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.

CVSS v3 3.7 LOW

3.7^/10

CVSS v3 : LOW

Vector :

Exploitability : 2.2 / Impact : 1.4

Attack Vector NETWORK

Attack Complexity HIGH

Privileges Required NONE

User Interaction NONE

Confidentiality Impact NONE

Integrity Impact LOW

Availability Impact NONE

Scope UNCHANGED

References

Link	Resource
https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m	Vendor Advisory

Configurations

Configuration 1 (hide)

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

History

25 Oct 2026, 23:10

Type	Values Removed	Values Added
Summary		(es) Python-Multipart es un analizador multipart de streaming para Python. Antes de la versión 0.0.30, QuerystringParser trataba el ; como un separador de campos en cuerpos application/x-www-form-urlencoded, además del &. El estándar de URL de WHATWG, los navegadores modernos y urllib.parse de Python (desde la corrección de CVE-2021-23336) tratan solo el & como un separador. Esto crea un diferencial de analizador: los mismos bytes son tokenizados en campos diferentes de los que produciría un intermediario compatible con WHATWG, permitiendo a un atacante introducir de contrabando campos de formulario adicionales más allá de un componente de inspección de cuerpo ascendente. Esta vulnerabilidad está corregida en la versión 0.0.30.

Information

Published : 2026-06-22 18:16

Updated : 2026-10-25 23:10

NVD link : CVE-2026-53538

Mitre link : CVE-2026-53538

CVE.ORG link : CVE-2026-53538

JSON object : View

Products Affected

fastapiexpert

python-multipart

CWE

CWE-436

Interpretation Conflict

CWE-444

Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

{"id": "CVE-2026-53538", "cveTags": [], "metrics": {"ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"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"}}], "cvssMetricV31": [{"type": "Secondary", "source": "security-advisories@github.com", "cvssData": {"scope": "UNCHANGED", "version": "3.1", "baseScore": 3.7, "attackVector": "NETWORK", "baseSeverity": "LOW", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", "integrityImpact": "LOW", "userInteraction": "NONE", "attackComplexity": "HIGH", "availabilityImpact": "NONE", "privilegesRequired": "NONE", "confidentialityImpact": "NONE"}, "impactScore": 1.4, "exploitabilityScore": 2.2}]}, "affected": [{"source": "security-advisories@github.com", "affectedData": [{"vendor": "Kludex", "product": "python-multipart", "versions": [{"status": "affected", "version": "< 0.0.30"}]}]}], "published": "2026-06-22T18:16:44.223", "references": [{"url": "https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m", "tags": ["Vendor Advisory"], "source": "security-advisories@github.com"}], "vulnStatus": "Analyzed", "weaknesses": [{"type": "Secondary", "source": "security-advisories@github.com", "description": [{"lang": "en", "value": "CWE-436"}, {"lang": "en", "value": "CWE-444"}]}], "descriptions": [{"lang": "en", "value": "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."}, {"lang": "es", "value": "Python-Multipart es un analizador multipart de streaming para Python. Antes de la versi\u00f3n 0.0.30, QuerystringParser trataba el ; como un separador de campos en cuerpos application/x-www-form-urlencoded, adem\u00e1s del &. El est\u00e1ndar de URL de WHATWG, los navegadores modernos y urllib.parse de Python (desde la correcci\u00f3n de CVE-2021-23336) tratan solo el & como un separador. Esto crea un diferencial de analizador: los mismos bytes son tokenizados en campos diferentes de los que producir\u00eda un intermediario compatible con WHATWG, permitiendo a un atacante introducir de contrabando campos de formulario adicionales m\u00e1s all\u00e1 de un componente de inspecci\u00f3n de cuerpo ascendente. Esta vulnerabilidad est\u00e1 corregida en la versi\u00f3n 0.0.30."}], "lastModified": "2026-10-25T23:10:00.167", "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"}