Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks

Exploitation

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

CVSS

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

EPSS

• CVE-2026-115927: 0.38% estimated exploitation activity in the next 30 days; percentile 32.50 (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 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…

Published by the source

28 Oct 2026, 22:43 UTC

Last modified by the source

28 Oct 2026, 23:00 UTC

Source last checked

29 Oct 2026, 06:10 UTC

1. Revision 2 · amended · 29 Oct 2026

   Amended from osv record GHSA-r4j5-9xpp-3985

2. Revision 1 · created · 28 Oct 2026

   Published from ghsa record GHSA-r4j5-9xpp-3985

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# Patch Next advisory brief: CVE-2026-115927

Advisory: Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks
IDs: CVE-2026-115927, GHSA-r4j5-9xpp-3985
Record: https://patchnext.com/advisories/cve-2026-115927 (revision 2, source checked 29 Oct 2026, 06:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: tornado 6.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI tornado: affected < 6.6; fixed in 6.6

## 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-r4j5-9xpp-3985 (OSV record GHSA-r4j5-9xpp-3985 lists affected ranges: PyPI tornado >=0 <6.6 (fixed))
- Verified: https://github.com/advisories/GHSA-r4j5-9xpp-3985 (GitHub-reviewed advisory GHSA-r4j5-9xpp-3985 lists affected ranges: PyPI tornado >=0 <6.6)

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