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Response snapshot Oct 29, 2026, 02:31 UTC

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VulnerabilityCVE-2026-115927

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

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.

At a glance

Exploited
Not known
Severity
5.3 Medium (CVSS 3.1)
Chance of exploitation
0.4% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: tornado: 6.6. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-r4j5-9xpp-3985 (opens a new tab)

What happened

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.

Continue reading

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 package ranges reported by GitHub: tornado: < 6.6

Scores and how they were measured

CVSS base severity

5.3

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.4%

Scored Percentile 32.3%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-r4j5-9xpp-3985 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-r4j5-9xpp-3985, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-115927, scored 2026-10-28. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 28). Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks (CVE-2026-115927). https://kevwire.com/record/CVE-2026-115927

MLA

"Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks (CVE-2026-115927)." KEVWire, 28 Oct. 2026, kevwire.com/record/CVE-2026-115927.

Chicago

KEVWire. "Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks (CVE-2026-115927)." Last modified October 28, 2026. https://kevwire.com/record/CVE-2026-115927.

Evidence summary (plain text)

Tornado: Event loop blocking and excess memory use from chunked bodies with many small chunks (Oct 28, 2026).
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.
CVSS 5.3 (version 3.1; evidence as of Oct 28, 2026; https://github.com/advisories/GHSA-r4j5-9xpp-3985).
EPSS 0.4% for the following 30 days, scored Oct 28, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-115927&date=2026-10-28).
Sources:
GitHub-reviewed advisory GHSA-r4j5-9xpp-3985: https://github.com/advisories/GHSA-r4j5-9xpp-3985
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-115927&date=2026-10-28
Record ID: CVE-2026-115927.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 29, 2026, 02:16 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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