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High

Dulwich: Symlink directory traversal in filter-branch index_filter via cross-commit state persistence

  • Disclosed: 02.10.2026
  • Updated: 11.10.2026 15:18

Summary

Dulwich's filter_branch.py CommitFilter._apply_index_filter() is vulnerable to symlink directory traversal. When processing commit history, materialized tree entries (including symlinks) persist in the working directory between commits, allowing a symlink from an ancestor commit to redirect file writes from a descendant commit to arbitrary filesystem locations.

Root Cause

_apply_index_filter() at dulwich/filter_branch.py:212 calls build_index_from_tree(".", tmp_index_path, ...) which materializes all tree entries to the current working directory. The finally block (line 229-230) only cleans up the temporary index file (os.unlink(tmp_index_path)) — NOT the filesystem files written to CWD. When process_commit() processes parents recursively first (line 260), files materialized from ancestor commits persist and affect processing of descendant commits.

On dulwich 1.2.7, build_file_from_blob() has no symlink protection, and validate_path_element only validates name patterns, not filesystem state.

Impact

An attacker can craft a malicious repository where running filter_branch with an index filter writes attacker-controlled content to arbitrary filesystem locations via symlink traversal. This achieves RCE if the write targets .git/hooks/.

Attack Scenario

  1. Attacker creates a repository where commit history (linearized) has:
    • Ancestor commit: tree entry evil (mode 120000, symlink → /target_dir)
    • Descendant commit: tree entry evil/payload (mode 100644, attacker content)
  2. Victim clones repository and runs filter_branch with an index filter
  3. process_commit() processes ancestor first → materializes evil as symlink to /target_dir in CWD
  4. CWD is NOT cleaned between commits
  5. Processing descendant: os.path.exists("./evil") → True (symlink exists). build_file_from_blob(blob, mode, "./evil/payload") → open("./evil/payload", "wb") follows intermediate symlink → writes to /target_dir/payload

Suggested Fix

Clean the CWD between commit iterations in _apply_index_filter(), or verify that no intermediate path components are symlinks before writing files.

Reported by zx (Jace)

Affected software

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PlatformPackageVulnerable versionsFixed in
Python (PyPI) dulwich >= 0.23.1, < 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-22, CWE-59
Platform
Python (PyPI)
Package
dulwich
Disclosed
02.10.2026
Updated
11.10.2026 15:18

Sources and licenses

OSV / GitHub Advisory Database - GHSA-5fqc-mrg8-w798 - 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
    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 this entry
    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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