What’s Vulnerable › Vulnerabilities
CVE-2026-53538: Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling
- Severity: low (CVSS 3.7 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.3% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-06-22 · updated 2026-10-25
- Weakness: CWE-436
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.
Affected products
- Kludex python-multipart: before 0.0.30 — fixed in 0.0.30
How to fix
Upgrade to a fixed version: Kludex python-multipart 0.0.30. Check the vendor advisory for mitigations.
References
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