CVE-2026-59973: FrontMCP and mcp-from-openapi have bypass of OpenAPI external $ref SSRF fix

The published fix for GHSA-v6ph-xcq9-qxxj / CVE-2026-39885 added a direct hostname denylist for OpenAPI external $ref dereferencing, but the latest patched dependency mcp-from-openapi 2.3.0 still makes backend-origin requests to loopback when the target is reached through hostname resolution, redirects, or IPv4-mapped IPv6 syntax.

FrontMCP latest release v1.2.1 and current main still call OpenAPIToolGenerator.fromURL() and OpenAPIToolGenerator.fromJSON() from mcp-from-openapi 2.3.0 when loading OpenAPI adapters. An attacker who can cause a hosted or multi-user FrontMCP deployment to load an untrusted OpenAPI spec can trigger requests from the server to localhost or private services during tool generation.

This is a latest-version bypass of the previous fix. A direct http://127.0.0.1 $ref control is now denied and produces zero canary hits, while semantically equivalent loopback targets still reach the canary.