GHSA-g794-3fmp-753h AsyncSSH `AuthorizedKeysFile %u` path traversal allows attacker-selected authorized keys to authenticate a traversal username

• PyPI asyncssh: >= 2.22.0, < 2.23.0 → fixed in 2.23.0

Summary 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. Affected Product • Package: asyncssh • Ecosystem: pip • Affected versions: confirmed on 2.22.0; exact lower bound not finalized • Tested version: 2.22.0 • Audit commit/tag: tag `v2.22.0`, commit `af5a81e669633d83d535163f93b6bf3f957c9238` • PyPI sdist SHA256: `c3ce72b01be4f97b40e62844dd384227e5ff5a401a3793007c42f86a5c8eb537` Vulnerability Details • CWE: CWE-22: Improper Limitation of a Pathname to a Restricted Directory • Component: AsyncSSH server config reload and public-key authentication (`asyncssh/config.py`, `asyncssh/connection.py`, `asyncssh/auth_keys.py`, `asyncssh/misc.py`) • Root cause: `%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. • Security boundary violated: the configured authorized-keys directory and public-key authentication trust boundary. • Direct impact: public-key authentication succeeds using an attacker-selected authorized-keys file outside the intended directory. • Chain impact, if any: none claimed; direct authentication impact is primary. Attack Preconditions • The AsyncSSH server uses a config or equivalent pattern where `AuthorizedKeysFile` contains `%u`, for example `AuthorizedKeysFile authorized_keys/%u`. • Public-key authentication is enabled. • The attacker can place or reference a readable authorized-keys-format file outside the intended directory, such as a file in a world-writable or application-writable location. • The application does not separately reject usernames containing `/`, `\`, or `..` before AsyncSSH uses the username for key-file selection. Reproduction The run-scoped evidence contains a safe localhost proof: 1. Start the proof harness saved at harness_app.py 2. Run exploit_proof.py through run_proof.sh 3. 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. 4. Expected result: the normal username `victim` fails, while the traversal username authenticates with the same attacker key. Observed proof output: ```text [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 ```