BadHost – CVE-2026-48710: Starlette Host-Header Authentication Bypass Vulnerability

CVE-2026-48710 has been reported, a vulnerability occurring in versions of Starlette below 1.0.1 where the Host header is used without validation when generating request.url.

An attacker can craft a manipulated Host header to generate a request.url.path that differs from the actual request path, thereby bypassing path-based authentication middleware. For example, if an attacker sends a header such as Host: example.com/health?x=, they can access the /protected path that requires authentication while the system recognizes it as the /health path.

Numerous FastAPI and Starlette-based AI infrastructures, including vLLM, LiteLLM, MCP servers, and AI agent frameworks, are exposed to this vulnerability. In particular, MCP servers may be especially vulnerable to attack due to their characteristic of using unauthenticated OAuth discovery endpoints.

Countermeasures:

• Update Starlette: Update to the latest version, 1.0.1 or higher, so that Host headers containing invalid characters are ignored.
• Improve authentication method: Instead of path-based authentication middleware, use security methods applied to the endpoint itself, such as FastAPI's Depends() or Starlette's requires().
• Introduce a reverse proxy: Place Nginx, Caddy, Traefik, etc. in front to validate and normalize the Host header.
• Modify code: In middleware, use scope["path"], which cannot be manipulated, instead of request.url.path.