Medium
SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
Type: advisorySources: OSV
Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisory ID: MMSA-2026-00635
- Products
- Not specified
- Affected versions
- Not specified
- Fixed versions
- Not specified
- CVEs
- CVE-2026-4339
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 3 Nov 2026, 15:27 UTC · Revision 1
Source evidence
SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
core-osv · Original sourcePublished 26 Jun 2026, 14:44 UTC
Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisory ID: MMSA-2026-00635
osv:CVE-2026-4339
Complete source record
{
"id": "c507c7190794e7d915d00ddb6e16a35c712cd7fa8aeaa2d4beee5bdef1f27f6b",
"url": "https://osv.dev/vulnerability/CVE-2026-4339",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server",
"summary": "Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisory ID: MMSA-2026-00635",
"evidence": "osv:CVE-2026-4339",
"products": [],
"severity": "medium",
"structured": {
"id": "CVE-2026-4339",
"aliases": [
"CVE-2026-4339"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N"
}
],
"affected": [
{
"ranges": [
{
"repo": "https://github.com/mattermost/mattermost",
"type": "GIT",
"events": [
{
"introduced": "5b955468ea1ea8b56c0fb10feb258a36a767d7bb"
},
{
"fixed": "4439c6121d009e1b7a16bad504e62627572cf157"
},
{
"introduced": "9f2616376582893035a7e3cb38b0fab648bba4e8"
},
{
"fixed": "7395f2ba2f17aa328fde4856101bb927aa80dc1f"
},
{
"introduced": "d4f4e78835e86e2e76f76bfe797623076f5c9835"
},
{
"fixed": "374d6843170180aea130e92a4925b26cbdafca5d"
}
],
"database_specific": {
"extracted_events": [
{
"cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "10.11.0"
},
{
"fixed": "10.11.19"
}
],
"source": "CPE_RANGE"
},
{
"cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "11.5.0"
},
{
"fixed": "11.5.7"
}
],
"source": "CPE_RANGE"
},
{
"cpe": "cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:*",
"range": [
{
"introduced": "11.6.0"
},
{
"fixed": "11.6.4"
}
],
"source": "CPE_RANGE"
}
]
}
}
],
"versions": [
"v10.11.19-rc1",
"v11.6.3",
"v10.11.18",
"v11.5.6",
"v11.6.2",
"v11.5.5",
"v10.11.17",
"v10.11.16",
"v10.11.15-rc1",
"v10.11.15",
"v11.5.4",
"v11.6.1-rc1",
"v11.6.1",
"v11.5.3",
"v10.11.14",
"v11.5.2-rc3",
"v11.5.2",
"v11.6.0",
"v10.11.14-rc1",
"v11.5.0",
"v11.5.2-rc2",
"v11.5.2-rc1",
"v10.11.13",
"v11.5.1",
"v10.11.13-rc1",
"v10.11.12",
"v10.11.11-rc2",
"v10.11.11",
"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-2026-4339.json"
}
}
],
"modified": "2026-10-28T02:30:29.814476683Z",
"provider": "osv",
"references": [
"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/4xxx/CVE-2026-4339.json",
"https://mattermost.com/security-updates",
"https://nvd.nist.gov/vuln/detail/CVE-2026-4339"
],
"provider_metadata": {
"cwe_ids": [
"CWE-918"
],
"cna_assigner": "Mattermost",
"unresolved_ranges": [
{
"extracted_events": [
{
"range": [
{
"introduced": "10.11.0"
},
{
"last_affected": "10.11.18"
},
{
"introduced": "11.6.0"
},
{
"last_affected": "11.6.3"
},
{
"introduced": "11.5.0"
},
{
"last_affected": "11.5.6"
}
],
"source": "AFFECTED_FIELD",
"strategy": "StandardRange"
}
]
}
],
"osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/4xxx/CVE-2026-4339.json"
}
},
"related_cves": [
{
"value": "CVE-2026-4339",
"evidence": "/id"
}
],
"security_relevant": true,
"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 496816,
"published_at": "2026-06-26T14:44:58.655+00:00"
}