Medium
Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
Type: advisorySources: OSV
Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2026-28759
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 7 Nov 2026, 20:48 UTC · Revision 1
Source evidence
Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
core-osv · Original sourcePublished 18 May 2026, 06:50 UTC
Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576
osv:CVE-2026-28759
Complete source record
{
"id": "976f07d423dd3083feb01fd016766eff84e875408b74d0e950078340bba568e2",
"url": "https://osv.dev/vulnerability/CVE-2026-28759",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels",
"summary": "Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576",
"evidence": "osv:CVE-2026-28759",
"products": [],
"severity": "medium",
"structured": {
"id": "CVE-2026-28759",
"aliases": [
"CVE-2026-28759",
"GHSA-8h9w-w78c-vvr3",
"GO-2026-5256"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/mattermost/mattermost",
"type": "GIT",
"events": [
{
"introduced": "5b955468ea1ea8b56c0fb10feb258a36a767d7bb"
},
{
"fixed": "b21ef302025ee100f04f358c16301dc75fea91d9"
},
{
"introduced": "2a1ac98bd3fae587927adf9340ba266c8c02018e"
},
{
"fixed": "37b08cfe7154241c2c6a8af0fd500e0785bbc900"
},
{
"introduced": "9f2616376582893035a7e3cb38b0fab648bba4e8"
},
{
"fixed": "cc4ef6863ee114bc96be4e34478ec2f0db3eac4e"
}
],
"database_specific": {
"extracted_events": [
{
"cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "10.11.0"
},
{
"fixed": "10.11.14"
}
],
"source": "CPE_RANGE"
},
{
"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": [
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"v11.5.0",
"v11.5.2-rc2",
"v11.5.2-rc1",
"v11.4.3",
"v10.11.13",
"v11.5.1",
"v10.11.13-rc1",
"v11.4.2",
"v10.11.12",
"v11.4.1-rc2",
"v11.4.1-rc1",
"v11.4.1",
"v10.11.11-rc2",
"v10.11.11",
"v11.4.0-rc3",
"v11.4.0",
"mattermost-redux@11.4.0",
"@mattermost/types@11.4.0",
"@mattermost/client@11.4.0",
"v10.11.11-rc1",
"v10.11.10",
"v10.11.9",
"v10.11.9-rc1",
"v10.11.8",
"v10.11.7",
"v10.11.6",
"v10.11.5",
"v10.11.4-rc3",
"v10.11.4",
"v10.11.4-rc2",
"v10.11.4-rc1",
"v10.11.3",
"v10.11.2-rc2",
"v10.11.2",
"v10.11.2-rc1",
"v10.11.1",
"v10.11.1-rc1",
"v10.11.0-rc3",
"v10.11.0",
"mattermost-redux@10.11.0",
"@mattermost/types@10.11.0",
"@mattermost/client@10.11.0"
],
"database_specific": {
"source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-28759.json"
}
}
],
"modified": "2026-10-28T02:30:51.476149650Z",
"provider": "osv",
"references": [
"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/28xxx/CVE-2026-28759.json",
"https://mattermost.com/security-updates",
"https://nvd.nist.gov/vuln/detail/CVE-2026-28759"
],
"provider_metadata": {
"cwe_ids": [
"CWE-863"
],
"cna_assigner": "Mattermost",
"unresolved_ranges": [
{
"extracted_events": [
{
"range": [
{
"introduced": "11.5.0"
},
{
"last_affected": "11.5.1"
},
{
"introduced": "10.11.0"
},
{
"last_affected": "10.11.13"
},
{
"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/2026/28xxx/CVE-2026-28759.json"
}
},
"related_cves": [
{
"value": "CVE-2026-28759",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 520362,
"published_at": "2026-05-18T06:50:07.346+00:00"
}