CVE-2026-31958: Tornado Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-qjxf-f2mg-c6mc
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CVSS 7.5
CWE-400
Published
Advisory Summary

CVE-2026-31958 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 tornado is vulnerable to dos due to too many multipart parts. Remediation is available by upgrading affected packages to version 6.5.5.

CVE-2026-31958 at a Glance

CVE Identifier CVE-2026-31958
GHSA Identifier GHSA-qjxf-f2mg-c6mc
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-2026-31958

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: 6.5.5 
pip install tornado>=6.5.5
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-2026-31958, 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 6.5.5), 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

In versions of Tornado prior to 6.5.5, the only limit on the number of parts in `multipart/form-data` is the `max_body_size` setting (default 100MB). Since parsing occurs synchronously on the main thread, this creates the possibility of denial-of-service due to the cost of parsing very large multipart bodies with many parts. Tornado 6.5.5 introduces new limits on the size and complexity of multipart bodies, including a default limit of 100 parts per request. These limits are configurable if needed; see `tornado.httputil.ParseMultipartConfig`. It is also now possible to disable `multipart/form-data` parsing entirely if it is not required for the application.

CVE-2026-31958 Affected Tornado Versions

1 component(s)
Fixed Versions:
6.5.5
Product / Package Ecosystem Affected Versions Fixed Version
tornado
pypi/tornado
PyPI
< 6.5.5
6.5.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/tornadoweb/tornado/security/advisories/GHSA-qjxf-f2mg-c6mc
NVD — CVE-2026-31958 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-31958
Security Fix Commit FIX COMMIT
https://github.com/tornadoweb/tornado/commit/119a195e290c43ad2d63a2cf012c29d43d6ed839
Tornado Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2026-140.yaml
Tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado
Tornado Release RELEASE
https://github.com/tornadoweb/tornado/releases/tag/v6.5.5
lists.debian.org/debian-lts-announce... WEB
https://lists.debian.org/debian-lts-announce/2026/04/msg00000.html

Frequently Asked Questions

What is CVE-2026-31958?
CVE-2026-31958 is a security vulnerability in Tornado involving Tornado is vulnerable to DoS due to too many multipart parts under CWE-400: Uncontrolled Resource Consumption.
Which Tornado versions are affected?
Affected versions include releases prior to 6.5.5 across affected packages: tornado.
What version fixes CVE-2026-31958?
Upgrading to 6.5.5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is tornado affected?
Yes, tornado is affected in releases prior to 6.5.5. Upgrade your dependency to 6.5.5 or later.
What is the CVSS score of CVE-2026-31958?
CVE-2026-31958 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-2026-31958?
CVE-2026-31958 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-2026-31958?
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-qjxf-f2mg-c6mc",
  "modified" : "2026-06-08T19:51:12Z",
  "published" : "2026-03-12T14:19:52Z",
  "aliases" : [ "CVE-2026-31958" ],
  "summary" : "Tornado is vulnerable to DoS due to too many multipart parts",
  "details" : "In versions of Tornado prior to 6.5.5, the only limit on the number of parts in `multipart/form-data` is the `max_body_size` setting (default 100MB). Since parsing occurs synchronously on the main thread, this creates the possibility of denial-of-service due to the cost of parsing very large multipart bodies with many parts. \n\nTornado 6.5.5 introduces new limits on the size and complexity of multipart bodies, including a default limit of 100 parts per request. These limits are configurable if needed; see `tornado.httputil.ParseMultipartConfig`. It is also now possible to disable `multipart/form-data` parsing entirely if it is not required for the application.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
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      "name" : "tornado"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.5.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.5.4"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/security/advisories/GHSA-qjxf-f2mg-c6mc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-31958"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/commit/119a195e290c43ad2d63a2cf012c29d43d6ed839"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2026-140.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/tornadoweb/tornado"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/releases/tag/v6.5.5"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2026/04/msg00000.html"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-12T14:19:52Z",
    "nvd_published_at" : "2026-03-11T20:16:16Z"
  }
}