Expat Denial of Service via storeAtts() Quadratic Complexity

{
  "id": "6c9aedfa2f0205a0397cb476026cc17a6d7ad50bbd552e9ba8211299537dcbfa",
  "url": "https://osv.dev/vulnerability/CVE-2026-66046",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Expat Denial of Service via storeAtts() Quadratic Complexity",
    "summary": "Expat through 2.8.3 contains a denial of service vulnerability caused by quadratic algorithmic complexity in the storeAtts() function in xmlparse.c, where processing N specified attributes with non-normalized values triggers an O(N^2) linear scan of elementType->defaultAtts to determine CDATA status. A remote unauthenticated attacker can supply a single well-formed XML document of a few megabytes to an application parsing untrusted XML to cause excessive CPU consumption, resulting in denial of service without requiring authentication, external entity resolution, or non-default parser options.",
    "evidence": "osv:CVE-2026-66046",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-66046",
      "aliases": [
        "CVE-2026-66046"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/libexpat/libexpat",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "last_affected": "92810461043fce37e70079b37ab1f04490a8f039"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "last_affected": "2.8.3"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
                  },
                  {
                    "cpe": "cpe:2.3:a:libexpat_project:libexpat:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "last_affected": "2.8.3"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "R_2_8_3",
            "R_2_8_2",
            "R_2_8_1",
            "R_2_8_0",
            "R_2_7_5",
            "R_2_7_4",
            "R_2_7_3",
            "R_2_7_2",
            "R_2_7_1",
            "R_2_7_0",
            "R_2_6_4",
            "R_2_6_3",
            "R_2_6_2",
            "R_2_6_1",
            "R_2_6_0",
            "R_2_5_0",
            "R_2_4_9",
            "R_2_4_8",
            "R_2_4_7",
            "R_2_4_6",
            "R_2_4_5",
            "R_2_4_4",
            "R_2_4_3",
            "R_2_4_2",
            "R_2_4_1",
            "R_2_4_0",
            "R_2_3_0",
            "R_2_2_10",
            "R_2_2_9",
            "R_2_2_8",
            "R_2_2_7",
            "R_2_2_6",
            "R_2_2_5",
            "R_2_2_4",
            "R_2_2_3",
            "R_2_2_2",
            "R_2_2_1",
            "R_2_2_0",
            "R_2_1_1",
            "R_2_1_0",
            "R_2_0_1",
            "R_2_0_0",
            "R_1_95_8",
            "R_1_95_7",
            "R_1_95_6",
            "R_1_95_5",
            "R_1_95_4",
            "R_1_95_3",
            "R_1_95_2",
            "R_1_95_0",
            "libexpat-alpha-1",
            "start",
            "sourceforge_init",
            "jclark-orig",
            "V20000512",
            "V19991013",
            "V19990728",
            "V19990709",
            "V19990626",
            "V1_1",
            "V19990425",
            "V1990307",
            "V19990109",
            "V19981231",
            "V19981122",
            "V1_0",
            "beta4",
            "beta3",
            "beta2",
            "REC1_0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-66046.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:25.809378862Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/66xxx/CVE-2026-66046.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-66046",
        "https://www.vulncheck.com/advisories/expat-denial-of-service-via-storeatts-quadratic-complexity",
        "https://github.com/libexpat/libexpat/pull/1321",
        "https://github.com/libexpat/libexpat"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-407"
        ],
        "cna_assigner": "VulnCheck",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/66xxx/CVE-2026-66046.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-66046",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 492549,
  "published_at": "2026-08-18T14:10:05.176+00:00"
}