Description
Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to validate that a username returned during bot registration belongs to a bot account, which allows an unprivileged attacker to intercept private messages sent by plugins via direct message channels by pre-registering a user account with a predictable plugin bot username.. Mattermost Advisory ID: MMSA-2026-00649
Affected products
| Vendor | Product | Affected | Fixed |
|---|---|---|---|
| Mattermost | Mattermost | 11.6.0 – ≤ 11.6.1, 11.5.0 – ≤ 11.5.4, 10.11.0 – ≤ 10.11.15, 10.11.0 – ≤ 10.11.16 | 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.
SUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| SLES for SAP 16.0 | govulncheck-vulndb | 0.0.20260827T195228-160000.1.1 | SUSE-SU-2026:23625-1 |
| SLES 16.0 | govulncheck-vulndb | 0.0.20260827T195228-160000.1.1 | SUSE-SU-2026:23625-1 |
openSUSE
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| openSUSE Leap 16.0 | govulncheck-vulndb | 0.0.20260827T195228-160000.1.1 | openSUSE-SU-2026:21761-1 |
Affected packages
| Ecosystem | Package | Affected | Fixed | Record |
|---|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | >= 11.6.0, < 11.6.1 | 11.6.1 | GHSA-3vmp-whvv-5v9v |
| Go | github.com/mattermost/mattermost-server | >= 11.5.0, < 11.5.5 | 11.5.5 | GHSA-3vmp-whvv-5v9v |
| Go | github.com/mattermost/mattermost-server | >= 10.11.0, < 10.11.17 | 10.11.17 | GHSA-3vmp-whvv-5v9v |
| Go | github.com/mattermost/mattermost-server | >= 10.11.0+incompatible, < 10.11.17+incompatible · >= 11.5.0+incompatible, < 11.5.5+incompatible · >= 11.6.0+incompatible, < 11.6.1+incompatible | 10.11.17+incompatible, 11.5.5+incompatible, 11.6.1+incompatible | GO-2026-6279 |
| Go | github.com/mattermost/mattermost-server/v5 | all versions | — | GO-2026-6279 |
| Go | github.com/mattermost/mattermost-server/v6 | all versions | — | GO-2026-6279 |
| Go | github.com/mattermost/mattermost/server/v8 | >= 8.0.0-20250731163400-5b955468ea1e, < 8.0.0-20260428151657-c79c3831061a | 8.0.0-20260428151657-c79c3831061a | GHSA-3vmp-whvv-5v9v |
Advisories 3
- SUSE-SU-2026:23625-1 Security update for govulncheck-vulndb SUSE / openSUSE Security Updates · Sep 29, 2026
- openSUSE-SU-2026:21761-1 Security update for govulncheck-vulndb SUSE / openSUSE Security Updates · Sep 9, 2026
- WID-SEC-2026-1529 Mattermost Server: Mehrere Schwachstellen ermöglichen nicht spezifizierten Angriff BSI CERT-Bund (WID) · Jun 14, 2026
References
- MMSA-2026-00649 vendor-advisory