Missing request origin validation on burn-on-read reveal endpoint

{
  "id": "46f306cdbc530ceddfb769a925d79d858e11d6b4fd075e91dc4c697ccaee63cf",
  "url": "https://osv.dev/vulnerability/CVE-2026-6339",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Missing request origin validation on burn-on-read reveal endpoint",
    "summary": "Mattermost versions 11.5.x <= 11.5.1, 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint which allows an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag.. Mattermost Advisory ID: MMSA-2026-00636",
    "evidence": "osv:CVE-2026-6339",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-6339",
      "aliases": [
        "CVE-2026-6339",
        "GHSA-xvcx-mgpc-5xh3",
        "GO-2026-5769"
      ],
      "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": "2a1ac98bd3fae587927adf9340ba266c8c02018e"
                },
                {
                  "fixed": "37b08cfe7154241c2c6a8af0fd500e0785bbc900"
                },
                {
                  "introduced": "9f2616376582893035a7e3cb38b0fab648bba4e8"
                },
                {
                  "fixed": "cc4ef6863ee114bc96be4e34478ec2f0db3eac4e"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "11.4.0"
                      },
                      {
                        "fixed": "11.4.4"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "11.5.0"
                      },
                      {
                        "fixed": "11.5.2"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v11.5.0",
            "v11.5.2-rc2",
            "v11.5.2-rc1",
            "v11.4.3",
            "v11.5.1",
            "v11.4.2",
            "v11.4.1-rc2",
            "v11.4.1-rc1",
            "v11.4.1",
            "v11.4.0-rc3",
            "v11.4.0",
            "mattermost-redux@11.4.0",
            "@mattermost/types@11.4.0",
            "@mattermost/client@11.4.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-6339.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:25.501042056Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/6xxx/CVE-2026-6339.json",
        "https://mattermost.com/security-updates",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-6339"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-346"
        ],
        "cna_assigner": "Mattermost",
        "unresolved_ranges": [
          {
            "extracted_events": [
              {
                "range": [
                  {
                    "introduced": "11.5.0"
                  },
                  {
                    "last_affected": "11.5.1"
                  },
                  {
                    "introduced": "11.4.0"
                  },
                  {
                    "last_affected": "11.4.3"
                  }
                ],
                "source": "AFFECTED_FIELD",
                "strategy": "StandardRange"
              }
            ]
          }
        ],
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/6xxx/CVE-2026-6339.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-6339",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 492168,
  "published_at": "2026-05-18T08:05:30.925+00:00"
}