Description
Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645
Affected products
| Vendor | Product | Affected | Fixed |
|---|---|---|---|
| Mattermost | Mattermost | 11.5.0 – ≤ 11.5.1 | 11.6.0 |
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 | < 5.3.2-0.20260401090745-f4d1abe7e8f5 | 5.3.2-0.20260401090745-f4d1abe7e8f5 | GHSA-8r89-8w26-cq32 |
| Go | github.com/mattermost/mattermost-server | >= 11.5.0+incompatible, < 11.5.2+incompatible | 11.5.2+incompatible | GO-2026-5263 |
| Go | github.com/mattermost/mattermost-server/v5 | all versions | — | GO-2026-5263 |
| Go | github.com/mattermost/mattermost-server/v6 | all versions | — | GO-2026-5263 |
| Go | github.com/mattermost/mattermost/server/v8 | >= 11.5.0, < 11.5.2 | 11.5.2 | GHSA-8r89-8w26-cq32 |
| Go | github.com/mattermost/mattermost/server/v8 | < 8.0.0-20260401090745-f4d1abe7e8f5 | 8.0.0-20260401090745-f4d1abe7e8f5 | GHSA-8r89-8w26-cq32 |
Advisories 2
- openSUSE-SU-2026:21483-1 Security update for govulncheck-vulndb SUSE / openSUSE Security Updates · Jul 31, 2026
- WID-SEC-2026-1154 Mattermost: Mehrere Schwachstellen ermöglichen nicht spezifizierten Angriff BSI CERT-Bund (WID) · May 18, 2026
References
- MMSA-2026-00645 vendor-advisory