CVE-2026-45309: Asyncssh Path Traversal Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-g794-3fmp-753h",
  "modified" : "2026-05-27T21:35:06Z",
  "published" : "2026-05-27T21:35:06Z",
  "aliases" : [ "CVE-2026-45309" ],
  "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```",
  "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "asyncssh"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.22.0"
      }, {
        "fixed" : "2.23.0"
      } ]
    } ],
    "versions" : [ "2.22.0" ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/ronf/asyncssh/security/advisories/GHSA-g794-3fmp-753h"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ronf/asyncssh"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-27T21:35:06Z",
    "nvd_published_at" : null
  }
}