Tornado has out-of-bounds memory access via C extension

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

3.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

EPSS

• CVE-2026-49854: 0.42% estimated exploitation activity in the next 30 days; percentile 36.29 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Tornado's optional native extension tornado.speedups implements websocketmask without validating that the mask argument is exactly four bytes long. The C function reads four bytes from mask unconditionally, even when Python passes a shorter byte string. This can read beyond the provided buffer, exposing up to 3 bytes of uninitialized memory. The behavior is reachable from Tornado's XSRF token decoder when xsrfcookies=True and the native extension is active. Mitigations This bug is fixed in Tornado 6.5.6. Prior to upgrading to this version, setting the environment variable TORNADOEXTENSION=0 will disable the vulnerable code (at the expense of reducing websocket performance).

Published by the source

12 Jun 2026, 18:30 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:45 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-cx3h-4qpv-8hc9

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-cx3h-4qpv-8hc9

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# Patch Next advisory brief: CVE-2026-49854

Advisory: Tornado has out-of-bounds memory access via C extension
IDs: CVE-2026-49854, GHSA-cx3h-4qpv-8hc9
Record: https://patchnext.com/advisories/cve-2026-49854 (revision 2, source checked 26 Oct 2026, 13:45 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: tornado 6.5.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI tornado: affected < 6.5.6; fixed in 6.5.6

## Suggested inspection (read-only)
- `python -m pip show tornado`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on tornado.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-cx3h-4qpv-8hc9 (OSV record GHSA-cx3h-4qpv-8hc9 lists affected ranges: PyPI tornado >=0 <6.5.6 (fixed))
- Verified: https://github.com/advisories/GHSA-cx3h-4qpv-8hc9 (GitHub-reviewed advisory GHSA-cx3h-4qpv-8hc9 lists affected ranges: PyPI tornado >=0 <6.5.6)

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