CVE-2026-4339

published 2026-06-26

CVE-2026-4339: 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…

Priority P4 33 medium 6.5 CVSS 3.1

AV L AC L PR L UI N S C C H I N A N

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

Affected

6 ranges

Vendor	Product	Version range	Fixed in
mattermost	mattermost	10.11.0 – 10.11.18	—
mattermost	mattermost	11.5.0 – 11.5.6	—
mattermost	mattermost	11.6.0 – 11.6.3	—
mattermost	mattermost_server	>= 10.11.0 < 10.11.19	10.11.19
mattermost	mattermost_server	>= 11.5.0 < 11.5.7	11.5.7
mattermost	mattermost_server	>= 11.6.0 < 11.6.4	11.6.4

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CWE-918