not affect the security of the protocols we studied,since these handshakes are precisely used to avoid maintaining
state between old and new connections.
In [16], Bock et al. discovered that some TLS servers were using
static nonces. This was caused by a faulty implementation of the TLS
record layer protocol. That is, it was not caused by a reinstallationof an already-in-use key. Additionally, some servers used randomly
generated nonces, which means in practice nonce reuse is likely tooccur due to the birthday paradox. In contrast, key reinstallation
attacks allow an adversary to force nonce reuse on demand by
replaying handshake message(s), and are caused by flaws in thespecification (or implementation) of the handshake protocol.
McGrew wrote a survey of best practices for generating IVs
and nonces, and summarizes how they are generated and used inseveral protocols [51]. However, in the discussion of security risks,(variations of) key reinstallation attacks are not mentioned.

Discussion

There are some important lessons that can be learned from ourresults. First, the specification of a protocol should be sufficiently
precise and explicit. For example, when attacking the 4-way handshake in Section 3.3, we observed that the 802.11 standard is ambiguous as to which replay counter values should be accepted. A
more precise or formal specification would avoid any such potential
incorrect interpretations.
Second, it is not because a protocol has been formally provensecure, that implementations of it are also secure. In our case, themodel of the 4-way handshake used in formal proofs did not fullyreflect reality. This is because it did not define when the negotiatedsession key should be installed. As a result, there was no guaranteethat a session key is installed just once. Only by reading real code

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Mathy Vanhoef and Frank Piessens

Another somewhat related work is that of Beurdouche et al. [14]and that of de Ruiter and Poll [27]. They discovered that several
TLS implementations contained faulty state machines. In particular,
certain implementations wrongly allowed handshake messages tobe repeated. However, they were unable to come up with example
attacks that exploited the ability to repeat messages. We conjecturethat an adversary can repeat certain messages to trick an endpoint
into reinstalling the TLS session keys, i.e., a key reinstallation attack might be possible. We consider it interesting future work to
determine whether this leads to practical attacks.
Reuse of IVs is also an issue in the broken WEP protocol [17, 18].
In particular, Borisov et al. discovered that certain wireless network cards initialized the WEP IV to zero each time they were(re)initialized. Consequently, keystreams corresponding to small
IVs are likely to be reused [18]. However, in contrast to key reinstallation attacks, these IV resets cannot be triggered remotely.

7.2

The GCM cipher is known to be weak when short authentication
tags are used [29], and when nonces are reused [43]. Böck et al. empirically investigate nonce reuse when GCM is used in TLS [16], anddiscovered several servers that reuse nonces. Our attack on GCMPin 802.11 is unique because we can control when an endpoint reusesa nonce, and because GCMP uses the same (authentication) key inboth communication directions. Several cryptographers recently
referred to GCM as fragile [35, 56].
Finally, other works highlighted security issues in either Wi-Fi
implementations or surrounding technologies. For example, design
flaws were discovered in Wi-Fi Protected Setup (WPS) [73], vulnerabilities were found in drivers [13, 20], routers were found to beusing predictable pre-shared keys [45], and and so on.

8

CONCLUSION

Despite the security proof of both the 4-way and group key handshake, we showed that they are vulnerable to key reinstallation
attacks. These attacks do not violate the security properties of theformal proofs, but highlight limitations of the models employed bythem. In particular, the models do not specify when a key should beinstalled for usage by the data-confidentiality protocol. Additionally, we showed that the PeerKey and fast BSS transition handshakeare vulnerable to key reinstallation attacks.
All Wi-Fi clients we tested were vulnerable to our attack against
the group key handshake. This enables an adversary to replay
broadcast and multicast frames. When the 4-way or fast BSS transition handshake is attacked, the precise impact depends on thedata-confidentiality protocol being used. In all cases though, it ispossible to decrypt frames and thus hijack TCP connections. This
enables the injection of data into unencrypted HTTP connections.
Moreover, against Android 6.0 our attack triggered the installationof an all-zero key, completely voiding any security guarantees.
Rather worryingly, our key reinstallation attack even occurs
spontaneously if certain handshake messages are lost due to background noise. This means that under certain conditions, implementations are reusing nonces without an adversary being present.
An interesting future research direction is to determine whether
other protocol implementations are also vulnerable to key reinstallation attacks. Protocols that appear particularly vulnerable arethose that must take into account that messages may be lost. Afterall, these protocols are explicitly designed to process retransmitted
frames, and are possibly reinstalling keys while doing so.

Wi-Fi and Network Protocol Security

In one of the first formal analysis of the 4-way handshake, He
and Mitchell discovered a denial-of-service vulnerability [38, 55].
This led to the standardization of a slightly improved 4-way handshake [1]. In 2005, He et al. presented a formal correctness proofof both the 4-way handshake and the group key handshake [39].
However, they did not explicitly model cipher selection and downgrade protection. This enabled Vanhoef and Piessens to carry out adowngrade attack against the 4-way handshake [72]. In their attack,the AP is tricked into using RC4 to encrypt the group key when
it is transported in message 3. This attack is only possible if the
network supports WPA-TKIP, which was already known to be aweak cipher [66, 69]. Additionally, the models employed in [39] do
not define when to install the negotiated session key or transported
group key. However, we showed this timing is in fact essential,
since otherwise key reinstallation attacks may be possible.
The FT handshake is based on the 4-way handshake [5], but there
are no formal security analysis of it. Instead, existing works focus
on the performance of the handshake, examples being [11, 46].
Several works study authentication mechanisms which negotiate
master keys (PMKs) [19, 21, 59, 75]. Some of these mechanisms rely
on first establishing a secure TLS session [9]. As a result, recent
attacks on TLS also affect these mechanisms, examples being [10, 14,
15, 27, 62]. In this paper we did not study mechanisms that negotiate
master keys, but instead focused on handshakes that derive freshsession keys from such a negotiated or pre-shared master key.
Regarding data-confidentiality protocols, the first practical attack
on WPA-TKIP was found by Beck and Tews [66]. They showed
how to decrypt a small TKIP packet, recovered the MIC key, andsubsequently forged packets. Their attack was further improvedin several works [36, 67, 69, 70]. Researchers also attacked theweak per-packet key construction of TKIP by exploiting biases in
RC4 [6, 57, 71]. Nowadays TKIP is deprecated by the Wi-Fi Alliancedue to its security issues [74].
Although CCMP received some criticism [60], it has been provento provide security guarantees similar to modes such as OCB [42].
In [31], Fouque et al. discusses theoretic message forging attacks
when nonces are repeated in CCMP.

ACKNOWLEDGMENTS
This research is partially funded by the Research Fund KU Leuvenand by the imec High Impact Initiative Distributed Trust project.

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CCS’17, October 30-November 3, 2017, Dallas, TX, USA

CCS’17, October 30–November 3, 2017, Dallas, TX, USA.

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