CVE-2026-55245

Bifrost is an enterprise AI gateway for routing requests to model providers. Prior to 1.5.17, the isPublicIP function in core/providers/utils/fetch.go, reached through FetchAndEncodeURL for Bedrock and Vertex image or document URLs, classifies Carrier-Grade NAT 100.64.0.0/10, IPv6 6to4 2002::/16, NAT64 64:ff9b::/96 and 64:ff9b:1::/48, and deprecated IPv6 site-local fec0::/10 addresses as public. A remote attacker who controls a multimodal request URL can make the gateway fetch internal services, including a cloud instance metadata endpoint encoded through 6to4 or NAT64. This issue is fixed in version 1.5.17.

Bifrost is an enterprise AI gateway for routing requests to model providers. Prior to 1.5.17, the isPublicIP function in core/providers/utils/fetch.go, reached through FetchAndEncodeURL for Bedrock and Vertex image or document URLs, classifies Carrier-Grade NAT 100.64.0.0/10, IPv6 6to4 2002::/16, NAT64 64:ff9b::/96 and 64:ff9b:1::/48, and deprecated IPv6 site-local fec0::/10 addresses as public. A remote attacker who controls a multimodal request URL can make the gateway fetch internal services, including a cloud instance metadata endpoint encoded through 6to4 or NAT64. This issue is fixed in version 1.5.17.

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

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…

Bifrost is an enterprise AI gateway for routing requests to model providers. Prior to 1.5.17, the isPublicIP function in core/providers/utils/fetch.go, reached through FetchAndEncodeURL for Bedrock and Vertex image or document URLs, classifies Carrier-Grade NAT 100.64.0.0/10, IPv6 6to4 2002::/16, NAT64 64:ff9b::/96 and 64:ff9b:1::/48, and deprecated IPv6 site-local fec0::/10 addresses as public. A remote attacker who controls a multimodal request URL can make the gateway fetch internal services, including a cloud instance metadata endpoint encoded through 6to4 or NAT64. This issue is fixed in version 1.5.17.

Bifrost is an enterprise AI gateway for routing requests to model providers. Prior to 1.5.17, the isPublicIP function in core/providers/utils/fetch.go, reached through FetchAndEncodeURL for Bedrock and Vertex image or document URLs, classifies Carrier-Grade NAT 100.64.0.0/10, IPv6 6to4 2002::/16, NAT64 64:ff9b::/96 and 64:ff9b:1::/48, and deprecated IPv6 site-local fec0::/10 addresses as public. A remote attacker who controls a multimodal request URL can make the gateway fetch internal services, including a cloud instance metadata endpoint encoded through 6to4 or NAT64. This issue is fixed in version 1.5.17.

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

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…