ContextForge: DNS TOCTOU race condition causes SSRF protection bypass (`/admin/gateways/test`)

Summary

The /admin/gateways/test endpoint validates submitted URLs by resolving the hostname at validation time and blocking private address ranges. The HTTP client independently re-resolves DNS at connection time with no IP binding between the two operations, creating a TOCTOU window exploitable via DNS rebinding. The source code explicitly acknowledges this limitation in two separate locations.

Details

validate_gateway_test_url() in mcpgateway/common/validators.py (lines 1527–1710) calls socket.getaddrinfo() on the submitted hostname, checks whether the resolved IP falls in private, loopback, link-local, or cloud-metadata ranges (including 169.254.169.254, 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16), and accepts the URL if the result is clean. The validated URL is then passed to the HTTP client as the original hostname string, not as the validated IP address.

The HTTP client (httpx, via ResilientHttpClient) performs its own independent DNS resolution at connection time. No mechanism bridges the two resolutions:

• The validated IP address is never passed to the HTTP client.

• Only the original hostname is forwarded, triggering a second independent lookup.

• No TTL enforcement, mandatory DNS-cache reuse, or IP-level socket binding is

implemented.

The configuration options ssrf_blocked_networks (default: enabled, covers 169.254.169.254/32, link-local ranges, etc.) and ssrf_dns_fail_closed (default: True) apply exclusively at validation time. They share the same TOCTOU gap because they operate on the validation-time resolution result, not on the connection-time resolution performed by the HTTP client.

Two independent acknowledgements in the source code

Location 1 — mcpgateway/common/validators.py, lines 1537–1543 (function docstring of validate_gateway_test_url):

** "DNS TOCTOU Limitation: This validation resolves DNS at validation time, but

the HTTP client will re-resolve DNS at connection time. An attacker controlling

DNS can return a public IP during validation and a private IP during connection

(DNS rebinding). True mitigation requires pinning the validated IP into the

connection (custom resolver/transport, or IP allowlist check at connect

callback). This is tracked as a known limitation for future improvement."

Location 2 — mcpgateway/admin.py, lines 14025–14029 (call site comment):

** "TODO(ICACF-15): DNS rebinding risk — allowlist and SSRF checks resolve DNS,

but the actual ResilientHttpClient request resolves DNS a third time. An

attacker-controlled DNS server could return a public IP during validation and a

private IP during the actual request. Consider pinning the resolved IP for

outbound requests (custom transport) or caching DNS resolution across

validation and request phases."

The existence of a named TODO ticket (ICACF-15) confirms the maintainers consider this an open, tracked defect.

Prerequisites

• MCPGATEWAY_ADMIN_API_ENABLED=true (not the default; must be explicitly

enabled by an operator).

• The attacker holds a credential with explicit gateways.read permission

assigned via a database role.

Regarding prerequisite 2: the endpoint is decorated with @require_permission("gateways.read", allow_admin_bypass=False). The allow_admin_bypass=False flag explicitly disables the platform-admin shortcut, meaning even a platform admin must hold an explicit database-backed role assignment that carries gateways.read. A credential produced solely via the platform-admin bootstrap bypass described in the companion advisory (GHSA-m8rv-5m6m-32ff) — a virtual identity with no database record — is rejected with HTTP 403 at this endpoint because no role lookup can succeed without a database row. An attacker who has forged a JWT via that bootstrap path does not automatically gain access to this endpoint; they still require a separately provisioned account with an appropriate role.

Proof of Concept

Setup

cd /opt/mcp-cf-test
MCPGATEWAY_ADMIN_API_ENABLED=true \
JWT_SECRET_KEY=my-test-key-but-now-longer-than-32-bytes \
uvicorn mcpgateway.main:app --host 0.0.0.0 --port 8000 &
sleep 5