Instituto Nacional de ciberseguridad. Sección Incibe
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CVE-2026-53538

Severity CVSS v4.0:
Pending analysis
Type:
Unavailable / Other
Publication date:
22/06/2026
Last modified:
26/06/2026

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.

Impact

Vector 3.x
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N CVSS v3.1 Severity and Metrics:

Base Score: 3.70 LOW
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Attack Vector (AV): Network
Attack Complexity (AC): High
Privileges Required (PR): None
User Interaction (UI): None
Scope (S): Unchanged
Confidentiality (C): None
Integrity (I): Low
Availability (A): None

Base Score 3.x
3.70
Severity 3.x
LOW

Vulnerable products and versions

CPE From Up to
cpe:2.3:a:fastapiexpert:python-multipart:*:*:*:*:*:python:*:* 0.0.30 (excluding)

To consult the complete list of CPE names with products and versions, see this page


References to Advisories, Solutions, and Tools