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GHSA-5fqc-mrg8-w798

HighCVSS 8.6 / 10
Published Oct 2, 2026·Last modified Oct 2, 2026
Affected Components(1)
PyPI logodulwich
0.23.1 – 1.2.8
Description

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)

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Risk Scores
Base Score
8.6

The vulnerability requires local access to the device to be exploited. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. The attacker needs the user to perform some action, like clicking a link. The vulnerability can affect other systems as well, not just the initial system. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a high impact on the availability of the system.

Threat Intelligence
7.9

Exploitation activity has been observed. Apply available patches or mitigations urgently.

EPSS
N/A

Probability that this vulnerability will be exploited in the wild within the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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High severity
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