Check the resolved parent directory before extracting old format gems - #9755
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Gem::Package::Old#extract_files only validated entry paths by string expansion, so a preexisting symlink in the extraction directory redirected writes outside of it. Gem::Package#extract_tar_gz already re-resolves the parent directory with File.realpath, so extract the shared check into verify_extraction_dir and call it from both. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Gem::Package::Old#extract_filesvalidated entry paths only by expanding them against the destination directory, so a symlinked subdirectory that already exists under the extraction root redirected writes outside of it.Gem::Package#extract_tar_gzalready re-resolves the parent directory withFile.realpath, so this makes the legacy reader consistent with the modern one rather than fixing a vulnerability. Planting that symlink already requires write access to the extraction directory.The shared check now lives in
verify_extraction_dirand both readers call it. In the old format reader theFileUtils.rm_rfof the destination runs after the check, so a rejected entry cannot delete a file outside the root on its way out.