An attacker can send a crafted request like GET /protected with a Host: example.com/health?x= header. The request will reach the /proteced path, but request.url would be https://example.com/health?x=/protected, and request.url.path would return /health instead of the real request path.

It's used a lot in the data heavy AI world for it's efficiency shipping large files. This includes lots and lots of production servers.

From the advisory: this includes LLM inference servers like vLLM, LLM proxy servers like LiteLLM, AI agent frameworks, MCP gateways, and custom APIs. MCP servers are especially at risk because the MCP spec mandates unauthenticated OAuth discovery endpoints, providing a reliable path for exploitation.