CVE-2026-4339

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Published Jun 26, 2026 CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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

Remediation

Vendor Solution

Update Mattermost to versions 11.7.0, 10.11.19, 11.6.4, 11.5.7 or higher.

Tracking

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Advisories

No advisories available
References
History
Type Values Removed Values Added
First Time appeared Mattermost
Mattermost mattermost
Vendors & Products Mattermost
Mattermost mattermost
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
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
Title SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server
Weaknesses CWE-918
References
Metrics cvssV3_1

{'score': 6.5, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
Mattermost
Published
Updated
Jun 26, 2026, 3:40 PM
Reserved
Mar 17, 2026, 2:57 PM
Status
Analyzed
Published
Jun 26, 2026, 3:16 PM
Modified
Jun 29, 2026, 7:26 PM