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Vulnerability record · CVE-2026-53538 · published 22 June 2026

CVE-2026-53538: Fastapiexpert python-multipart interpretation conflict vulnerability

Fastapiexpert · Python Multipart

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.

3.7 CVSS 3.1 Low EPSS 0.26% · top 84.6% CWE-436 · Interpretation conflictCWE-444 · HTTP request smuggling
3.7CVSS 3.1 base score
0.26%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
1References
26 Jun 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m Vendor Advisory

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Source: NIST National Vulnerability Database (record CVE-2026-53538), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.