Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

Summary

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

Timeline

1. May 22, 2026, 10:22 AM UTCCVE published
2. May 22, 2026, 12:14 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. May 22, 2026, 12:14 PM UTCRecord last updated

Affected versions

• Mattermost Mattermost

  From 11.6.0 · through 11.6.0; From 11.5.0 · through 11.5.3; From 11.4.0 · through 11.4.4; From 10.11.0 · through 10.11.14

• github.com/mattermost/mattermost-serverGo

  From 11.6.0 · fixed in 11.6.1

• github.com/mattermost/mattermost-serverGo

  From 11.5.0 · fixed in 11.5.4

• github.com/mattermost/mattermost-serverGo

  From 11.4.0 · fixed in 11.4.5

• github.com/mattermost/mattermost-serverGo

  From 10.11.0 · fixed in 10.11.15

• github.com/mattermost/mattermost/server/v8Go

  fixed in 8.0.0-20260410202636-17939826efa2

• github.com/mattermost/mattermost-serverGo

  From 10.11.0+incompatible · fixed in 10.11.15+incompatible; From 11.4.0+incompatible · fixed in 11.4.5+incompatible; From 11.5.0+incompatible · fixed in 11.5.4+incompatible; From 11.6.0+incompatible · fixed in 11.6.1+incompatible

• github.com/mattermost/mattermost-server/v5Go

  All versions

• github.com/mattermost/mattermost-server/v6Go

  All versions

References 1