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:

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:

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:

Reproduction:

Link to reproduction script: path_traversal_poc.zip

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: