Summary
Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections
Dulwich's porcelain.checkout(paths=[...]) code path writes files using raw os.open(file_path, O_WRONLY|O_CREAT|O_TRUNC, mode) followed by f.write(obj.data). This code path does NOT call build_file_from_blob() at all, completely bypassing any symlink protections (including the unreleased d09f8af fix). os.open without O_NOFOLLOW follows symlinks at both the target file and intermediate directories, allowing arbitrary file writes.
Root Cause
At dulwich/porcelain/__init__.py:5661-5675, the checkout(paths=[...]) implementation:
file_path = _checked_worktree_path(r, path)
os.makedirs(os.path.dirname(file_path), exist_ok=True)
flags = os.O_WRONLY | os.O_CREAT | os.O_TRUNC
with os.fdopen(os.open(file_path, flags, mode), "wb") as f:
f.write(obj.data)
_checked_worktree_path() (line 601-631) only performs name validation, checking that the path doesn't start with / or \\ and that components pass INVALID_DOTNAMES checks. It performs zero filesystem symlink detection.
Attack Scenario
- Attacker creates a repository where HEAD has
triggeras a symlink (mode 120000, content../../.git/hooks/post-checkout), and tagv1.0hastriggeras an executable file (mode 100755, content#!/bin/sh\nmalicious_payload) - Victim clones the repository, worktree has
trigger→../../.git/hooks/post-checkout(a symlink) - Victim runs
porcelain.checkout(repo, target="v1.0", paths=["trigger"])to restore a specific file from a tag _checked_worktree_path(r, "trigger")passes, name validation only, no symlink checkos.open("trigger", O_WRONLY|O_CREAT|O_TRUNC, 0o755)follows the symlink → opens.git/hooks/post-checkoutfor writingf.write(obj.data)writes the malicious payload to the hook- Next checkout operation triggers the hook → RCE
Reported by zx (Jace)
Impact
An attacker can craft a malicious repository that, when a victim clones it and runs checkout(paths=[...]), writes attacker-controlled content (with attacker-controlled permissions) to any filesystem location accessible to the user. Writing to .git/hooks/post-checkout achieves RCE on the next git checkout.
GHSA-8W8G-WQ8H-FQ33 has a CVSS score of 8.6 (High). The vector is requires local access, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.2.8); upgrading removes the vulnerable code path.
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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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Replace the raw os.open path with a call to build_file_from_blob (once that function is hardened against intermediate symlinks), or add explicit symlink detection: resolve the path with os.path.realpath() and verify it stays within the worktree root before opening.
Frequently Asked Questions
- What is GHSA-8W8G-WQ8H-FQ33? GHSA-8W8G-WQ8H-FQ33 is a high-severity security vulnerability in dulwich (pip), affecting versions >= 0.24.0, <= 1.2.7. It is fixed in 1.2.8.
- How severe is GHSA-8W8G-WQ8H-FQ33? GHSA-8W8G-WQ8H-FQ33 has a CVSS score of 8.6 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of dulwich are affected by GHSA-8W8G-WQ8H-FQ33? dulwich (pip) versions >= 0.24.0, <= 1.2.7 is affected.
- Is there a fix for GHSA-8W8G-WQ8H-FQ33? Yes. GHSA-8W8G-WQ8H-FQ33 is fixed in 1.2.8. Upgrade to this version or later.
- Is GHSA-8W8G-WQ8H-FQ33 exploitable, and should I be worried? Whether GHSA-8W8G-WQ8H-FQ33 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 GHSA-8W8G-WQ8H-FQ33 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 GHSA-8W8G-WQ8H-FQ33? Upgrade
dulwichto 1.2.8 or later.