Gotenberg has an SSRF deny-list bypass in IsPublicIP via IPv6 6to4 / NAT64 / site-local prefixes

### Summary `IsPublicIP` in `pkg/gotenberg/outbound.go` incorrectly classifies IPv6 6to4 / NAT64 / deprecated site-local addresses as public IPs, allowing an unauthenticated attacker to reach internal destinations (e.g., cloud metadata services at `169.254.169.254`) via a single crafted DNS AAAA record. This is a variant of CVE-2026-44430 (modelcontextprotocol/registry). ### Details `IsPublicIP` uses Go stdlib helpers (`IsLoopback`, `IsPrivate`, `IsLinkLocalUnicast`, etc.) to block internal IPs. However, these helpers do not recognize IPv6 prefixes that embed IPv4 addresses: | Prefix | RFC | Tunnels to | |--------|-----|-----------| | `2002::/16` | RFC 3056 (6to4) | IPv4 in bits 16-47 | | `64:ff9b::/96` | RFC 6052 (NAT64 well-known) | IPv4 in low 32 bits | | `64:ff9b:1::/48` | RFC 8215 (NAT64 local-use) | IPv4 in low 32 bits | | `fec0::/10` | RFC 3879 (deprecated site-local) | internal routing | `addr.Unmap()` only handles `::ffff:0:0/96` (IPv4-mapped) and has no effect on these prefixes. On dual-stack or NAT64-enabled cloud hosts, the OS kernel transparently routes these addresses to their embedded internal IPv4 destinations. Vulnerable code (`pkg/gotenberg/outbound.go` L53-69, commit `93d0103`): ```go func IsPublicIP(addr netip.Addr) bool { addr = addr.Unmap() // only handles ::ffff:x.x.x.x switch { case addr.IsLoopback(), addr.IsPrivate(), addr.IsLinkLocalUnicast(), ...: return false } return true // 6to4/NAT64/site-local incorrectly reaches here } ``` ### PoC ``` cd poc/ ./build.sh # docker build (~30s) ./run.sh # docker run — exits with code 1 (bug detected) ``` Expected output: `IsPublicIP(2002:a9fe:a9fe::) = true` — the function returns true for 3 addresses that wrap 169.254.169.254 (AWS IMDS). Full test file available via GHSA private comment on request. ### Impact An unauthenticated attacker controlling a DNS AAAA record can tunnel gotenberg's outbound HTTP client to AWS/GCP/Azure IMDS (169.254.169.254), leaking IAM credentials. The Chromium URL convert route returns the full response as a PDF (full-read SSRF). Affects all deployments with `WithDenyPrivateIPs(true)` on dual-stack or NAT64-enabled hosts. ### Suggested Fix Add explicit prefix checks after `addr.Unmap()`: ``` var blockedIPv6Prefixes = []netip.Prefix{ netip.MustParsePrefix("2002::/16"), netip.MustParsePrefix("64:ff9b::/96"), netip.MustParsePrefix("64:ff9b:1::/48"), netip.MustParsePrefix("fec0::/10"), } for _, p := range blockedIPv6Prefixes { if p.Contains(addr) { return false } } ``` Source Code Location: https://github.com/gotenberg/gotenberg Affected Packages: - go:github.com/gotenberg/gotenberg/v8, affected <= 8.32.0 CWEs: - CWE-184: Incomplete List of Disallowed Inputs - CWE-918: Server-Side Request Forgery (SSRF) CVSS: - Primary: score 7.5, CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:L/A:N - CVSS_V3: score 7.5, CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:L/A:N References: - https://github.com/gotenberg/gotenberg/security/advisories/GHSA-86m8-88fq-xfxp - https://github.com/advisories/GHSA-86m8-88fq-xfxp