Affected: PyPI: asyncssh

### Summary

AsyncSSH v2.14.1 and earlier is 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 AsyncSSH in the patched version.

**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@openssh.com`) and Encrypt-then-MAC (`*-etm@openssh.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…