CVE-2026-53539

Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. This vulnerability is fixed in 0.0.30.

CVSS v3 7.5 HIGH

7.5^/10

CVSS v3 : HIGH

Vector :

Exploitability : 3.9 / Impact : 3.6

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact NONE

Integrity Impact NONE

Availability Impact HIGH

Scope UNCHANGED

References

Link	Resource
https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf	Vendor Advisory

Configurations

Configuration 1 (hide)

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

History

No history.

Information

Published : 2026-06-22 18:16

Updated : 2026-06-26 19:50

NVD link : CVE-2026-53539

Mitre link : CVE-2026-53539

CVE.ORG link : CVE-2026-53539

JSON object : View

Products Affected

fastapiexpert

python-multipart

CWE

CWE-400

Uncontrolled Resource Consumption

CWE-407

Inefficient Algorithmic Complexity

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