asyncssh has SCP Path Traversal to Arbitrary File Write

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:

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