CVE-2026-53538: Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, QuerystringParser treated ; as a field separator in…

PriorityP415low3.7CVSS 3.1

AVNACHPRNUINSUCNILAN

EPSS

0.18%

7.3th percentile

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

2 ranges

Vendor	Product	Version range	Fixed in
fastapiexpert	python-multipart	< 0.0.30	0.0.30
kludex	python-multipart	>= 0 < 0.0.30	0.0.30

CVSS provenance

nvdv3.13.7LOWCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

ghsa5.9MEDIUM

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