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

{
  "id": "72203bfdead2553b68cf386fc6c70dfd9b76b6b9d34f99d3958179550d153234",
  "url": "https://osv.dev/vulnerability/CVE-2026-53539",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service",
    "summary": "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.",
    "evidence": "osv:CVE-2026-53539",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-53539",
      "aliases": [
        "CVE-2026-53539",
        "GHSA-5rvq-cxj2-64vf",
        "PYSEC-2026-3036"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/kludex/python-multipart",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "9d3ead568a259f222cff6425262ff63e88d930d4"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "0.0.30"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "cpe": "cpe:2.3:a:fastapiexpert:python-multipart:*:*:*:*:*:python:*:*",
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "0.0.30"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "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"
          }
        }
      ],
      "modified": "2026-10-28T02:30:30.246551125Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53539.json",
        "https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-53539"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-400",
          "CWE-407"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53539.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-53539",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 497277,
  "published_at": "2026-06-22T16:55:42.787+00:00"
}