GHSA-8R89-8W26-CQ32

Vulnerability from github – Published: 2026-05-18 09:31 – Updated: 2026-06-01 15:27
VLAI
Summary
Mattermost doesn't verify channel membership when processing AI-assisted message rewrites
Details

Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645

Severity
6.5 (Medium)
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
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{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/mattermost/mattermost/server/v8"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "11.5.0"
            },
            {
              "fixed": "11.5.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/mattermost/mattermost/server/v8"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "8.0.0-20260401090745-f4d1abe7e8f5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/mattermost/mattermost-server"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "5.3.2-0.20260401090745-f4d1abe7e8f5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-5163"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-862"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-01T15:27:00Z",
    "nvd_published_at": "2026-05-18T09:16:23Z",
    "severity": "MODERATE"
  },
  "details": "Mattermost versions 11.5.x \u003c= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645",
  "id": "GHSA-8r89-8w26-cq32",
  "modified": "2026-06-01T15:27:00Z",
  "published": "2026-05-18T09:31:48Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-5163"
    },
    {
      "type": "WEB",
      "url": "https://github.com/mattermost/mattermost/commit/f4d1abe7e8f545f1a87f463fa9fe451c731aebf8"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/mattermost/mattermost"
    },
    {
      "type": "WEB",
      "url": "https://mattermost.com/security-updates"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Mattermost doesn\u0027t verify channel membership when processing AI-assisted message rewrites"
}


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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

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Nomenclature

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  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.