Medium · published September 27, 2026
python-utcp (pip package utcp-http) before 1.1.12 does not verify whether tool URLs declared in a hand-written UTCP manual point at the agent's own loopback interface when that manual is discovered from a remote, non-loopback origin. Because ensure_secure_url intentionally permits loopback HTTP for local development and native manuals bypassed the loopback check performed by the OpenAPI converter, an attacker who can serve a UTCP manual that a victim registers can cause the client to issue requests to services bound only to 127.0.0.1 on the victim host and have the response bodies returned to the caller (server-side request forgery). The http, sse and streamable_http protocols are all affected. Reach is limited to loopback, and exploitation further requires a loopback service that answers unauthenticated requests with useful data. Fixed in utcp-http 1.1.12, which rejects manuals fetched from a non-loopback origin that declare loopback tool URLs, keyed off the final post-redirect discovery URL.