PYSEC-2026-1974

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.