Advisories for Pypi/Asyncssh package

A malicious SSH server can wedge an AsyncSSH client, and an authenticated client can wedge an AsyncSSH server, by sending a channel maximum packet size of 0 in SSH_MSG_CHANNEL_OPEN_CONFIRMATION (server→client) or SSH_MSG_CHANNEL_OPEN (client→server). AsyncSSH stores the peer-supplied value verbatim with no lower-bound check; the first time channel data is written, SSHChannel._flush_send_buf enters a synchronous infinite loop that cannot be interrupted by asyncio.wait_for or any timeout. The loop body has no …

| | | |—|—| | Product | asyncssh (all versions through 2.23.0) | | Related | CVE-2019-6111 (same class in OpenSSH) | | Fix | AsyncSSH 2.23.1 | A malicious SSH server can write arbitrary files on the asyncssh SCP client's filesystem by sending filenames containing ../ traversal sequences. The SCP receive path does not currently sanitize server-provided filenames. By chaining directory traversals via the D (directory) action, an attacker …

The fix for CVE-2026-45309 added a guard in SSHServerConfig._set_tokens (asyncssh/config.py:715-716) that rejects an SSH username containing /, , or equal to .., before it is substituted for the %u token in AuthorizedKeysFile: if self._user == '..' or '/' in self._user or '' in self._user: raise IllegalUserName('Unsafe username substitution') However, the %u-substituted value is subsequently passed through environment-variable expansion (_expand_val, config.py:145-149 — token expansion then env expansion) and, at file-open time, …

AsyncSSH 2.22.0 expands the OpenSSH-compatible AuthorizedKeysFile %u token with the raw SSH username during pre-authentication server config reload. A server configured with a documented per-user key pattern such as AuthorizedKeysFile authorized_keys/%u can be made to read an authorized-keys file outside the intended directory when the SSH username contains path traversal segments. If the attacker can place or reference a readable authorized-keys-format file containing their public key, the attacker can authenticate …

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 …

An issue in AsyncSSH v2.14.0 and earlier allows attackers to control the remote end of an SSH client session via packet injection/removal and shell emulation.

An issue in AsyncSSH v2.14.0 and earlier allows attackers to control the extension info message (RFC 8308) via a man-in-the-middle attack.

The SSH server implementation of AsyncSSH does not properly check whether authentication is completed before processing other requests. A customized SSH client can simply skip the authentication step.