Impact

ActiveStorage::Filename#base and #extension_with_delimiter, and therefore #extension, pass the stored filename unmodified to File.basename and File.extname, which raise ArgumentError when the name contains a NUL byte. Active Storage stores the client-supplied filename verbatim, so a user who is permitted to upload an attachment could persist such a filename on databases that accept NUL bytes in string columns, such as SQLite and MySQL/MariaDB. Each subsequent request that builds a variant or preview of the blob, opens it, or renders it as an Action Text attachment then raises, possibly resulting in a persistent DoS vulnerability for the pages that display the blob until it is purged or its filename is corrected. Applications that use PostgreSQL are not affected, because PostgreSQL rejects the row.

Details

Filename#base and #extension_with_delimiter now derive from Filename#sanitized, the form that Filename#to_s and the Content-Disposition header already use. For a filename that #sanitized changes, such as one containing a NUL byte, an invalid byte sequence, a path separator or another replaced character, the base and extension now agree with #to_s; the variant format inferred from such a name may change, and an existing variant may be regenerated once. Stored filenames are not modified, and no data migration is required.

Releases

The fixed releases are available at the normal locations.

Patches for those who cannot upgrade at once:

• 8.1: https://github.com/rails/rails/commit/60f22af88b71eae251f112d8c0237a08a8832692.patch
• 8.0: https://github.com/rails/rails/commit/9b0d4bef2211a2c08cab2e0ea17fa1d0efd5d47a.patch

Credit

This issue was found by the Anthropic OSS Scanner (ANT-2026-UQM5Z4YL).