CVE-2026-4339
Published:June 26, 2026
Updated:June 29, 2026
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 Packages
github.com/mattermost/mattermost (GO):
Affected version(s) >=v11.6.0 <v11.6.4Fix Suggestion:
Update to version v11.6.4github.com/mattermost/mattermost (GO):
Affected version(s) >=v11.5.0 <v11.5.7Fix Suggestion:
Update to version v11.5.7github.com/mattermost/mattermost (GO):
Affected version(s) >=v10.11.0 <v10.11.19Fix Suggestion:
Update to version v10.11.19Related Resources (1)
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Contact UsCVSS v4
Base Score:
8.2
Attack Vector
LOCAL
Attack Complexity
LOW
Attack Requirements
NONE
Privileges Required
LOW
User Interaction
NONE
Vulnerable System Confidentiality
HIGH
Vulnerable System Integrity
NONE
Vulnerable System Availability
NONE
Subsequent System Confidentiality
HIGH
Subsequent System Integrity
NONE
Subsequent System Availability
NONE
CVSS v3
Base Score:
6.5
Attack Vector
LOCAL
Attack Complexity
LOW
Privileges Required
LOW
User Interaction
NONE
Scope
CHANGED
Confidentiality
HIGH
Integrity
NONE
Availability
NONE
Weakness Type (CWE)
Server-Side Request Forgery (SSRF)
EPSS
Base Score:
0.10