GHSA-2wxc-x7rj-hg8f asyncssh has SCP Path Traversal to Arbitrary File Write

• PyPI asyncssh: < 2.23.1 → fixed in 2.23.1

| | | |---|---| | Product | asyncssh (all versions through 2.23.0) | | Related | CVE-2019-6111 (same class in OpenSSH) | | Fix | AsyncSSH 2.23.1 | A 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. --- Steps to exploit: Step 1 - Normal usage: Application calls `await asyncssh.scp((conn, 'file'), '/home/user/downloads/')`. This is the standard, documented API. Step 2 - SCP protocol: asyncssh opens an SSH exec channel, runs `scp -f file`. The server controls the filename field: ``` C0644 100 ../pwned.txt\n (simple traversal) D0755 0 ..\n (traverse up, repeat as needed) C0644 47 .bashrc\n (write payload) E\n ``` Step 3 - `_parse_cd_args` (`scp.py:134-142`) returns the filename verbatim: ```python def _parse_cd_args(args: bytes) -> Tuple[int, int, bytes]: permissions, size, name = args.split(None, 2) return int(permissions, 8), int(size), name # no sanitization ``` The returned `name` is not passed through `basename()` and is not checked for `..` or `/` components. Step 4 - `_recv_files` (`scp.py:706-713`) joins the unsanitized name: ```python new_dstpath = posixpath.join(dstpath, name) ``` With `dstpath=b'/home/user/downloads/subdir'` and `name=b'../pwned.txt'`, this resolves to `/home/user/downloads/pwned.txt`, outside the target. 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. Step 6 - RCE chains: | Target | Execution trigger | Reliability | |---|---|---| | `~/.bashrc` | Next terminal open | High | | `~/.profile` | Next login | High | | `~/.ssh/rc` | Next SSH connection (requires sshd) | High | | `~/.ssh/authorized_keys` | Attacker logs in with `command=` | Medium | --- Reproduction: Link to reproduction script: path_traversal_poc.zip ```bash docker build -t asyncssh-scp-traversal -f Dockerfile . docker run --rm asyncssh-scp-traversal ``` The attached `poc_scp_traversal.py` starts a malicious SSH server in-process using asyncssh's own API, then downloads from it via `asyncssh.scp()`. *Expected Output:*