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Not in CISA KEV
HIGH severity

CVE-2026-28356 — multipart

HIGH

CVE-2026-28356 is a high-severity (CVSS 7.5) CWE-1333 vulnerability in multipart. A fix is available for multipart — see the affected versions and patch details below.

ReDoS in multipart 1.3.0 - `parse_options_header()`

Also known asGHSA-p2m9-wcp5-6qw3PYSEC-2026-2670
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Nov 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-28356.

EPSS Exploitation Probability

via FIRST.org ↗
0.8%probability of exploitation in next 30 days
Lower Risk+0.08%
Lower risk than most CVEs58th percentile — riskier than 58% of all scored CVEsHighest risk
0.11%0.58%1.05%1.53%0.8%0.8%May 26Sep 26Nov 26

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

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected
🐍multipart🐍multipart

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

Summary

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.

Impact

Any WSGI or ASGI application using multipart.parse_form_data() directly or indirectly (e.g. while parsing multipart/form-data streams) is vulnerable. The slow-down is significant enough to block request handling threads for multiple seconds per request.

Affected versions

All versions up to and including 1.3.0 are affected. The issue is fixed in 1.2.2, 1.3.1 and 1.4.0-dev.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPImultipart≥ 1.3.0&&< 1.3.11.3.1pip install --upgrade 'multipart==1.3.1'
🐍PyPImultipartall versions1.2.2pip install --upgrade 'multipart==1.2.2'

Detection & mitigation playbook

Open-source dependency

  1. Detect

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

  2. Fix

    Update multipart to 1.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-28356 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 issue is only exploitable in applications using the multipart.parse_form_data function directly or indirectly when parsing request headers or multipart/form-data streams. Also, the security impact of this flaw is limited to a denial of service, there is no corruption of memory or arbitrary code execution. Due to…

Workaround published by Red Hat
Red Hat has investigated whether a possible mitigation exists for this issue, and has not been able to identify a practical example. Please update the affected package as soon as possible.
Source: Red Hat security advisory for CVE-2026-28356 (CC BY 4.0)
ProductFixed inAdvisory
Red Hat AI Inference Server 3.2rhaiis/vllm-cuda-rhel9:1774351144RHSA-2026:5809
Red Hat AI Inference Server 3.2rhaiis/model-opt-cuda-rhel9:1774547384RHSA-2026:6761
Red Hat AI Inference Server 3.2rhaiis/vllm-rocm-rhel9:1775252598RHSA-2026:6762
Red Hat OpenShift AI 2.25rhoai/odh-caikit-tgis-serving-rhel9:1776247907RHSA-2026:10184
Red Hat OpenShift AI 2.25rhoai/odh-vllm-cpu-rhel9:1782397826RHSA-2026:42644
Red Hat OpenShift AI 3.3rhoai/odh-vllm-cpu-rhel9:1782996080RHSA-2026:37275
Red Hat OpenShift AI 3.3rhoai/odh-vllm-cpu-rhel9:1790274961RHSA-2026:73987

Frequently Asked Questions

## Summary 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. ## Impact Any WSGI or ASGI application using `multipart.parse_form_data()` directly or indirectly (e.g. while parsing `multipart/form-data` streams) is vulnerable. The slow-down is significa
O3 Security · Impact-Aware SCA

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CVE-2026-28356: multipart DoS — Fixed in 1.3.1