---
id: "CVE-2026-115927"
kind: "vulnerability"
aliases: ["CVE-2026-115927"]
title: "Tornado: chunked request bodies can stall the event loop and exhaust memory"
severity: "medium"
cvss: 5.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["package", "python", "self-hosted", "tornado"]
not_affected_if: ["no-chunked-input", "trusted-clients-only", "version-6.6.0-or-later"]
published_at: "2026-10-26T20:17:52+00:00"
first_seen_at: "2026-10-26T20:50:17+00:00"
updated_at: "2026-10-26T20:51:54+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-115927", "first_seen_at": "2026-10-26T20:50:17+00:00"}]
---

# Tornado: chunked request bodies can stall the event loop and exhaust memory

Tornado decodes HTTP/1.1 chunked bodies without yielding to the event loop, so a remote peer sending millions of one-byte chunks blocks every other connection in that process. For handlers that do not use @stream_request_body, each chunk is kept as a separate object, using roughly twenty times the body size in memory.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- `pip show tornado` or `python -c "import tornado; print(tornado.version)"` to read the installed version
- Check whether any handler uses `@stream_request_body` and whether the app runs `SimpleAsyncHTTPClient` against untrusted servers

## Fix

- Upgrade Tornado to `6.6.0` or later
- Until upgraded, lower `max_body_size` only as a partial mitigation, since it limits duration but not the stall

## Timeline

- 2026-10-26T20:50:17+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-115927

## From the sources

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.

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