Does CoreSSH Server Mitigate the SSH Terrapin Attack

Question:

Does CoreSSH Server Mitigate the SSH Terrapin Attack (CVE-2023-48795)? For further details, please see the official NIST CVE notice and the attack website terrapin-attack.com.

Answer:

CoreSSH Server mitigates this attack by default. The product has been updated with support for the "strict KEX" protocol extension. This attack requires Man-in-the-Middle capabilities and that the connection be secured with ChaCha20-Poly1305 or CBC with Encrypt-then-MAC. The support is configurable through the UseStrictKeyExchange setting.

UseStrictKeyExchange

This extension is defined in OpenSSH 9.6. By default, this extension is enabled in CoreSSH Server. If the client supports this extension, the attack is mitigated. Since both client and server must implement strict key exchange to effectively mitigate the Terrapin attack, the server provides options to further control the behavior in different scenarios.

When UseStrictKeyExchange is set to 1 (default), the server will indicate support for strict key exchange by automatically including the pseudo-algorithm kex-strict-c-v00@openssh.com for client components and kex-strict-s-v00@openssh.com in the list of supported key exchange algorithms during SSH negotiation.

To change the desired behavior set UseStrictKeyExchange to one of the following values:

`0`	Disabled. Strict key exchange is not supported in the server.
`1`(default)	Enabled, but not enforced. This setting enables strict key exchange, but if the remote host does not support strict key exchange the connection is still allowed to continue.
`2`	Enabled, but reject affected algorithms if the remote host does not support strict key exchange. If the remote host supports strict key exchange all algorithms may be used. If the remote host does not support strict key exchange the connection will only continue if the selected encryption and MAC algorithms are not affected by the Terrapin attack.
`3`	Required. If the remote host does not support strict key exchange the connection will fail.

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