{"id":"PYSEC-2026-2384","summary":"AsyncSSH `AuthorizedKeysFile %u` path traversal allows attacker-selected authorized keys to authenticate a traversal username","details":"## Summary\nAsyncSSH 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.\n\n## Affected Product\n- Package: asyncssh\n- Ecosystem: pip\n- Affected versions: confirmed on 2.22.0; exact lower bound not finalized\n- Tested version: 2.22.0\n- Audit commit/tag: tag `v2.22.0`, commit `af5a81e669633d83d535163f93b6bf3f957c9238`\n- PyPI sdist SHA256: `c3ce72b01be4f97b40e62844dd384227e5ff5a401a3793007c42f86a5c8eb537`\n\n## Vulnerability Details\n- CWE: CWE-22: Improper Limitation of a Pathname to a Restricted Directory\n- Component: AsyncSSH server config reload and public-key authentication (`asyncssh/config.py`, `asyncssh/connection.py`, `asyncssh/auth_keys.py`, `asyncssh/misc.py`)\n- 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.\n- Security boundary violated: the configured authorized-keys directory and public-key authentication trust boundary.\n- Direct impact: public-key authentication succeeds using an attacker-selected authorized-keys file outside the intended directory.\n- Chain impact, if any: none claimed; direct authentication impact is primary.\n\n## Attack Preconditions\n- The AsyncSSH server uses a config or equivalent pattern where `AuthorizedKeysFile` contains `%u`, for example `AuthorizedKeysFile authorized_keys/%u`.\n- Public-key authentication is enabled.\n- 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.\n- The application does not separately reject usernames containing `/`, `\\`, or `..` before AsyncSSH uses the username for key-file selection.\n\n## Reproduction\nThe run-scoped evidence contains a safe localhost proof:\n\n1. Start the proof harness saved at \n[harness_app.py](https://github.com/user-attachments/files/27232526/harness_app.py)\n\n2. Run \n[exploit_proof.py](https://github.com/user-attachments/files/27232538/exploit_proof.py)\nthrough \n[run_proof.sh](https://github.com/user-attachments/files/27232545/run_proof.sh)\n\n3. 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.\n4. Expected result: the normal username `victim` fails, while the traversal username authenticates with the same attacker key.\n\nObserved proof output:\n\n```text\n[CONTROL] username=victim success=False\n[ATTACK] username=../../../asyncssh-proof-exploit-proof-8b2bd23daeeb.pub success=True\n[ATTACK] output=AUTH_BYPASS_SUCCESS username=../../../asyncssh-proof-exploit-proof-8b2bd23daeeb.pub\nPASS: traversal username authenticated with attacker-controlled authorized_keys file\n```","aliases":["CVE-2026-45309","GHSA-g794-3fmp-753h"],"modified":"2026-07-13T16:49:54.110302741Z","published":"2026-07-13T15:19:12.785321Z","references":[{"type":"WEB","url":"https://github.com/ronf/asyncssh/security/advisories/GHSA-g794-3fmp-753h"},{"type":"PACKAGE","url":"https://github.com/ronf/asyncssh"},{"type":"PACKAGE","url":"https://pypi.org/project/asyncssh"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-g794-3fmp-753h"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-45309"}],"affected":[{"package":{"name":"asyncssh","ecosystem":"PyPI","purl":"pkg:pypi/asyncssh"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.22.0"},{"fixed":"2.23.0"}]}],"versions":["2.22.0"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/asyncssh/PYSEC-2026-2384.yaml"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:P"}]}