CVE-2026-53538 Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling
Attacker can smuggle form fields by exploiting parser differences in Python-Multipart, bypassing upstream security checks.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-53538?
CVE-2026-53538 is a low-severity 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, QuerystringParser treated ; as a field separator in application/x-www-form-urlencoded bodies, in addition to &. The WHATWG URL standard, modern browsers, and Python's urllib.parse (since the CVE-2021-23336 fix) treat only & as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component. This vulnerability is fixed in 0.0.30.
Which versions are affected by CVE-2026-53538?
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-2026-53538?
Upgrade python-multipart to 0.0.30 or later.
Is CVE-2026-53538 being exploited?
There is no known exploitation of CVE-2026-53538 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-2026-53539 Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service
- 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
Sources
Also known as CLEANSTART-2026-BI20445, CLEANSTART-2026-KU79465, DEBIAN-CVE-2026-53538, ECHO-1ad6-2748-b07b, GHSA-6jv3-5f52-599m, MINI-3x2w-39vg-5rp8 and 18 more.