CVE-2026-4339: SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 11.7.0 - Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 10.11.19 - Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 11.6.4 - Upgrade
Upgrade
Mattermostto a version that resolves this vulnerability.Fixed in 11.5.7 - Compensating control
Until Mattermost is upgraded, restrict access to the Mattermost Agents plugin MCP server in stdio mode so only trusted users/processes can reach it (to reduce attacker access needed to supply internal URLs as attachment targets).
Event History
Frequently Asked Questions
What is the severity of CVE-2026-4339?
The severity of CVE-2026-4339 is medium, rated at 6.5.
How do I fix CVE-2026-4339?
To fix CVE-2026-4339, update your Mattermost server to versions above 10.11.18, 11.6.3, or 11.5.6.
What exploit is associated with CVE-2026-4339?
CVE-2026-4339 is associated with a server-side request forgery (SSRF) vulnerability.
Which Mattermost versions are affected by CVE-2026-4339?
Mattermost versions 10.11.x up to 10.11.18, 11.6.x up to 11.6.3, and 11.5.x up to 11.5.6 are affected by CVE-2026-4339.
Can CVE-2026-4339 lead to a compromise of my system?
Yes, CVE-2026-4339 can allow an attacker to exploit SSRF, potentially compromising your system if accessed.