{"id":"PYSEC-2026-3808","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) -\u003e 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\u003cimg width=\"1400\" height=\"815\" alt=\"image\" src=\"https://github.com/user-attachments/assets/496745e4-d11d-4ed8-bddd-d15dd13d1751\" /\u003e","aliases":["CVE-2026-54591","GHSA-2wxc-x7rj-hg8f"],"modified":"2026-09-10T12:15:06.100986067Z","published":"2026-09-10T09:44:57.655635Z","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"},{"type":"PACKAGE","url":"https://pypi.org/project/asyncssh"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-2wxc-x7rj-hg8f"}],"affected":[{"package":{"name":"asyncssh","ecosystem":"PyPI","purl":"pkg:pypi/asyncssh"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.23.1"}]}],"versions":["0.8.1","0.8.2","0.8.3","0.8.4","0.9.0","0.9.1","0.9.2","1.0.0","1.0.1","1.1.0","1.1.1","1.10.0","1.10.1","1.11.0","1.11.1","1.12.0","1.12.1","1.12.2","1.13.0","1.13.1","1.13.2","1.13.3","1.14.0","1.15.0","1.15.1","1.16.0","1.16.1","1.17.0","1.17.1","1.18.0","1.2.0","1.2.1","1.3.0","1.3.1","1.3.2","1.4.0","1.4.1","1.5.0","1.5.1","1.5.2","1.5.3","1.5.4","1.5.5","1.5.6","1.6.0","1.6.1","1.6.2","1.7.1","1.7.2","1.7.3","1.8.0","1.8.1","1.9.0","2.0.0","2.0.1","2.1.0","2.10.0","2.10.1","2.11.0","2.12.0","2.13.0","2.13.1","2.13.2","2.14.0","2.14.1","2.14.2","2.15.0","2.16.0","2.17.0","2.18.0","2.19.0","2.2.0","2.2.1","2.20.0","2.21.0","2.21.1","2.22.0","2.23.0","2.3.0","2.4.0","2.4.1","2.4.2","2.5.0","2.6.0","2.7.0","2.7.1","2.7.2","2.8.0","2.8.1","2.9.0"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/asyncssh/PYSEC-2026-3808.yaml"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H"}]}