BadHost (CVE-2026-48710): Critical Starlette Vulnerability Exposes Millions of AI Agents to Host-Header Auth Bypass

Ars Technica·high signal

A critical vulnerability in Starlette (325 million weekly downloads), the routing core of FastAPI, allows attackers to bypass path-based authorization by injecting a single character into the HTTP Host header. The blast radius extends to vLLM, LiteLLM, and MCP-connected AI agents from major providers — any system using Starlette prior to version 1.0.1 is vulnerable. OSTIF disclosed the flaw; a free scanner is available at badhost.org.