CVE-2026-28356 — multipart
HIGHCVE-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()`
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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-28356 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
multipart🐍multipartReal-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | multipart | ≥ 1.3.0&&< 1.3.1 | 1.3.1pip install --upgrade 'multipart==1.3.1' |
| 🐍PyPI | multipart | all versions | 1.2.2pip install --upgrade 'multipart==1.2.2' |
Detection & mitigation playbook
Open-source dependencyDetect
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.
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.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-28356 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-28356. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
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…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat AI Inference Server 3.2 | rhaiis/vllm-cuda-rhel9:1774351144 | RHSA-2026:5809 |
| Red Hat AI Inference Server 3.2 | rhaiis/model-opt-cuda-rhel9:1774547384 | RHSA-2026:6761 |
| Red Hat AI Inference Server 3.2 | rhaiis/vllm-rocm-rhel9:1775252598 | RHSA-2026:6762 |
| Red Hat OpenShift AI 2.25 | rhoai/odh-caikit-tgis-serving-rhel9:1776247907 | RHSA-2026:10184 |
| Red Hat OpenShift AI 2.25 | rhoai/odh-vllm-cpu-rhel9:1782397826 | RHSA-2026:42644 |
| Red Hat OpenShift AI 3.3 | rhoai/odh-vllm-cpu-rhel9:1782996080 | RHSA-2026:37275 |
Frequently Asked Questions
Is CVE-2026-28356 in your dependencies?
O3 Security finds CVE-2026-28356 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.