CVE-2026-53538: CWE-436: Interpretation Conflict in Kludex python-multipart
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.
AI Analysis
Technical Summary
The python-multipart library's QuerystringParser prior to version 0.0.30 incorrectly treated the semicolon character as a field separator in application/x-www-form-urlencoded bodies, diverging from the WHATWG URL standard and modern browser behavior, which recognize only the ampersand as a separator. This interpretation conflict creates a parser differential that can be exploited to smuggle additional form fields past upstream body inspection components. The issue is addressed and fixed in version 0.0.30 of python-multipart.
Potential Impact
This vulnerability allows an attacker to bypass upstream body inspection by injecting extra form fields that are not recognized by intermediaries following the WHATWG URL standard. While it does not directly compromise confidentiality or availability, it can lead to integrity issues by enabling unauthorized data injection into form submissions.
Mitigation Recommendations
Upgrade python-multipart to version 0.0.30 or later, where this vulnerability is fixed. No other mitigation is required as the fix is available and addresses the issue directly.
CVE-2026-53538: CWE-436: Interpretation Conflict in Kludex python-multipart
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.
CVSS v3.1
Score 3.7low
Affected software
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AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The python-multipart library's QuerystringParser prior to version 0.0.30 incorrectly treated the semicolon character as a field separator in application/x-www-form-urlencoded bodies, diverging from the WHATWG URL standard and modern browser behavior, which recognize only the ampersand as a separator. This interpretation conflict creates a parser differential that can be exploited to smuggle additional form fields past upstream body inspection components. The issue is addressed and fixed in version 0.0.30 of python-multipart.
Potential Impact
This vulnerability allows an attacker to bypass upstream body inspection by injecting extra form fields that are not recognized by intermediaries following the WHATWG URL standard. While it does not directly compromise confidentiality or availability, it can lead to integrity issues by enabling unauthorized data injection into form submissions.
Mitigation Recommendations
Upgrade python-multipart to version 0.0.30 or later, where this vulnerability is fixed. No other mitigation is required as the fix is available and addresses the issue directly.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-06-09T18:13:07.263Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a39735aeed863c81e396230
Added to database: 06/22/2026, 17:39:38 UTC
Last enriched: 06/29/2026, 21:51:01 UTC
Last updated: 08/07/2026, 00:41:15 UTC
Views: 104
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