Tornado vulnerable to excessive logging caused by malformed multipart form data

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.