GHSA-2wxc-x7rj-hg8f

| | | |---|---| | 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.

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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 - `_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 |

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**Reproduction:**

Link to reproduction script: [path_traversal_poc.zip](https://github.com/user-attachments/files/28160665/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:*

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