CVE-2026-53539 Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service
An attacker can cause CPU exhaustion and denial of service by submitting crafted querystring bodies with semicolon separators to applications using Python-Multipart.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-53539?
CVE-2026-53539 is a high-severity denial of service vulnerability in Kludex python-multipart, published on 22 Jun 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 0.0.30, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. This vulnerability is fixed in 0.0.30.
Which versions are affected by CVE-2026-53539?
3 open source packages ship an affected version, including python-multipart. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
python-multipart PyPI | All versions | 0.0.30 |
python-multipart PyPI | All versions | 0.0.30 |
python-multipart PyPI | All versions | 0.0.30 |
Products include Fastapiexpert Python-Multipart.
How do I fix CVE-2026-53539?
Upgrade python-multipart to 0.0.30 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-2026-53539 being exploited?
There is no known exploitation of CVE-2026-53539 in the wild and no public exploit code.
Other vulnerabilities in Kludex python-multipart
- CVE-2026-24486 Python-Multipart has Arbitrary File Write via Non-Default Configuration
- CVE-2024-24762 python-multipart vulnerable to content-type header Regular expression Denial of Service
- CVE-2026-40347 Python-Multipart affected by Denial of Service via large multipart preamble or epilogue data
- CVE-2026-53537 Python-Multipart: Content-Disposition parameter smuggling via RFC 2231/5987 extended parameters
- CVE-2026-53538 Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling
Sources
Also known as CLEANSTART-2026-EB34508, CLEANSTART-2026-SR16406, DEBIAN-CVE-2026-53539, ECHO-1942-6e43-27d8, GHSA-5rvq-cxj2-64vf, MINI-2r4m-69f3-x4pm and 18 more.