Vulnerability · Published May 22, 2026 · Assigned by Mattermost
CVE-2026-5740Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server
High 7.5
In plain English
A high-severity security flaw in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 28% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 10.11.15 | GHSA-w9m8-p4cc-4qj9 |
| Go | github.com/mattermost/mattermost-server | 11.6.1 | GHSA-w9m8-p4cc-4qj9 |
| Go | github.com/mattermost/mattermost-server | 11.4.5 | GHSA-w9m8-p4cc-4qj9 |
| Go | github.com/mattermost/mattermost-server | 11.5.4 | GHSA-w9m8-p4cc-4qj9 |
| Go | github.com/mattermost/mattermost-server | 10.11.15+incompatible, 11.4.5+incompatible, 11.5.4+incompatible, 11.6.1+incompatible | GO-2026-5838 |
| Go | github.com/mattermost/mattermost-server/v5 | no fix listed | GO-2026-5838 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-5838 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260410202636-17939826efa2 | GO-2026-5838 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260410202636-17939826efa2 | GHSA-w9m8-p4cc-4qj9 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.6.0 – through 11.6.011.5.0 – through 11.5.311.4.0 – through 11.4.410.11.0 – through 10.11.14 |
Weakness: CWE-789 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
References
- MMSA-2026-00647 vendor-advisory