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 plugi...
Date: 2026-06
Affected: mattermost/mattermost_server
Attack vector: ssrf
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2016-11077
CVE-2017-18892
CVE-2017-18909
CVE-2017-18918
CVE-2023-2783
CVE-2023-35075
CVE-2023-5967
CVE-2024-37182
CVE-2025-13523
CVE-2025-24839
CVE-2025-27715
CVE-2025-31363
CVE-2025-3913
CVE-2025-47700
CVE-2025-49810
CVE-2025-55074
CVE-2025-9072
CVE-2026-2461
CVE-2026-25783
CVE-2026-28735
CVE-2026-3108
CVE-2026-3637
CVE-2026-4053
CVE-2026-4339
CVE-2026-5163
CVE-2026-9699
CWE: CWE-116
CWE-1287
CWE-150
CWE-22
CWE-276
CWE-311
CWE-347
CWE-532
CWE-601
CWE-639
CWE-672
CWE-693
CWE-732
CWE-74
CWE-754
CWE-79
CWE-862
CWE-863
CWE-918
CVSS: 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


Mitigations:

Architectural layers (MAESTRO):