Gotenberg ExifTool group-prefix bypass

Summary

Gotenberg exposes an endpoint for writing PDF metadata through ExifTool. The intended protection blocked dangerous pseudo-tags such as FileName, Directory, HardLink, and SymLink. The check compared the submitted key directly against the blocklist.

ExifTool also accepts group-prefixed keys, for example File:FileName. ExifTool strips that prefix before resolving the tag, but Gotenberg's validation allowed colons and did not normalize the key before comparing it with the dangerous-tag list. This made the blocklist incomplete.

Impact

The advisory describes a pre-authentication attack against default Gotenberg deployments. An attacker could rename or move generated files, or create hardlinks and symlinks inside the server environment. In deployments with mounted volumes or non-containerized operation, this can become a file-read or file-overwrite primitive through link and directory manipulation.

Why this mattered

This was a bypass of an earlier Gotenberg fix for dangerous ExifTool metadata writes. The root cause was not ExifTool itself, but a mismatch between Gotenberg's input filtering and ExifTool's accepted key syntax. Security filters around delegated tools need to validate the canonical form that the downstream tool will actually execute.

Disclosure

The issue was reported by JohannesLks and published by the Gotenberg maintainers on April 30, 2026. The public advisory is available on GitHub.