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CVE-2026-53538

3.7 low SSVC exploitation: none Analyzed
NVD · CVE.org

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.

Fix

Patch, vendor advisory and mitigation references on NVD 1

Every line above is quoted from NVD, CISA, Microsoft, the vendor's own advisory or DISA; nothing is generated.

Change history 1

  1. First seenCVSS 3.7 · fastapiexpertNVD modified 2026-10-25 23:10

Affected (CPE criteria) 1

  • cpe:2.3:a:fastapiexpert:python-multipart:*:*:*:*:*:python:*:*

References 1