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.parseqs in tornado/escape.py without passing maxnumfields. RequestHandler.execute in tornado/web.py parses the body before handler dispatch through HTTPServerRequest.parsebody and parsebodyarguments 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 maxbuffersize, which defaults to 104857600 bytes. This issue is fixed in version 6.5.8.

Summary

When Tornado's multipart/form-data parser encounters certain errors, it logs a warning but continues trying to parse the remainder of the data. This allows remote attackers to generate an extremely high volume of logs, constituting a DoS attack. This DoS is compounded by the fact that the logging subsystem is synchronous.

Affected versions

All versions of Tornado prior to 6.5 are affected. The vulnerable parser is enabled by default.

Solution

Upgrade to Tornado version 6.5. In the meantime, risk can be mitigated by blocking Content-Type: multipart/form-data in a proxy.