---
id: CVE-2026-115927
aliases:
  - GHSA-r4j5-9xpp-3985
title: >-
  Tornado: Event loop blocking and excess memory use from chunked bodies with
  many small chunks
summary: >-
  Tornado: Event loop blocking and excess memory use from chunked bodies with
  many small chunks
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
vendor: tornado
product: tornado
ecosystem: pip
affected:
  - tornado < 6.6
patched:
  - tornado 6.6
published: '2026-10-28'
updated: '2026-10-28'
sourceUpdated: '2026-10-28T23:00:04.082411607Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-r4j5-9xpp-3985'
references:
  - url: >-
      https://github.com/tornadoweb/tornado/security/advisories/GHSA-r4j5-9xpp-3985
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-115927'
  - url: 'https://github.com/tornadoweb/tornado/pull/3773'
  - url: >-
      https://github.com/tornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b01c8
  - url: 'https://github.com/tornadoweb/tornado'
  - url: 'https://github.com/tornadoweb/tornado/releases/tag/v6.6.0'
  - url: 'https://github.com/advisories/GHSA-r4j5-9xpp-3985'
tags:
  - osv
  - pip
  - ghsa
  - cve.org
epss: 0.00384
epssPercentile: 0.32384
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-27T13:45:56.157686Z'
cwe:
  - CWE-400
  - CWE-770
ingestedAt: '2026-10-26T20:39:42.649Z'
---

## Overview

### 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 passed to `data_received` no longer correspond to HTTP chunk boundaries.

### Workarounds

- Run Tornado behind a reverse proxy that buffers request bodies and forwards them with a `Content-Length` header instead of chunked encoding.
- Lower `max_body_size` (for the whole server or for individual handlers) to limit the impact of each request.

### Credit

Reported by [uttej-badwane](https://github.com/uttej-badwane).

## Affected packages

- `tornado < 6.6`

## Remediation

Upgrade to a patched release:

- `tornado 6.6`