CVE-2026-53538
Description
A flaw was found in Python-Multipart, a tool used for processing web form data. A remote attacker could exploit a vulnerability where the software incorrectly interprets certain characters as separators in web form data. This difference in interpretation compared to standard web practices allows an attacker to bypass security checks and inject additional data fields. This could lead to unauthorized information disclosure.
Statement
A flaw was found in python-multipart. The 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 only treat & as a separator. This parser differential allows an attacker to smuggle extra form fields past an upstream body-inspecting component (e.g., a WAF or API gateway), resulting in HTTP parameter pollution. This is the same class of issue as CVE-2021-23336 in CPython's urllib.parse.
Mitigation
Upgrade to python-multipart 0.0.30 or later, which treats only & as a field separator per the WHATWG URL standard. ; is parsed as ordinary field data, matching urllib.parse, browsers, and other compliant parsers.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 4.8 | N/A | 3.7 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | Low | N/A | None |
| Integrity Impact | Low | N/A | Low |
| Availability Impact | None | N/A | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
Understanding the Weakness (CWE)
Other
Technical Impact: Varies by Context
Frequently Asked Questions
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