GHSA-7cx3-6m66-7c5m

### 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.