GHSA-8423-8fgw-73vq: tornado Denial of Service Vulnerability

MODERATE
Share
CWE-770
Published
Advisory Summary

GHSA-8423-8fgw-73vq is a MODERATE denial of service vulnerability (CWE-770: Allocation of Resources Without Limits or Throttling) affecting tornado libraries in the PyPI ecosystem. The issue involves multipart split() creates huge temp list before max_parts check -> memory amplification dos (httputil.py:34). Remediation is available by upgrading affected packages to version 6.5.8.

GHSA-8423-8fgw-73vq at a Glance

CVE Identifier N/A
GHSA Identifier GHSA-8423-8fgw-73vq
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-770: Allocation of Resources Without Limits or Throttling
Affected Product tornado
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix GHSA-8423-8fgw-73vq

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: 6.5.8 
pip install tornado>=6.5.8
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 GHSA-8423-8fgw-73vq, 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 6.5.8), 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-770: Allocation of Resources Without Limits or Throttling, where the product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. 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

## Description ### Summary `parse_multipart_form_data` (httputil.py:34) calls `data.split(b"--"+boundary+b"\r\n")` **before** the `max_parts` check (:35). A 600KB body with 100k parts creates a 100k-element transient list first, then rejects transient memory amplification (each split element is a copy). Pre-auth HTTP DoS. ### Root cause ```python parts = data[:final_boundary_index].split(b"--" + boundary + b"\r\n") # :34 huge list first if len(parts) > config.max_parts: # :35 check after raise HTTPInputError("multipart/form-data has too many parts") ``` ### PoC gist: https://gist.github.com/afldl/649861f25d39b53b7edbe0298e171617 `poc.py` + `output.txt` (100k parts from 600KB transient list). ### Fix Count separators without materializing the list (e.g. `data.count(b"--"+boundary)` first). ### Credit Reported by afldl, 2026-07.

GHSA-8423-8fgw-73vq Affected tornado Versions

1 component(s)
Fixed Versions:
6.5.8
Product / Package Ecosystem Affected Versions Fixed Version
tornado
pypi/tornado
PyPI
< 6.5.8
6.5.8

References

GitHub Advisory / Wiki ADVISORY
https://github.com/tornadoweb/tornado/security/advisories/GHSA-8423-8fgw-73vq
tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado/pull/3704
Security Fix Commit FIX COMMIT
https://github.com/tornadoweb/tornado/commit/de85b3f87446e323e881bbaa3d5a74f4b76e5f05
tornado Source Repository REPOSITORY
https://gist.github.com/afldl/649861f25d39b53b7edbe0298e171617
tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado
tornado Release RELEASE
https://github.com/tornadoweb/tornado/releases/tag/v6.5.8

Frequently Asked Questions

What is GHSA-8423-8fgw-73vq?
GHSA-8423-8fgw-73vq is a security vulnerability in tornado involving multipart split() creates huge temp list before max_parts check -> memory amplification DoS (httputil.py:34) under CWE-770: Allocation of Resources Without Limits or Throttling.
Which tornado versions are affected?
Affected versions include releases prior to 6.5.8 across affected packages: tornado.
What version fixes GHSA-8423-8fgw-73vq?
Upgrading to 6.5.8 (or the latest patched release for your respective branch) resolves the vulnerability.
Is tornado affected?
Yes, tornado is affected in releases prior to 6.5.8. Upgrade your dependency to 6.5.8 or later.
What CWE is associated with GHSA-8423-8fgw-73vq?
GHSA-8423-8fgw-73vq is classified under CWE-770: Allocation of Resources Without Limits or Throttling (The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.).
How can I check PyPI dependencies for GHSA-8423-8fgw-73vq?
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-8423-8fgw-73vq",
  "modified" : "2026-09-01T20:17:38Z",
  "published" : "2026-09-01T20:17:38Z",
  "aliases" : [ ],
  "summary" : "tornado: multipart split() creates huge temp list before max_parts check -> memory amplification DoS (httputil.py:34)",
  "details" : "## Description\n\n### Summary\n\n`parse_multipart_form_data` (httputil.py:34) calls\n`data.split(b\"--\"+boundary+b\"\\r\\n\")` **before** the `max_parts` check (:35).\nA 600KB body with 100k parts creates a 100k-element transient list first,\nthen rejects   transient memory amplification (each split element is a copy).\nPre-auth HTTP DoS.\n\n### Root cause\n\n```python\nparts = data[:final_boundary_index].split(b\"--\" + boundary + b\"\\r\\n\")  # :34  huge list first\nif len(parts) > config.max_parts:                                       # :35  check after\n    raise HTTPInputError(\"multipart/form-data has too many parts\")\n```\n\n### PoC\n\ngist: https://gist.github.com/afldl/649861f25d39b53b7edbe0298e171617\n`poc.py` + `output.txt` (100k parts from 600KB   transient list).\n\n### Fix\n\nCount separators without materializing the list (e.g. `data.count(b\"--\"+boundary)` first).\n\n### Credit\n\nReported by afldl, 2026-07.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
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      "name" : "tornado"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
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      }, {
        "fixed" : "6.5.8"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.5.7"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/security/advisories/GHSA-8423-8fgw-73vq"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/pull/3704"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/commit/de85b3f87446e323e881bbaa3d5a74f4b76e5f05"
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    "url" : "https://gist.github.com/afldl/649861f25d39b53b7edbe0298e171617"
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    "type" : "PACKAGE",
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  }, {
    "type" : "WEB",
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  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-01T20:17:38Z",
    "nvd_published_at" : null
  }
}