CVE-2026-53539 | Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service

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.

Published: 2026-06-22 Last update: 2026-06-26 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-53539 is rated Low Risk (36.1/100): CVSS High severity, with low exploitation likelihood (EPSS 0.26%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-53539

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-23 — 0.26% —

Full EPSS history (1 record total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-53539

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2026-53539

GitHub Security Advisory for CVE-2026-53539

GHSA-5rvq-cxj2-64vf · Severity: high · Ecosystem: pip — python-multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service

OS Trackers for CVE-2026-53539

vendor priority summary link
debian not yet assigned CVE-2026-53539 not yet assigned priority: Debian including 1 source packages (python-multipart), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): open 5. https://security-tracker.debian.org/tracker/CVE-2026-53539
suse high — https://www.suse.com/security/cve/CVE-2026-53539/
ubuntu medium CVE-2026-53539 medium priority: Ubuntu including 1 source packages (python-multipart), 5 status rows across 5 suites (jammy, noble, questing, resolute, upstream): needs-triage 5. https://ubuntu.com/security/CVE-2026-53539

Affected software / configurations for CVE-2026-53539

Vendor Product Version Raw CPE
fastapiexpert python-multipart < 0.0.30 cpe:2.3:a:fastapiexpert:python-multipart:*:*:*:*:*:python:*:*

References for CVE-2026-53539

cvelogic Threat Intelligence