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PuTTY vulnerability vuln-terrapin

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Security researchers at the Ruhr University Bochum have identified a vulnerability in the SSH protocol (specifically, in widely-used OpenSSH protocol extensions, exposed by weakness in the underlying SSH protocol design), exploitable by an active attacker to break the integrity of an encrypted SSH connection in specific ways. They have named this the 'Terrapin' attack, and it has been assigned CVE-2023-48795.

PuTTY 0.80 will fully mitigate the protocol vulnerability when connecting to a server which also has mitigations (such as OpenSSH 9.6), or else will warn before proceeding with a connection where the attack would be possible; either way, an attack can no longer be performed without there being an opportunity to avoid the risk.

Even in the absence of mitigations, the vulnerability can only be exploited when certain cryptographic algorithms are in use. With PuTTY's default cipher configuration and the most common server configurations, connections would not be vulnerable in any case. (So it's quite possible that users will not notice any difference with PuTTY 0.80, even if connecting to a server with no mitigations for this vulnerability.)

However, the Terrapin attack is possible if PuTTY's cipher configuration has been changed from the default in a specific way, and/or the remote server doesn't offer to use certain algorithms that aren't vulnerable (either because it doesn't implement them, or has been configured not to use them).

In more detail, the affected algorithms that PuTTY supports are:

• ChaCha20-Poly1305 (which PuTTY calls 'ChaCha20').
  By default, PuTTY prefers the much more common AES family of algorithms to ChaCha20, so if the server supports both, one of the AES family will be used.
  However, if PuTTY has been reconfigured to prefer ChaCha20, then that's likely to be used; or, if the server doesn't offer AES (apart from AES-GCM), again, ChaCha20 is likely to be used. These are the most likely scenarios for exposing the vulnerability.
• AES-CBC in encrypt-then-MAC ('ETM') mode.
  While AES is in common use, and 'AES' is at the top of PuTTY's default cipher preference list, PuTTY unconditionally prefers its CTR mode to CBC, and the standard encrypt-after-MAC mode to encrypt-then-MAC; this is not configurable. So the vulnerable mode of AES can only be chosen if it's the only one the server offers (other than AES-GCM), which would be unusual (especially as CTR mode was specified for the SSH protocol before ETM was).

To mitigate the vulnerability, the OpenSSH project has defined a SSH extension called 'strict KEX' (documented in their PROTOCOL document), which PuTTY 0.80 implements.

When PuTTY ≥0.80 connects to an upgraded server that implements strict KEX, the vulnerability is avoided, regardless of the algorithms in use. (This will require both PuTTY and the server to have been upgraded after this vulnerability was made public.)
(If strict KEX is in use, the message "Enabling strict key exchange semantics" will be in PuTTY's Event Log.)

Otherwise, when PuTTY ≥0.80 connects to a server without the strict-KEX mitigation, and one of the vulnerable algorithms mentioned above would be used for the connection, the PuTTY tools will issue a warning similar to the following:

The client-to-server cipher selected for this session is ChaCha20-Poly1305, which, with this server, is vulnerable to the 'Terrapin' attack CVE-2023-48795, potentially allowing an attacker to modify the encrypted session.

Upgrading, patching, or reconfiguring this SSH server is the best way to avoid this vulnerability, if possible.

To accept the risk and continue, press "Yes". To abandon the connection, press "No".

"No" is the safest choice here.

(The name of the cipher, and the direction, could be different.)

If the server's offered algorithms are such that a simple change to PuTTY's cipher configuration can avoid the vulnerability, the warning will additionally advise what exactly needs changing. (The SSH protocol design is such that the cipher configuration must be changed before starting the connection.)

(We have not provided a way to suppress this warning, on the assumption that it will be rare at worst to encounter a server with which it's unavoidable, and in any case, such a server should soon be upgraded or reconfigured now that the Terrapin attack is public knowledge. However, if you find that there's some SSH server that it's not practical to upgrade or reconfigure for some reason, and reconfiguring PuTTY doesn't help, please let us know. If you believe your need to connect to such a server outweighs the risk of attack, you can always use an older version of PuTTY to connect to that specific server in the short term; but also, let us know.)

If the Terrapin attack is used to compromise the integrity of an SSH connection, the attacker can only modify the traffic on the supposedly-secure channel in a limited way ('prefix-truncation attack'). The researchers' paper describes some specific kinds of mischief that can be perpetrated as a result, but others may be discovered. We do not recommend proceeding past a warning of a Terrapin-vulnerable SSH connection, even if none of the descriptions below worry you.

That said, here is how the main application-level attacks, all based on the suppression of extensions reported via the ext-info mechanism, affect PuTTY specifically:

• server-sig-algs is the only extension whose suppression affects PuTTY. It allows the attacker to force use of the weak hash SHA-1 in public-key user authentication. This does not directly lead to a compromise, but could possibly allow a well-resourced attacker to remove the meddler-in-the-middle protection that public key authentication usually provides against an intercepting proxy using an already-spoofed host key.
  (Or, with modern (≥8.8) OpenSSH in its default configuration, cause a denial of service, since SHA-1 won't be accepted.)
• publickey-hostbound@openssh.com makes no difference to PuTTY/Pageant, since they do not implement host-bound keys in the first place.
• ping@openssh.com doesn't make any difference to PuTTY either, since it doesn't implement the keystroke timing obfuscation that this extension is used for.

(The attacks against the AsyncSSH client described in the paper are not relevant to the PuTTY tools.)

This protocol vulnerability was pre-disclosed to us by Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk, on 17 November 2023. For full details of their report, see their dedicated website about the Terrapin attack.