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GHSA-555p-6grf-mh7f
No affected components available
Impact
dulwich.porcelain.format_patch(outdir=...) derives each patch filename from the commit's subject line. Prior to this fix, get_summary only replaced spaces with dashes - path separators (/, ), parent-directory components (..), and other filename-hostile characters (e.g. :) were preserved verbatim and passed straight into os.path.join(outdir, f"{i:04d}-{summary}.patch").
A malicious commit subject could therefore direct the generated patch file outside the requested outdir. Reduced examples:
- x/../../x produced <outdir>/0001-x/../../x.patch, resolving two directories above outdir.
- x....\x produced the equivalent escape on Windows, here \ is also a path separator.
Related issues from the same root cause:
- Subjects containing characters that are illegal in Windows filenames (e.g. :) caused format_patch to fail outright on Windows, where git would have succeeded.
- Very long subjects produced excessively long filenames that could exceed filesystem limits; git truncates them.
Anyone calling porcelain.format_patch (or the dulwich format-patch CLI) against untrusted commits - for example, a service that runs format-patch over user-supplied repositories or pull requests - could have patch files written to attacker-chosen locations within the process's write permissions.
Patches
Fixed in Dulwich 1.2.5. Users should upgrade.
dulwich.patch.get_summary now mirrors git's format_sanitized_subject: only [A-Za-z0-9._] are kept, runs of other characters collapse to a single -, consecutive . collapse to a single ., trailing ./- are stripped, and the result is length-limited. This makes the returned string safe to embed as a filename component, so format_patch can no longer be steered out of outdir via the commit subject.
Workarounds
Until upgrading, callers that pass untrusted commits to porcelain.format_patch can:
- Use stdout=True and write the patch to a destination they control, rather than letting format_patch choose the filename.
- Validate the chosen path before opening - e.g. compare os.path.realpath(returned_path) against os.path.realpath(outdir) and reject any patch whose resolved path is not inside outdir.
- Pre-screen commits and refuse to format any whose subject's first line contains /, , .., or other characters that are not safe on the target filesystem.
Resources
- Fix commit: https://github.com/jelmer/dulwich/commit/c2446e51b
- Affected API: dulwich.porcelain.format_patch / dulwich format-patch
- Reference behavior: git's format_sanitized_subject in pretty.c
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 impact is confined to the system where the vulnerability exists. There is a low impact on the integrity of the data.
Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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