fix: replace string-prefix check in safe_extractall with commonpath - #324
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Gi-ron merged 1 commit intoJul 28, 2026
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`startswith` is the wrong primitive for "is target inside directory base": once a trailing separator drops anywhere along the way, sibling directories whose names extend base match. We could spot-fix by re-appending the separator before the check, but `commonpath` makes the directory-boundary intent explicit and removes the failure mode entirely. Fixes GHSA-6cmm-8875-5pcw. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com> (cherry picked from commit 97de058)
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Summary
_is_bad_pathwithos.path.commonpath, closing a path traversal insafe_extractallContext
The official fix landed on Ulmo, Verawood and master but did not include Teak. Since
extract_archive.pyis unchanged in our Teak branch relative to upstream, this cherry-pick applies cleanly with no conflicts._is_bad_pathvalidated extraction targets withresolved(joinpath(base, path)).startswith(base).safe_extractallappends a trailing separator tooutput_path, but_checkmembersimmediately callsresolved(base), which normalizes it away. The check then matched/data/<base64>as a string prefix of/data/<base64>evil/..., letting a crafted.tar.gzentry such as../<base64>evil/filewrite outside the intended extraction directory.Reached via the Studio course/library import flow (
import_olx), which requires theCOURSES_IMPORT_COURSEpermission — so the attacker must be a course author or staff. Only.tar.gzis practically exploitable, sinceZipFile.extractallstrips..segments independently. Realistic impact is limited filesystem corruption and cross-tenant pollution of course import staging directories.The fix uses
commonpathrather than re-appending the separator, making the directory-boundary intent explicit and removing the failure mode entirely. AValueErrorguard treats incomparable paths (mixed absolute/relative, different drives) as bad.Cherry-picked with
-x; applied cleanly with no conflicts