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
Weakness
The product allocates memory based on an untrusted, large size value, but it does not ensure that the size is within expected limits, allowing arbitrary amounts of memory to be allocated.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Mattermost_server | Mattermost | 10.11.0 (including) | 10.11.15 (excluding) |
| Mattermost_server | Mattermost | 11.4.0 (including) | 11.4.5 (excluding) |
| Mattermost_server | Mattermost | 11.5.0 (including) | 11.5.4 (excluding) |
| Mattermost_server | Mattermost | 11.6.0 (including) | 11.6.1 (excluding) |