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

26 Jun 2026, 19:50

Type	Values Removed	Values Added
CPE		cpe:2.3:a:fastapiexpert:python-multipart::::::python::*
References	~~() https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf -~~	() https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf - Vendor Advisory
First Time		Fastapiexpert python-multipart Fastapiexpert

22 Jun 2026, 18:16

Type	Values Removed	Values Added
New CVE

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

{"id": "CVE-2026-53539", "cveTags": [], "metrics": {"ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2026-53539", "role": "CISA Coordinator", "options": [{"exploitation": "none"}, {"automatable": "yes"}, {"technicalImpact": "partial"}], "version": "2.0.3", "timestamp": "2026-06-23T14:05:50.901666Z"}}], "cvssMetricV31": [{"type": "Secondary", "source": "security-advisories@github.com", "cvssData": {"scope": "UNCHANGED", "version": "3.1", "baseScore": 7.5, "attackVector": "NETWORK", "baseSeverity": "HIGH", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H", "integrityImpact": "NONE", "userInteraction": "NONE", "attackComplexity": "LOW", "availabilityImpact": "HIGH", "privilegesRequired": "NONE", "confidentialityImpact": "NONE"}, "impactScore": 3.6, "exploitabilityScore": 3.9}]}, "affected": [{"source": "security-advisories@github.com", "affectedData": [{"vendor": "Kludex", "product": "python-multipart", "versions": [{"status": "affected", "version": "< 0.0.30"}]}]}], "published": "2026-06-22T18:16:44.360", "references": [{"url": "https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf", "tags": ["Vendor Advisory"], "source": "security-advisories@github.com"}], "vulnStatus": "Analyzed", "weaknesses": [{"type": "Secondary", "source": "security-advisories@github.com", "description": [{"lang": "en", "value": "CWE-400"}, {"lang": "en", "value": "CWE-407"}]}], "descriptions": [{"lang": "en", "value": "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."}], "lastModified": "2026-06-26T19:50:56.510", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:fastapiexpert:python-multipart:*:*:*:*:*:python:*:*", "vulnerable": true, "matchCriteriaId": "381A8F1D-278D-409F-974A-898BE65DB20C", "versionEndExcluding": "0.0.30"}], "operator": "OR"}]}], "sourceIdentifier": "security-advisories@github.com"}