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tornado: multipart split() creates huge temp list before max_parts check -> memory amplification DoS (httputil.py:34)GHSA-8423-8fgw-73vq

Fix: tornadoweb/tornado#3704

GHSA-8423-8fgw-73vq is a CWE-770 vulnerability in tornado. A fix is available for tornado — see the affected versions and patch details below.

Also known asCVE-2026-91990
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 15, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

  • CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for GHSA-8423-8fgw-73vq.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk-0.06%
Lower risk than most CVEs35th percentile — riskier than 35% of all scored CVEsHighest risk
0.00%0.33%0.66%0.99%0.5%0.4%Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected
🐍tornado

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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

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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPItornadoall versions6.5.8pip install --upgrade 'tornado==6.5.8'

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for tornado, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update tornado to 6.5.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-8423-8fgw-73vq is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

This is an Important severity flaw. Applications using the Tornado web framework on Red Hat platforms are vulnerable to a denial of service due to a memory amplification issue in the `parse_multipart_form_data` function. A remote, unauthenticated attacker can send a crafted multipart request, causing the server to…

Workaround published by Red Hat
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Source: Red Hat security advisory for GHSA-8423-8fgw-73vq (CC BY 4.0)
ProductFixed inAdvisory
Red Hat Enterprise Linux 10python-tornado-0:6.5.8-0.el10_2.1RHSA-2026:67147
Red Hat Enterprise Linux 10.0 Extended Update Supportpython-tornado-0:6.5.8-0.el10_0.1RHSA-2026:80741
Red Hat Enterprise Linux 9python-tornado-0:6.5.8-0.el9_8.1RHSA-2026:67146
Red Hat Ansible Automation Platform 2.7ansible-automation-platform-27/lightspeed-chatbot-rhel9:1791250620RHSA-2026:79378

Frequently Asked Questions

## 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 p
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