---
id: GHSA-8423-8fgw-73vq
title: >-
  tornado: multipart split() creates huge temp list before max_parts check ->
  memory amplification DoS (httputil.py:34)
summary: >-
  tornado: multipart split() creates huge temp list before max_parts check ->
  memory amplification DoS (httputil.py:34)
severity: medium
vendor: tornado
product: tornado
ecosystem: pip
affected:
  - tornado < 6.5.8
patched:
  - tornado 6.5.8
published: '2026-09-01'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:51:15.579433219Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-8423-8fgw-73vq'
references:
  - url: >-
      https://github.com/tornadoweb/tornado/security/advisories/GHSA-8423-8fgw-73vq
  - url: 'https://github.com/tornadoweb/tornado/pull/3704'
  - url: >-
      https://github.com/tornadoweb/tornado/commit/de85b3f87446e323e881bbaa3d5a74f4b76e5f05
  - url: 'https://gist.github.com/afldl/649861f25d39b53b7edbe0298e171617'
  - url: 'https://github.com/tornadoweb/tornado'
  - url: 'https://github.com/tornadoweb/tornado/releases/tag/v6.5.8'
  - url: 'https://github.com/advisories/GHSA-8423-8fgw-73vq'
tags:
  - osv
  - pip
  - ghsa
cwe:
  - CWE-770
ingestedAt: '2026-09-01T20:32:02.179Z'
---

## Overview

## 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.

## Affected packages

- `tornado < 6.5.8`

## Remediation

Upgrade to a patched release:

- `tornado 6.5.8`