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Response snapshot Oct 29, 2026, 01:26 UTC

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VulnerabilityCVE-2026-116577

Tornado: Quadratic CPU usage when parsing HTTP headers with line folding

Tornado's HTTP header parser takes quadratic time (O(n²)) when a header value is continued across many "obsolete line folding" (obs-fold) lines. An unauthenticated attacker can send a request whose header block is mostly short continuation lines.

At a glance

Exploited
Not known
Severity
5.3 Medium (CVSS 3.1)
Chance of exploitation
Not estimated
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-pcg2-g6cj-q92c (opens a new tab)

What happened

Summary

Tornado's HTTP header parser takes quadratic time (O(n²)) when a header value is continued across many "obsolete line folding" (obs-fold) lines. An unauthenticated attacker can send a request whose header block is mostly short continuation lines. Parsing it uses a large amount of CPU on the IOLoop thread and blocks every other connection that process is serving.

This issue is closely related to CVE-2025-67725 (GHSA-c98p-7wgm-6p64). The fix for that advisory covered repeated headers but not line-folded continuations.

Impact

Continue reading

All Tornado HTTP servers are affected. Header parsing runs before any application code, so no authentication or particular handler is required.

How bad it gets depends on max_header_size:

- Default (64 KB): each malicious request blocks the event loop for tens of milliseconds. Repeated requests can degrade service. - Raised max_header_size: the cost grows with the square of the header size. A single 1 MB request can block the process for several seconds.

Only availability is affected. Confidentiality and integrity are not.

Patches

Fixed in Tornado 6.6 by removing support for obsolete line folding completely; header lines that begin with whitespace are now rejected.

Workarounds

- Keep max_header_size at its default value or lower it. This limits the impact but doesn't remove it. - Run Tornado behind a reverse proxy that rejects or normalizes obsolete line folding before forwarding requests.

Credit

Reported by @manus-pi.

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

Not available

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-pcg2-g6cj-q92c (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-pcg2-g6cj-q92c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

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: Quadratic CPU usage when parsing HTTP headers with line folding (CVE-2026-116577). https://kevwire.com/record/CVE-2026-116577

MLA

"Tornado: Quadratic CPU usage when parsing HTTP headers with line folding (CVE-2026-116577)." KEVWire, 28 Oct. 2026, kevwire.com/record/CVE-2026-116577.

Chicago

KEVWire. "Tornado: Quadratic CPU usage when parsing HTTP headers with line folding (CVE-2026-116577)." Last modified October 28, 2026. https://kevwire.com/record/CVE-2026-116577.

Evidence summary (plain text)

Tornado: Quadratic CPU usage when parsing HTTP headers with line folding (Oct 28, 2026).
Tornado's HTTP header parser takes quadratic time (O(n²)) when a header value is continued across many "obsolete line folding" (obs-fold) lines. An unauthenticated attacker can send a request whose header block is mostly short continuation lines. Parsing it uses a large amount of CPU on the IOLoop thread and blocks every other connection that process is serving.

This issue is closely related to CVE-2025-67725 (GHSA-c98p-7wgm-6p64). The fix for that advisory covered repeated headers but not line-folded continuations.

All Tornado HTTP servers are affected.
CVSS 5.3 (version 3.1; evidence as of Oct 28, 2026; https://github.com/advisories/GHSA-pcg2-g6cj-q92c).
Sources:
GitHub-reviewed advisory GHSA-pcg2-g6cj-q92c: https://github.com/advisories/GHSA-pcg2-g6cj-q92c
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-116577.
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, 00:55 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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