CVE-2026-53538
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.
How to fix
- fastapiexpert python-multipart0.0.30 or later
- python-multipart0.0.30 or later
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| fastapiexpert python-multipart | v < 0.0.30 | 0.0.30 |
| python-multipartPyPI | v < 0.0.30 | 0.0.30 |
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Attack Surface Management
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