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 products
| Vendor | Product | Affected | Fixed |
|---|---|---|---|
| Mattermost | Mattermost | 11.6.0 – ≤ 11.6.0, 11.5.0 – ≤ 11.5.3, 11.4.0 – ≤ 11.4.4, 10.11.0 – ≤ 10.11.14 | 11.7.0 |
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
openSUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| openSUSE Leap 16.0 | govulncheck-vulndb | 0.0.20260723T184607-160000.1.1 | openSUSE-SU-2026:21483-1 |
Affected packages
| Ecosystem | Package | Affected | Fixed | Record |
|---|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | >= 11.6.0, < 11.6.1 | 11.6.1 | GHSA-w9m8-p4cc-4qj9 |
| Go | github.com/mattermost/mattermost-server | >= 11.5.0, < 11.5.4 | 11.5.4 | GHSA-w9m8-p4cc-4qj9 |
| Go | github.com/mattermost/mattermost-server | >= 11.4.0, < 11.4.5 | 11.4.5 | GHSA-w9m8-p4cc-4qj9 |
| Go | github.com/mattermost/mattermost-server | >= 10.11.0, < 10.11.15 | 10.11.15 | GHSA-w9m8-p4cc-4qj9 |
| Go | github.com/mattermost/mattermost-server | >= 10.11.0+incompatible, < 10.11.15+incompatible · >= 11.4.0+incompatible, < 11.4.5+incompatible · >= 11.5.0+incompatible, < 11.5.4+incompatible · >= 11.6.0+incompatible, < 11.6.1+incompatible | 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 | all versions | — | GO-2026-5838 |
| Go | github.com/mattermost/mattermost-server/v6 | all versions | — | GO-2026-5838 |
| Go | github.com/mattermost/mattermost/server/v8 | < 8.0.0-20260410202636-17939826efa2 | 8.0.0-20260410202636-17939826efa2 | GHSA-w9m8-p4cc-4qj9 |
Advisories 2
- openSUSE-SU-2026:21483-1 Security update for govulncheck-vulndb SUSE / openSUSE Security Updates · Jul 31, 2026
- WID-SEC-2026-1243 Mattermost: Mehrere Schwachstellen ermöglichen nicht spezifizierten Angriff BSI CERT-Bund (WID) · May 25, 2026
References
- MMSA-2026-00647 vendor-advisory