CVE-2026-42561 Python-Multipart: Denial of Service via unbounded multipart part headers
An attacker can cause denial of service in applications using Python-Multipart by sending requests with excessive or oversized multipart headers.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-42561?
CVE-2026-42561 is a high-severity denial of service vulnerability in Kludex python-multipart, published on 13 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.27, python-multipart has a denial of service vulnerability in multipart part header parsing. When parsing multipart/form-data, MultipartParser previously had no limit on the number of part headers or the size of an individual part header. An attacker could send a request with either many repeated headers without terminating the header block or a single very large header value, causing excessive CPU work before request rejection or completion. This vulnerability is fixed in 0.0.27.
Which versions are affected by CVE-2026-42561?
3 open source packages ship an affected version, including python-multipart.
| Package | Affected | Fixed in |
|---|---|---|
python-multipart PyPI | All versions | 0.0.27 |
python-multipart PyPI | All versions | 0.0.27 |
python-multipart PyPI | All versions | 0.0.27 |
Linux distribution packages (1)
python-python-multipart openSUSE:Tumbleweed | All versions | 0.0.28-1.1 |
How do I fix CVE-2026-42561?
Upgrade python-multipart to 0.0.27 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-42561 being exploited?
There is no known exploitation of CVE-2026-42561 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-SC26416, CLSA-2026-1783904617, DEBIAN-CVE-2026-42561, ECHO-6701-0c72-de60, GHSA-pp6c-gr5w-3c5g, MINI-4hf2-jvm5-g9mq and 11 more.