Vulnerability Report

CVE-2026-53538

Title: Python-Multipart Parser Differential/Request Smuggling

Request Smuggling

Proof Of Concept

No public PoC currently indexed for CVE-2026-53538.

CWE Category CWE-436
Published Date Jun 22, 2026
Modified Date Jun 26, 2026
Exploit Status Not Found
Score 3.7 CVSS v3.1
Exploit Probability (EPSS)
0.18%

Vulnerability Summary

CVE-2026-53538: 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.

Impacted Vendors

F
fastapiexpert 1

Reference Links

https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m
CVSS v3.1
Source Entity [email protected]
Severity LOW
3.7
Attack Vector
NETWORK
Complexity
HIGH
Privileges
N/A
Interaction
NONE
Confidentiality
N/A
Integrity
N/A
Availability
N/A
Scope
UNCHANGED
RAW VECTOR CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Associated Attack Patterns (CAPEC)

Total: Patterns

CVE-2026-53538 Exploits & PoCs (Proof Of Concept)

No public PoCs found in our database for this CVE.

MODIFIED

Vulnerability data updated via NVD.

MODIFIED

Vulnerability data updated via NVD.

MODIFIED

Vulnerability data updated via NVD.

MODIFIED

Vulnerability data updated via NVD.

Attack Vector Matrix

Access Vector NETWORK
Complexity HIGH
Privileges N/A
Interaction NONE
CVSS Vector String CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Affected Stack

No specific products linked.