CVE-2026-29790: dbt-common's commonprefix() doesn't protect against path traversal

What kind of vulnerability is it? Who is impacted?

A path traversal vulnerability exists in dbt-common’s safe_extract() function used when extracting tarball archives. The function uses os.path.commonprefix() to validate that extracted files remain within the intended destination directory. However, commonprefix() compares paths character-by-character rather than by path components, allowing a malicious tarball to write files to sibling directories with matching name prefixes.

For example, when extracting to /tmp/packages, a crafted tarball could write files to /tmp/packagesevil/ by exploiting the character-based prefix matching.

This vulnerability affects users who:

• Install dbt packages from untrusted sources
• Process tarball archives through dbt-common’s extraction utilities

The practical risk is limited because:

• Exploitation requires a malicious tarball to be processed
• File writes are restricted to sibling directories with matching prefixes (not arbitrary paths)
• Packages from trusted sources (dbt Hub) are not affected

This is similar to CVE-2026-1703 in pip, which had a CVSS score of 3.9 (Low).