| Package | Affected versions | Patched versions |
|---|---|---|
| asyncssh | ≥ 2.22.0, < 2.23.0 | 2.23.0 |
| asyncssh | < 2.23.0 | 2.23.0 |
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 over SSH as the traversal username.
v2.22.0, commit af5a81e669633d83d535163f93b6bf3f957c9238c3ce72b01be4f97b40e62844dd384227e5ff5a401a3793007c42f86a5c8eb537asyncssh/config.py, asyncssh/connection.py, asyncssh/auth_keys.py, asyncssh/misc.py)%u in AuthorizedKeysFile is expanded from the remote username without rejecting path separators or .. segments, and the resulting path is opened without constraining it to the intended authorized-keys directory.AuthorizedKeysFile contains %u, for example AuthorizedKeysFile authorized_keys/%u./, \, or .. before AsyncSSH uses the username for key-file selection.The run-scoped evidence contains a safe localhost proof:
Start the proof harness saved at harness_app.py
Run exploit_proof.py through run_proof.sh
The harness creates sshd_config with AuthorizedKeysFile authorized_keys/%u, writes the attacker's public key to a file outside authorized_keys/, starts a real AsyncSSH server, and attempts two SSH logins.
Expected result: the normal username victim fails, while the traversal username authenticates with the same attacker key.
Observed proof output:
[CONTROL] username=victim success=False
[ATTACK] username=../../../asyncssh-proof-exploit-proof-8b2bd23daeeb.pub success=True
[ATTACK] output=AUTH_BYPASS_SUCCESS username=../../../asyncssh-proof-exploit-proof-8b2bd23daeeb.pub
PASS: traversal username authenticated with attacker-controlled authorized_keys file
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