{
"id": "GHSA-w9m8-p4cc-4qj9",
"aliases": [
"CVE-2026-5740"
],
"details": "Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647",
"summary": "Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation",
"affected": [
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "11.6.0"
},
{
"fixed": "11.6.1"
}
]
}
],
"package": {
"name": "github.com/mattermost/mattermost-server",
"ecosystem": "Go"
},
"versions": [
"11.6.0"
]
},
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "11.5.0"
},
{
"fixed": "11.5.4"
}
]
}
],
"package": {
"name": "github.com/mattermost/mattermost-server",
"ecosystem": "Go"
}
},
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "11.4.0"
},
{
"fixed": "11.4.5"
}
]
}
],
"package": {
"name": "github.com/mattermost/mattermost-server",
"ecosystem": "Go"
}
},
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "10.11.0"
},
{
"fixed": "10.11.15"
}
]
}
],
"package": {
"name": "github.com/mattermost/mattermost-server",
"ecosystem": "Go"
}
},
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
},
{
"fixed": "8.0.0-20260410202636-17939826efa2"
}
]
}
],
"package": {
"name": "github.com/mattermost/mattermost/server/v8",
"ecosystem": "Go"
}
}
],
"modified": "2026-06-29T22:52:00Z",
"severity": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
}
],
"published": "2026-05-26T13:30:15Z",
"references": [
{
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-5740",
"type": "ADVISORY"
},
{
"url": "https://github.com/mattermost/mattermost/commit/17939826efa20a97f087b3d390ec5136df350bae",
"type": "WEB"
},
{
"url": "https://github.com/mattermost/mattermost",
"type": "PACKAGE"
},
{
"url": "https://mattermost.com/security-updates",
"type": "WEB"
}
],
"schema_version": "1.4.0",
"database_specific": {
"cwe_ids": [
"CWE-789"
],
"severity": "HIGH",
"github_reviewed": true,
"nvd_published_at": "2026-05-22T11:16:23Z",
"github_reviewed_at": "2026-06-29T22:52:00Z"
}
}