CVE-2026-4339

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

State: Published
Severity: 6.5 Medium
Published: Jun 26, 2026
Updated: Jun 26, 2026
Reserved: Mar 17, 2026

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

Scores

CVSS	Score	Vector	Source
3.1	6.5 Medium	CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:N/A:N	Mattermost

Weaknesses

CWE-918 Server-Side Request Forgery (SSRF)

Affected Products

Product	Versions
MattermostMattermost	10.11.0 through 10.11.18Affected 11.6.0 through 11.6.3Affected 11.5.0 through 11.5.6Affected 11.7.0Not affected 10.11.19Not affected 11.6.4Not affected 11.5.7Not affected

References

MMSA-2026-00635vendor-advisory

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