CVE-2026-115927 — Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks

Affected: tornadoweb tornado

Tornado is a Python web framework and asynchronous networking library. Prior to 6.6.0, Tornado decodes HTTP/1.1 chunked transfer encoding by processing body data that has already arrived without yielding to the event loop. A remote unauthenticated peer can send millions of one-byte chunks to monopolize the process event loop and stall every other connection served by that process, while lowering max_body_size only limits the duration of each request. For handlers that do not use @stream_request_body, retaining each chunk as a separate object can consume roughly twenty times the received body size, and SimpleAsyncHTTPClient has the same event-loop behavior when reading a response from a malicious server. This issue is fixed in version 6.6.0.

GHSA-r4j5-9xpp-3985 — Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks

Affected: PyPI: tornado

### Summary

When Tornado reads an HTTP/1.1 body that uses chunked transfer encoding, it processes all chunks that have already arrived without yielding to the event loop. A client can send a body made of millions of 1-byte chunks, which blocks the event loop (and therefore every other connection served by that process) for the entire time it takes to parse the body. A body near the default `max_body_size` of 100 MB can block the process for close to a minute, and the attacker can repeat or parallelize these requests.

Bodies made of tiny chunks also consumed far more memory than their size: a handler that does not use `@stream_request_body` stored each chunk as a separate object, using roughly 20 times as much memory as the bytes received.

### Impact

All Tornado HTTP servers are affected when they receive a request body for a handler that reads it, which includes any handler that does not reject the request before the body is read. No authentication is required. Lowering `max_body_size` reduces the maximum blocking time and memory use for each request but does not remove the issue.

`SimpleAsyncHTTPClient` is affected in the same way when it reads a response from a malicious server.

### Patches

Fixed in Tornado 6.6. Tornado now yields to the event loop periodically while reading any body, combines small chunks before passing them to `data_received`, and stores non-streaming bodies more compactly.

Applications using `@stream_request_body` should note that the pieces pas…

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CVE-2026-115927 — Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks

GHSA-r4j5-9xpp-3985 — Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks