CVE-2026-53538
Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling
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.
INFO
Published Date :
June 22, 2026, 6:16 p.m.
Last Modified :
June 26, 2026, 7:52 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | LOW | MITRE-CVE | ||||
| CVSS 3.1 | LOW | [email protected] |
Solution
- Update Python-Multipart to version 0.0.30.
- Ensure upstream components are WHATWG compliant.
Public PoC/Exploit Available at Github
CVE-2026-53538 has a 1 public
PoC/Exploit available at Github.
Go to the Public Exploits tab to see the list.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-53538.
| URL | Resource |
|---|---|
| https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-53538 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-53538
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
MVidarr - The Music Video Management System
HTML Python Shell JavaScript CSS TypeScript Batchfile Dockerfile Mako Go Template
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-53538 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-53538 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Jun. 26, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:fastapiexpert:python-multipart:*:*:*:*:*:python:*:* versions up to (excluding) 0.0.30 Added Reference Type GitHub, Inc.: https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m Types: Vendor Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jun. 23, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-53538', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-06-23T14:49:11.715345Z'} -
New CVE Received by [email protected]
Jun. 22, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'Kludex', 'product': 'python-multipart', 'versions': [{'status': 'affected', 'version': '< 0.0.30'}]}] Added 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. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N Added CWE CWE-436 Added CWE CWE-444 Added Reference https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m