CVE-2026-4339 — SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Description (CNA)

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 (CNA)

Vendor / product	Versions
Mattermost / Mattermost	affected 10.11.0 through 10.11.18; affected 11.6.0 through 11.6.3; affected 11.5.0 through 11.5.6; unaffected 11.7.0; unaffected 10.11.19; unaffected 11.6.4; unaffected 11.5.7

CWE

CWE-918

References

https://mattermost.com/security-updates

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.