Tornado is a Python web framework and asynchronous networking library. Prior to 6.5.8, Tornado parses application/x-www-form-urlencoded request bodies with urllib.parse.parse qs in tornado/escape.py without passing max num fields. RequestHandler. execute in tornado/web.py parses the body before handler dispatch through HTTPServerRequest. parse body and parse body arguments in tornado/httputil.py, so an unauthenticated request body containing millions of separator-delimited fields can synchronously stall the single-threaded event loop and delay every connection. The body is bounded only by max buffer size, which defaults to 104857600 bytes. This issue is fixed in version 6.5.8.