CVE-2025-67725Network attack vector

Service interruption via quadratic repeated-header coalescing

Published Dec 12, 2025 · Updated Dec 12, 2025

Uncontrolled resource consumption in tornadoweb Tornado before 6.5.3 allows remote attackers to block the event loop via repeated HTTP headers. HTTPHeaders.add repeatedly concatenates values for the same header name, so immutable Python strings force full copies and quadratic processing time. One unauthenticated request can stall the single event loop; the default 64 KB header limit bounds the effect, while larger max_header_size settings increase it.

CVSS severity7.5
High
EPSS probability0.56%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Uncontrolled resource consumption in tornadoweb Tornado before 6.5.3 allows remote attackers to block the event loop via repeated HTTP headers. HTTPHeaders.add repeatedly concatenates values for the same header name, so immutable Python strings force full copies and quadratic processing time. One unauthenticated request can stall the single event loop; the default 64 KB header limit bounds the effect, while larger max_header_size settings increase it.

The record

CVE
CVE-2025-67725
Published
Dec 12, 2025
Updated
Dec 12, 2025
Vendor
tornadoweb
Product
Tornado
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 12, 2025CVE publishedPublication date reported by the CVE source.
  2. Dec 12, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 6.5.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

tornadoweb · Tornadoversion=< 6.5.3

Published CVSS scores

7.5GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.004Endpoint Denial of Service: Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo