mcp-from-openapi: Bypass of OpenAPI external $ref SSRF fix in latest FrontMCP and mcp-from-openapi

Summary

FrontMCP is a TypeScript-first framework for the Model Context Protocol (MCP). From mcp-from-openapi 2.3.0 until 2.5.0 and from frontmcp and @frontmcp/adapters 1.2.1 until 1.5.0, libs/adapters/src/openapi/openapi.adapter.ts loadOpenAPISpec() forwards untrusted OpenAPI url and spec inputs and loadOptions.refResolution to OpenAPIToolGenerator.fromURL() and OpenAPIToolGenerator.fromJSON(). The external $ref guard checks parsed hostname strings without resolving addresses, pinning validated addresses, revalidating redirect targets, or normalizing IPv4-mapped IPv6. An authenticated user who can import or configure an OpenAPI specification in a hosted or multi-user deployment can use DNS-to-loopback resolution, redirect-to-loopback behavior, or IPv4-mapped IPv6 loopback forms to cause backend-origin requests to internal services. This can expose internal administrative APIs, metadata-like services, and other private network endpoints. The practical impact is lower when only a trusted local administrator can configure OpenAPI specs, and disabling external reference protocols prevents the external $ref request. This issue is fixed in mcp-from-openapi 2.5.0 and frontmcp and @frontmcp/adapters 1.5.0.

Timeline

1. Sep 15, 2026, 3:13 PM UTCCVE published
2. Sep 16, 2026, 4:06 PM UTCCISA SSVC assessment

   Exploitation: poc · Automatable: no · Technical impact: partial

3. Sep 16, 2026, 4:06 PM UTCRecord last updated

Affected versions

• agentfront frontmcp

  fixed in 1.5.0

• agentfront mcp-from-openapi

  fixed in 2.5.0

• @frontmcp adapters

  fixed in 1.5.0

• mcp-from-openapinpm

  From 2.3.0 · fixed in 2.5.0

• @frontmcp/adaptersnpm

  From 1.2.1 · fixed in 1.5.0

• frontmcpnpm

  From 1.2.1 · fixed in 1.5.0

References 7