{"id":"CVE-2026-53539","summary":"Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service","details":"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.","aliases":["GHSA-5rvq-cxj2-64vf"],"modified":"2026-07-08T08:13:44.214891132Z","published":"2026-06-22T16:55:42.787Z","related":["CGA-2fjg-j4jj-x26w","SUSE-SU-2026:22372-1","openSUSE-SU-2026:21066-1"],"database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53539.json","cna_assigner":"GitHub_M","cwe_ids":["CWE-400","CWE-407"]},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53539.json"},{"type":"ADVISORY","url":"https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-53539"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/kludex/python-multipart","events":[{"introduced":"0"},{"fixed":"9d3ead568a259f222cff6425262ff63e88d930d4"}],"database_specific":{"source":["AFFECTED_FIELD","CPE_RANGE"],"cpe":"cpe:2.3:a:fastapiexpert:python-multipart:*:*:*:*:*:python:*:*","extracted_events":[{"introduced":"0"},{"fixed":"0.0.30"}]}}],"versions":["0.0.29","0.0.28","0.0.27","0.0.26","0.0.25","0.0.24","0.0.23","0.0.22","0.0.21","0.0.20","0.0.19","0.0.18","0.0.17","0.0.16","0.0.15","0.0.14","0.0.13","0.0.12","0.0.11","0.0.10","0.0.9","0.0.8","0.0.7","0.0.6","0.0.5","0.0.4","0.0.2"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-53539.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}