{"id":"CVE-2026-53538","summary":"Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling","details":"Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, 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 (since the CVE-2021-23336 fix) treat only & as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component. This vulnerability is fixed in 0.0.30.","aliases":["GHSA-6jv3-5f52-599m"],"modified":"2026-07-08T08:11:31.034434351Z","published":"2026-06-22T16:56:32.628Z","related":["CGA-844f-mf5p-f2jr","SUSE-SU-2026:22372-1","openSUSE-SU-2026:11099-1","openSUSE-SU-2026:21066-1"],"database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53538.json","cwe_ids":["CWE-436","CWE-444"],"cna_assigner":"GitHub_M"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53538.json"},{"type":"ADVISORY","url":"https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-53538"}],"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-2026-53538.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N"}]}