Rack::Sendfile header-based X-Accel-Mapping regex injection enables unauthorized X-Accel-Redirect

Summary Rack::Sendfilemapaccelpath interpolates the value of the X-Accel-Mapping request header directly into a regular expression when rewriting file paths for X-Accel-Redirect. Because the header value is not escaped, an attacker who can supply X-Accel-Mapping to the backend can inject regex...