GHSA-7cx3-6m66-7c5m Tornado vulnerable to excessive logging caused by malformed multipart form data

• PyPI tornado: < 6.5 → fixed in 6.5

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.