Unbounded post ID array in the bulk reactions endpoint allows denial of service

{
  "id": "7cce6e51d846201771cf20056c30bdf7e059f83686ee5f34af3819d3ddf29812",
  "url": "https://osv.dev/vulnerability/CVE-2026-95666",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Unbounded post ID array in the bulk reactions endpoint allows denial of service",
    "summary": "Mattermost versions 11.9.x <= 11.9.1, 11.8.x <= 11.8.5, 11.7.x <= 11.7.10, 11.10.x <= 11.10.1 fail to limit the length of the post ID array accepted by the bulk reactions endpoint which allows an authenticated user to cause excessive database load via a crafted request to {{POST /api/v4/posts/ids/reactions}}.. Mattermost Advisory ID: MMSA-2026-00771",
    "evidence": "osv:CVE-2026-95666",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-95666",
      "aliases": [
        "CVE-2026-95666"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/mattermost/mattermost",
              "type": "GIT",
              "events": [
                {
                  "introduced": "a9e574a82633915f22071f0d7ca2b006f249ec2a"
                },
                {
                  "fixed": "062eb3ba0e0f265bd0bc78d8c5ce5df06b7b6bc8"
                },
                {
                  "introduced": "583461af10fc9b2aef2871c722fc4376db695832"
                },
                {
                  "fixed": "8ed7691c22159afec427468b8a866aeeaf59a12c"
                },
                {
                  "introduced": "7f5bffb8cf3977db8237ce10158fa22aa0d2504f"
                },
                {
                  "fixed": "3a01fa64e5c292bd2503de37160cae10e10096de"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "11.7.0"
                      },
                      {
                        "fixed": "11.7.11"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "11.9.0"
                      },
                      {
                        "fixed": "11.9.2"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "11.10.0"
                      },
                      {
                        "fixed": "11.10.2"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v11.7.10",
            "v11.10.1",
            "v11.7.9",
            "v11.9.1-rc1",
            "v11.9.1",
            "v11.10.0-rc3",
            "v11.10.0",
            "v11.7.8",
            "v11.7.7",
            "v11.9.0",
            "mattermost-redux@11.9.0",
            "@mattermost/types@11.9.0",
            "@mattermost/shared@11.9.0",
            "@mattermost/client@11.9.0",
            "v11.7.6",
            "v11.7.5-rc1",
            "v11.7.5",
            "v11.7.4",
            "v11.7.3",
            "v11.7.2",
            "v11.7.1",
            "v11.7.0",
            "mattermost-redux@11.7.0",
            "@mattermost/types@11.7.0",
            "@mattermost/shared@11.7.0",
            "@mattermost/client@11.7.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-95666.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:30.830734847Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/95xxx/CVE-2026-95666.json",
        "https://mattermost.com/security-updates",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-95666"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-770"
        ],
        "cna_assigner": "Mattermost",
        "unresolved_ranges": [
          {
            "extracted_events": [
              {
                "range": [
                  {
                    "introduced": "11.9.0"
                  },
                  {
                    "last_affected": "11.9.1"
                  },
                  {
                    "introduced": "11.8.0"
                  },
                  {
                    "last_affected": "11.8.5"
                  },
                  {
                    "introduced": "11.7.0"
                  },
                  {
                    "last_affected": "11.7.10"
                  },
                  {
                    "introduced": "11.10.0"
                  },
                  {
                    "last_affected": "11.10.1"
                  }
                ],
                "source": "AFFECTED_FIELD",
                "strategy": "StandardRange"
              }
            ]
          }
        ],
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/95xxx/CVE-2026-95666.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-95666",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 497854,
  "published_at": "2026-09-22T13:11:13.798+00:00"
}