CVE-2026-4339
Medium
Description
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
SeverityMedium
CVSS6.5
Exploit probability—
Product
Mattermost Server
PublishedJun 26, 2026
First seenJun 26, 2026
Weakness typeCWE-918Server-side request forgery (SSRF)
How to fix
Update
- mattermost mattermost_server10.11.19, 11.5.7, 11.6.4 or later
Vendor advisories
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| mattermost mattermost_server | 10.11.0 ≤ v < 10.11.19, 11.5.0 ≤ v < 11.5.7, 11.6.0 ≤ v < 11.6.4 | 10.11.19, 11.5.7, 11.6.4 |
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