CVE-2026-53539
Description
A flaw was found in Python-Multipart, a streaming multipart parser. A remote attacker can exploit this vulnerability by submitting a specially crafted application/x-www-form-urlencoded body. This can cause the QuerystringParser to perform inefficient processing, leading to excessive CPU consumption. Consequently, a small number of concurrent requests can exhaust worker processes, resulting in a Denial of Service (DoS) for the affected system.
Statement
A flaw was found in python-multipart. When parsing application/x-www-form-urlencoded bodies, QuerystringParser used a two-step separator lookup that scanned the entire remaining buffer for & before falling back to ;. For bodies using ; as the separator with no &, this yields O(B²) byte comparisons per chunk. An attacker can submit a small crafted body to cause the parser to spend seconds of CPU per request, exhausting worker processes with a handful of concurrent requests. This is the same root cause as CVE-2026-53538 (parser differential), both fixed in python-multipart 0.0.30.
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.
Deployments that enforce request body size limits, rate limiting, and load balancing make exploitation much harder.
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 | 5.9 | N/A | 7.5 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | None | N/A | None |
| Availability Impact | High | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
cve.org: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Red Hat CVSS v3 Score Explanation
Lowered AC:L→H: effective exploitation requires large request bodies; typical RH deployments enforce body size limits via ingress controllers and reverse proxies, reducing exploitability.
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (CPU)
Attackers can create crafted inputs that intentionally cause the regular expression to use excessive backtracking in a way that causes the CPU consumption to spike.
Frequently Asked Questions
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