CVE-2026-4339

MEDIUM

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Title source: cna
STIX 2.1

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

References (1)

Core 1
Core References
Vendor Advisory vendor-advisory
MMSA-2026-00635
https://mattermost.com/security-updates

Scores

CVSS v3 6.5
EPSS 0.0010
EPSS Percentile 1.2%
Attack Vector LOCAL
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N

CISA SSVC

Vulnrichment
Exploitation none
Automatable no
Technical Impact partial

Details

CWE
CWE-918
Status published
Products (8)
Mattermost/Mattermost 10.11.0 - 10.11.18
Mattermost/Mattermost 10.11.19
Mattermost/Mattermost 11.5.0 - 11.5.6
Mattermost/Mattermost 11.5.7
Mattermost/Mattermost 11.6.0 - 11.6.3
Mattermost/Mattermost 11.6.4
Mattermost/Mattermost 11.7.0
mattermost/mattermost_server 10.11.0 - 10.11.19
Published Jun 26, 2026
Tracked Since Jun 26, 2026