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MEDIUM severity

CVE-2026-40347 — python-multipart

MEDIUM

CVE-2026-40347 is a medium-severity (CVSS 5.3) Uncontrolled Resource Consumption vulnerability in python-multipart. A fix is available for python-multipart — see the affected versions and patch details below.

Python-Multipart affected by Denial of Service via large multipart preamble or epilogue data

Also known asGHSA-mj87-hwqh-73pjPYSEC-2026-3038
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 10, 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-2026-40347.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs29th percentile — riskier than 29% of all scored CVEsHighest risk
0.00%0.31%0.61%0.92%0.0%0.4%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

1 pkg affected
🐍python-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

A denial of service vulnerability exists when parsing crafted multipart/form-data requests with large preamble or epilogue sections.

Details

Two inefficient multipart parsing paths could be abused with attacker-controlled input.

Before the first multipart boundary, the parser handled leading CR and LF bytes inefficiently while searching for the start of the first part. After the closing boundary, the parser continued processing trailing epilogue data instead of discarding it immediately. As a result, parsing time could grow with the size of crafted data placed before the first boundary or after the closing boundary.

Impact

An attacker can send oversized malformed multipart bodies that consume excessive CPU time during request parsing, reducing request-handling capacity and delaying legitimate requests. This issue degrades availability but does not typically result in a complete denial of service for the entire application.

Mitigation

Upgrade to version 0.0.26 or later, which skips ahead to the next boundary candidate when processing leading CR/LF data and immediately discards epilogue data after the closing boundary.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIpython-multipartall versions0.0.26pip install --upgrade 'python-multipart==0.0.26'

Affected Products

1 product · 1 configurations
Application
python-multipartfastapiexpert
< 0.0.26
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

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

  2. Fix

    Update python-multipart to 0.0.26 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40347 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 HatModerate
Workaround published by Red Hat
To mitigate this issue, restrict network access to services utilizing `python-multipart` to trusted clients only. Implement rate limiting and robust input validation at the application or network level to filter out malformed `multipart/form-data` requests before they reach the vulnerable component. This can help prevent the service from becoming unresponsive due to specially crafted requests.
Source: Red Hat security advisory for CVE-2026-40347 (CC BY 4.0)

Frequently Asked Questions

### Summary A denial of service vulnerability exists when parsing crafted `multipart/form-data` requests with large preamble or epilogue sections. ### Details Two inefficient multipart parsing paths could be abused with attacker-controlled input. Before the first multipart boundary, the parser handled leading CR and LF bytes inefficiently while searching for the start of the first part. After the closing boundary, the parser continued processing trailing epilogue data instead of discarding it immediately. As a result, parsing time could grow with the size of crafted data placed before the
O3 Security · Impact-Aware SCA

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CVE-2026-40347: python-multipart DoS — Fixed in 0.0.26