GHSA-w9m8-p4cc-4qj9HighCVSS 7.5

Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation

Published
May 26, 2026
Last Modified
June 29, 2026

🔗 CVE IDs covered (1)

CVE-2026-5740 →

📋 Description

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647

🎯 Affected products5

  • go/github.com/mattermost/mattermost-server:= 11.6.0
  • go/github.com/mattermost/mattermost-server:>= 11.5.0, < 11.5.4
  • go/github.com/mattermost/mattermost-server:>= 11.4.0, < 11.4.5
  • go/github.com/mattermost/mattermost-server:>= 10.11.0, < 10.11.15
  • go/github.com/mattermost/mattermost/server/v8:< 8.0.0-20260410202636-17939826efa2

🔗 References (4)