CVE-2026-53539: high-severity vulnerability in Kludex python-multipart
Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service
Published · Updated
E-mail alert when it lands in KEV, gets a public PoC or its score goes up.
21Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 7.5epss 0.5%
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.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Affected products
Kludex · python-multipartRelated CVEs — Kludex python-multipart
In the same product, most dangerous first.
CVE-2026-24486HIGHPython-Multipart has Arbitrary File Write via Non-Default ConfigurationEPSS 2.2%CVE-2024-24762HIGHpython-multipart vulnerable to content-type header Regular expression Denial of ServiceEPSS 1.5%CVE-2026-42561HIGHPython-Multipart: Denial of Service via unbounded multipart part headersEPSS 0.7%CVE-2024-53981HIGHpython-multipart has a Denial of service (DoS) via deformation `multipart/form-data` boundaryEPSS 0.6%CVE-2026-40347MEDIUMPython-Multipart affected by Denial of Service via large multipart preamble or epilogue dataEPSS 0.4%CVE-2026-53540LOWPython-Multipart: Negative Content-Length in parse_form buffers the entire body in memoryEPSS 0.3%