CVE-2026-54591: Asyncssh Path Traversal Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2wxc-x7rj-hg8f",
  "modified" : "2026-08-26T15:34:22Z",
  "published" : "2026-08-26T15:34:22Z",
  "aliases" : [ "CVE-2026-54591" ],
  "summary" : "asyncssh has SCP Path Traversal to Arbitrary File Write",
  "details" : "| | |\n|---|---|\n| Product | asyncssh (all versions through 2.23.0) |\n| Related | CVE-2019-6111 (same class in OpenSSH) |\n| Fix | AsyncSSH 2.23.1 |\n\nA 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 can escape any target directory and overwrite `~/.bashrc`, `~/.ssh/rc`, or `~/.ssh/authorized_keys`, achieving code execution. This is the same vulnerability class as CVE-2019-6111. The mitigation applied in OpenSSH does not appear to have been adopted in asyncssh.\n\n---\n\n**Steps to exploit:**\n\n**Step 1 - Normal usage:** Application calls `await asyncssh.scp((conn, 'file'), '/home/user/downloads/')`. This is the standard, documented API.\n\n**Step 2 - SCP protocol:** asyncssh opens an SSH exec channel, runs `scp -f file`. The server controls the filename field:\n\n```\nC0644 100 ../pwned.txt\\n       (simple traversal)\n\nD0755 0 ..\\n                   (traverse up, repeat as needed)\nC0644 47 .bashrc\\n             (write payload)\nE\\n\n```\n\n**Step 3 - `_parse_cd_args`** (`scp.py:134-142`) returns the filename verbatim:\n\n```python\ndef _parse_cd_args(args: bytes) -> Tuple[int, int, bytes]:\n    permissions, size, name = args.split(None, 2)\n    return int(permissions, 8), int(size), name  # no sanitization\n```\n\nThe returned `name` is not passed through `basename()` and is not checked for `..` or `/` components.\n\n**Step 4 - `_recv_files`** (`scp.py:706-713`) joins the unsanitized name:\n\n```python\nnew_dstpath = posixpath.join(dstpath, name)\n```\n\nWith `dstpath=b'/home/user/downloads/subdir'` and `name=b'../pwned.txt'`, this resolves to `/home/user/downloads/pwned.txt`, outside the target.\n\n**Step 5 - File write:** `_recv_file` opens the traversed path via `self._fs.open(dstpath, 'wb')` and writes attacker-controlled content. The resolved path is not checked against the target directory boundary.\n\n**Step 6 - RCE chains:**\n\n| Target | Execution trigger | Reliability |\n|---|---|---|\n| `~/.bashrc` | Next terminal open | High |\n| `~/.profile` | Next login | High |\n| `~/.ssh/rc` | Next SSH connection (requires sshd) | High |\n| `~/.ssh/authorized_keys` | Attacker logs in with `command=` | Medium |\n\n---\n\n**Reproduction:**\n\nLink to reproduction script: [path_traversal_poc.zip](https://github.com/user-attachments/files/28160665/path_traversal_poc.zip)\n\n```bash\ndocker build -t asyncssh-scp-traversal -f Dockerfile .\ndocker run --rm asyncssh-scp-traversal\n```\n\nThe attached `poc_scp_traversal.py` starts a malicious SSH server in-process using asyncssh's own API, then downloads from it via `asyncssh.scp()`.\n\n*Expected Output:*\n\n<img width=\"1400\" height=\"815\" alt=\"image\" src=\"https://github.com/user-attachments/assets/496745e4-d11d-4ed8-bddd-d15dd13d1751\" />",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "asyncssh"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.23.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.23.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/ronf/asyncssh/security/advisories/GHSA-2wxc-x7rj-hg8f"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-54591"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ronf/asyncssh/commit/d730803b8e4e94c20c7580d90f94d1e05f9f58de"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ronf/asyncssh"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ronf/asyncssh/releases/tag/v2.23.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-26T15:34:22Z",
    "nvd_published_at" : "2026-07-08T21:16:49Z"
  }
}