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CVE PROGRAM SOURCE RECORD6.5 Medium

CVE-2026-4339

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

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Status
Published
Published
Updated
Assigning organization
Mattermost

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

Affected products

Version statuses are supplied by the assigning organization. Consult the original record for detailed ranges and platform conditions.

Mattermost Mattermost

Default version status: unaffected

  • 10.11.0 to ≤ 10.11.18affected
  • 11.6.0 to ≤ 11.6.3affected
  • 11.5.0 to ≤ 11.5.6affected
  • 11.7.0unaffected
  • 10.11.19unaffected
  • 11.6.4unaffected
  • 11.5.7unaffected

References