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High

Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections

  • Disclosed: 02.10.2026
  • Updated: 11.10.2026 15:18

Summary

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:

python
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.

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.

Attack Scenario

  1. Attacker creates a repository where HEAD has trigger as a symlink (mode 120000, content ../../.git/hooks/post-checkout), and tag v1.0 has trigger as an executable file (mode 100755, content #!/bin/sh\nmalicious_payload)
  2. Victim clones the repository — worktree has trigger → ../../.git/hooks/post-checkout (a symlink)
  3. Victim runs porcelain.checkout(repo, target="v1.0", paths=["trigger"]) to restore a specific file from a tag
  4. _checked_worktree_path(r, "trigger") passes — name validation only, no symlink check
  5. os.open("trigger", O_WRONLY|O_CREAT|O_TRUNC, 0o755) follows the symlink → opens .git/hooks/post-checkout for writing
  6. f.write(obj.data) writes the malicious payload to the hook
  7. Next checkout operation triggers the hook → RCE

Suggested Fix

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.

Reported by zx (Jace)

Affected software

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PlatformPackageVulnerable versionsFixed in
Python (PyPI) dulwich >= 0.24.0, < 1.2.8 1.2.8

References

CVSS score 8.6/10
CVSS vector
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
Weakness (CWE)
CWE-59, CWE-61
Platform
Python (PyPI)
Package
dulwich
Disclosed
02.10.2026
Updated
11.10.2026 15:18

Sources and licenses

OSV / GitHub Advisory Database - GHSA-8w8g-wq8h-fq33 - CC BY 4.0

Vulnerability history: dulwich

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (13) in the archive
Total entries
13
First → latest
03.2015 10.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • Critical 3
  • High 7
  • Medium 2
  • Low 1

Disclosures per year

earlier: 2

Severity:
  1. Dulwich: Arbitrary File Write (RCE) on Windows via Unvalidated Drive Letters in Tree Paths
    High · 8.8 ghsa-8mcx-5rqc-vhmf < 1.2.9 1.2.9
  2. Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections this entry
    High · 8.6 ghsa-8w8g-wq8h-fq33 >= 0.24.0, < 1.2.8 1.2.8
  3. Dulwich: Symlink directory traversal in filter-branch index_filter via cross-commit state persistence
    High · 8.6 ghsa-5fqc-mrg8-w798 >= 0.23.1, < 1.2.8 1.2.8
  4. Dulwich: Symlink directory traversal in stash pop allows arbitrary file write via intermediate directory symlinks
    High · 8.6 ghsa-cm62-gvxx-vmxx >= 0.22.5, < 1.2.8 1.2.8
  5. Dulwich's submodule path traversal in porcelain.submodule_update / porcelain.clone(recurse_submodules=True) yields RCE via attacker-dropped .git/hooks payload
    High · 7.5 CVE-2026-52726 >= 0.23.2, < 1.2.5 1.2.5
  6. Dulwich Vulnerable to Command Injection via Merge Driver Path
    High CVE-2026-42563 >= 0.24.0, < 1.2.5 1.2.5
  7. Dulwich has an arbitrary file write via NTFS-hostile tree entries on Windows
    High · 8.8 CVE-2026-42305 >= 0.10.0, < 1.2.5 1.2.5
  8. Dulwich before 0.18.5, when an SSH subprocess is used, allows remote attackers to execute arbitrary commands via an ssh URL with an initial dash character in the hostname, a related issue to CVE-2017-
    Critical · 9.8 CVE-2017-16228 < 0.18.5 0.18.5
  9. The build_index_from_tree function in index.py in Dulwich before 0.9.9 allows remote attackers to execute arbitrary code via a commit with a directory path starting with .git/, which is not properly h
    Critical · 9.8 CVE-2014-9706 < 0.9.9 0.9.9
  10. Buffer overflow in the C implementation of the apply_delta function in _pack.c in Dulwich before 0.9.9 allows remote attackers to execute arbitrary code via a crafted pack file.
    Critical · 9.8 CVE-2015-0838 < 0.9.9 0.9.9

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