Dulwich symlink directory traversal in filter-branch index_filter

Severity: high · CVSS 8.6 · Published 2026-10-02

Dulwich is a Python library for interacting with Git repositories. When running filter-branch with an index filter on a malicious repository, symlinks from earlier commits can persist in the working directory and redirect subsequent writes to arbitrary filesystem locations. An attacker can exploit this to write files to sensitive locations like Git hooks, potentially achieving remote code execution.

Technologies: Dulwich, dulwich (PyPI). Vendors: Dulwich, PyPI.

Quick answers

Is This vulnerability exploited in the wild?
No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.
How severe is This vulnerability?
High severity, with a CVSS score of 8.6, published 2 October 2026.
Which products and versions are affected?
Dulwich 1.2.7 and earlier; dulwich (PyPI) >= 0.23.1, <= 1.2.7.
Is there a fix for This vulnerability?
No fixed version is recorded yet. Clean the working directory between commit iterations in _apply_index_filter(), or validate that no intermediate path components are symlinks before writing files. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is This vulnerability attacked?
It is attacked over the network, in a software library.
Am I affected?
Check whether you run Dulwich or dulwich (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

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

Technical details

The vulnerability exists in CommitFilter._apply_index_filter() which materializes tree entries (including symlinks) to the working directory via build_index_from_tree() but only cleans up the temporary index file, not the written files. When processing commits recursively, ancestor symlinks persist and are followed when writing descendant files, enabling arbitrary writes. build_file_from_blob() lacks symlink protection and validate_path_element only validates name patterns, not filesystem state.

Affected products

  • Dulwich Dulwich 1.2.7 and earlier

Timeline

  • 2026-10-02: disclosed

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