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Date: Mon, 18 Dec 2023 17:08:14 +0100
From: Fabian Bäumer <fabian.baeumer@....de>
To: oss-security@...ts.openwall.com
Cc: Marcus Brinkmann <marcus.brinkmann@....de>
Subject: CVE-2023-48795: Prefix Truncation Attacks in SSH Specification
(Terrapin Attack)
### Summary
Parts of the SSH specification are vulnerable to a novel prefix
truncation attack (a.k.a. Terrapin attack), which allows a
man-in-the-middle attacker to strip an arbitrary number of messages
right after the initial key exchange, breaking SSH extension negotiation
(RFC8308) in the process and thus downgrading connection security.
### Mitigations
To mitigate this protocol vulnerability, OpenSSH suggested a so-called
"strict kex" which alters the SSH handshake to ensure a
Man-in-the-Middle attacker cannot introduce unauthenticated messages as
well as convey sequence number manipulation across handshakes. Support
for strict key exchange has been added to a variety of SSH
implementations, including OpenSSH itself, PuTTY, libssh, and more.
**Warning: To take effect, both the client and server must support this
countermeasure.**
As a stop-gap measure, peers may also (temporarily) disable the affected
algorithms and use unaffected alternatives like AES-GCM instead until
patches are available.
### Details
The SSH specifications of ChaCha20-Poly1305
(chacha20-poly1305@...nssh.com) and Encrypt-then-MAC (*-etm@...nssh.com
MACs) are vulnerable against an arbitrary prefix truncation attack
(a.k.a. Terrapin attack). This allows for an extension negotiation
downgrade by stripping the SSH_MSG_EXT_INFO sent after the first message
after SSH_MSG_NEWKEYS, downgrading security, and disabling attack
countermeasures in some versions of OpenSSH. When targeting
Encrypt-then-MAC, this attack requires the use of a CBC cipher to be
practically exploitable due to the internal workings of the cipher mode.
Additionally, this novel attack technique can be used to exploit
previously unexploitable implementation flaws in a Man-in-the-Middle
scenario.
The attack works by an attacker injecting an arbitrary number of
SSH_MSG_IGNORE messages during the initial key exchange and consequently
removing the same number of messages just after the initial key exchange
has concluded. This is possible due to missing authentication of the
excess SSH_MSG_IGNORE messages and the fact that the implicit sequence
numbers used within the SSH protocol are only checked after the initial
key exchange.
In the case of ChaCha20-Poly1305, the attack is guaranteed to work on
every connection as this cipher does not maintain an internal state
other than the message's sequence number. In the case of
Encrypt-Then-MAC, practical exploitation requires the use of a CBC
cipher; while theoretical integrity is broken for all ciphers when using
this mode, message processing will fail at the application layer for CTR
and stream ciphers.
For more details and a pre-print of the associated research paper, see
https://terrapin-attack.com.
### Impact
This attack targets the specification of ChaCha20-Poly1305
(chacha20-poly1305@...nssh.com) and Encrypt-then-MAC
(*-etm@...nssh.com), which are widely adopted by well-known SSH
implementations and can be considered de-facto standard. These
algorithms can be practically exploited; however, in the case of
Encrypt-Then-MAC, we additionally require the use of a CBC cipher. As a
consequence, this attack works against all well-behaving SSH
implementations supporting either of those algorithms and can be used to
downgrade (but not fully strip) connection security in case SSH
extension negotiation (RFC8308) is supported. The attack may also enable
attackers to exploit certain implementation flaws in a man-in-the-middle
(MitM) scenario.
--
M. Sc. Fabian Bäumer
Chair for Network and Data Security
Ruhr University Bochum
Universitätsstr. 150, Building MC 4/145
44780 Bochum
Germany
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