Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path

{
  "id": "83ae23bb6e06cf7162040de89a77c4a0fa7cc21695975103de23e3a095c5dc36",
  "url": "https://osv.dev/vulnerability/CVE-2026-84971",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Persistent client crash loop via undersized FLE2 insert-update ciphertext in decryption path",
    "summary": "Improper handling of an unexpected value size in the decryption path of a client-side encryption library can cause a failed internal check that terminates the process using the library. A party able to place a suitably formed encrypted value where an application will decrypt it, or able to control the responses the application receives, may cause that application to stop running.",
    "evidence": "osv:CVE-2026-84971",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-84971",
      "aliases": [
        "CVE-2026-84971"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/mongodb/libmongocrypt",
              "type": "GIT",
              "events": [
                {
                  "introduced": "76a17191ae96c24c1b984a763f4a2b8fe5935ce8"
                },
                {
                  "fixed": "ab45a792fd995db28b6fde3e730edf6a7a90fbf4"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:mongodb:libmongocrypt:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "1.7.0"
                      },
                      {
                        "fixed": "1.20.4"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "1.20.3",
            "1.20.2",
            "1.20.1",
            "1.20.0",
            "pymongocrypt-1.18.1",
            "pymongocrypt-1.18.0",
            "pymongocrypt-1.18.0.dev0",
            "pymongocrypt-1.17.0",
            "pymongocrypt-1.16.0",
            "pymongocrypt-1.15.1",
            "pymongocrypt-1.15.0",
            "pymongocrypt-1.14.1",
            "pymongocrypt-1.14.0",
            "pymongocrypt-1.13.0",
            "pymongocrypt-1.12.2",
            "pymongocrypt-1.12.2rc4",
            "pymongocrypt-1.12.2rc2",
            "pymongocrypt-1.12.2rc1",
            "pymongocrypt-1.2.2rc0",
            "pymongocrypt-1.12.1",
            "pymongocrypt-1.12.0",
            "pymongocrypt-1.11.0",
            "java-1.11.0",
            "csharp-v1.11.0",
            "pymongocrypt-1.10.1",
            "csharp-v1.10.0",
            "pymongocrypt-1.10.0",
            "1.10.0b1",
            "1.10.0b0",
            "pymongocrypt-1.9.1",
            "node-v6.0.1",
            "csharp-v1.9.0",
            "pymongocrypt-1.9.0",
            "csharp-v1.8.2",
            "csharp-v1.8.1",
            "pymongocrypt-1.8.0",
            "node-v6.0.0",
            "node-v6.0.0-alpha.3",
            "node-v6.0.0-alpha.2",
            "pymongocrypt-1.7.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-84971.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:25.383354615Z",
      "provider": "osv",
      "references": [
        "https://jira.mongodb.org/browse/MONGOCRYPT-971",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84971.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-84971"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-617"
        ],
        "cna_assigner": "mongodb",
        "unresolved_ranges": [
          {
            "extracted_events": [
              {
                "range": [
                  {
                    "introduced": "1.7.0"
                  },
                  {
                    "fixed": "1.20.4"
                  }
                ],
                "source": "AFFECTED_FIELD",
                "strategy": "StandardRange"
              }
            ]
          }
        ],
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84971.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-84971",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 492118,
  "published_at": "2026-09-03T14:57:35.521+00:00"
}