CVE-2026-54591: AsyncSSH: 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/authorizedkeys, 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.

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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 - parsecdargs (scp.py:134-142) returns the filename verbatim:

python def parsecdargs(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 - recvfiles (scp.py:706-713) joins the unsanitized name:

python newdstpath = 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: recvfile 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/authorizedkeys | Attacker logs in with command= | Medium |

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Reproduction:

Link to reproduction script: pathtraversalpoc.zip

bash docker build -t asyncssh-scp-traversal -f Dockerfile . docker run --rm asyncssh-scp-traversal

The attached pocscptraversal.py starts a malicious SSH server in-process using asyncssh's own API, then downloads from it via asyncssh.scp().

Expected Output:

<img width="1400" height="815" alt="image" src="https://github.com/user-attachments/assets/496745e4-d11d-4ed8-bddd-d15dd13d1751" />