CVE-2025-67726: Tornado Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-jhmp-mqwm-3gq8
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CVSS 7.5
CWE-400
Published
Advisory Summary

CVE-2025-67726 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Tornado libraries in the PyPI ecosystem. The issue involves quadratic dos via crafted multipart parameters. Remediation is available by upgrading affected packages to version 6.5.3.

CVE-2025-67726 at a Glance

CVE Identifier CVE-2025-67726
GHSA Identifier GHSA-jhmp-mqwm-3gq8
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Tornado
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2025-67726

To remediate this vulnerability, upgrade all affected Tornado dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

tornado fixed: 6.5.3 
pip install tornado>=6.5.3
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show tornado

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2025-67726, inspect your installed packages using pip:

pip show tornado

Analyze the terminal output to locate any version of Tornado in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 6.5.3), your application is affected.

Note: The pip show tornado command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In Tornado, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

## Summary The `_parseparam` function in Tornado's `httputil.py` is used to parse specific HTTP header values, such as those in `multipart/form-data`. This function uses an inefficient algorithm that repeatedly calls `string.count()` within a nested loop while processing quoted semicolons (e.g., `param=";"`). As a result, if an attacker sends a request with a large number of maliciously crafted parameters in a `Content-Disposition` header, the server's CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado's single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period, leading to a Denial of Service (DoS). **Severity: High**

CVE-2025-67726 Affected Tornado Versions

1 component(s)
Fixed Versions:
6.5.3
Product / Package Ecosystem Affected Versions Fixed Version
tornado
pypi/tornado
PyPI
< 6.5.3
6.5.3

References

Tornado Release RELEASE
https://github.com/tornadoweb/tornado/releases/tag/v6.5.3
GitHub Advisory / Wiki ADVISORY
https://github.com/tornadoweb/tornado/security/advisories/GHSA-jhmp-mqwm-3gq8
NVD — CVE-2025-67726 NVD
https://nvd.nist.gov/vuln/detail/CVE-2025-67726
Security Fix Commit FIX COMMIT
https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd
Tornado Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-267.yaml
Tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado

Frequently Asked Questions

What is CVE-2025-67726?
CVE-2025-67726 is a security vulnerability in Tornado involving Quadratic DoS via Crafted Multipart Parameters under CWE-400: Uncontrolled Resource Consumption.
Which Tornado versions are affected?
Affected versions include releases prior to 6.5.3 across affected packages: tornado.
What version fixes CVE-2025-67726?
Upgrading to 6.5.3 (or the latest patched release for your respective branch) resolves the vulnerability.
Is tornado affected?
Yes, tornado is affected in releases prior to 6.5.3. Upgrade your dependency to 6.5.3 or later.
What is the CVSS score of CVE-2025-67726?
CVE-2025-67726 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2025-67726?
CVE-2025-67726 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check PyPI dependencies for CVE-2025-67726?
Run the dependency tree command: 'pip show tornado'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jhmp-mqwm-3gq8",
  "modified" : "2026-07-20T18:57:54Z",
  "published" : "2026-07-20T18:57:54Z",
  "aliases" : [ "CVE-2025-67726" ],
  "summary" : "Tornado: Quadratic DoS via Crafted Multipart Parameters",
  "details" : "## Summary\n\nThe `_parseparam` function in Tornado's `httputil.py` is used to parse specific HTTP header values, such as those in `multipart/form-data`. This function uses an inefficient algorithm that repeatedly calls `string.count()` within a nested loop while processing quoted semicolons (e.g., `param=\";\"`).\n\nAs a result, if an attacker sends a request with a large number of maliciously crafted parameters in a `Content-Disposition` header, the server's CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado's single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period, leading to a Denial of Service (DoS).\n\n**Severity: High**",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "tornado"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.5.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/security/advisories/GHSA-jhmp-mqwm-3gq8"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2025-67726"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-267.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/tornadoweb/tornado"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/releases/tag/v6.5.3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-20T18:57:54Z",
    "nvd_published_at" : "2025-12-12T07:15:44Z"
  }
}