Dulwich: Symlink directory traversal in filter-branch index_filter via cross-commit state persistence
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
- 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)
- Ancestor commit: tree entry
- Victim clones repository and runs
filter_branchwith an index filter process_commit()processes ancestor first → materializesevilas symlink to/target_dirin CWD- CWD is NOT cleaned between commits
- 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)
Software afectado
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| Plataforma | Paquete | Versiones vulnerables | Corregida en |
|---|---|---|---|
| Python (PyPI) | dulwich | >= 0.23.1, < 1.2.8 | 1.2.8 |
Referencias
- Vector CVSS
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H- Debilidad (CWE)
CWE-22, CWE-59- Plataforma
- Python (PyPI)
- Paquete
dulwich- Publicada
- 02.10.2026
- Actualizado
- 21.10.2026 14:17
Fuentes y licencias
OSV / GitHub Advisory Database - GHSA-5fqc-mrg8-w798 - CC BY 4.0
Historial de vulnerabilidades: dulwich
Todas las vulnerabilidades publicadas de este paquete (Python (PyPI)), de la más reciente a la más antigua. Muestra si es un fallo aislado o un paquete que hay que vigilar.
- Entradas en total
- 13
- Primera → última
- 03.2015 10.2026
- Sin corrección
- 0
- Explotada en ataques
- 0
Distribución por gravedad
- Crítica 3
- Alta 7
- Media 2
- Baja 1
Divulgaciones por año
- 2017: 1
- 2018: 0
- 2019: 0
- 2020: 0
- 2021: 0
- 2022: 0
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 10
antes: 2
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