Description
Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590
Affected products
| Vendor | Product | Affected | Fixed |
|---|---|---|---|
| Mattermost | Mattermost | 11.4.0 – ≤ 11.4.0, 11.3.0 – ≤ 11.3.1, 11.2.0 – ≤ 11.2.3, 10.11.0 – ≤ 10.11.11 | 11.5.0 |
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 | >= 11.4.0-rc1, < 11.4.1 | 11.4.1 | GHSA-fg35-5rf6-qg3g |
| Go | github.com/mattermost/mattermost-server | >= 11.3.0-rc1, < 11.3.2 | 11.3.2 | GHSA-fg35-5rf6-qg3g |
| Go | github.com/mattermost/mattermost-server | >= 11.2.0-rc1, < 11.2.4 | 11.2.4 | GHSA-fg35-5rf6-qg3g |
| Go | github.com/mattermost/mattermost-server | >= 10.11.0-rc1, < 10.11.12 | 10.11.12 | GHSA-fg35-5rf6-qg3g |
| Go | github.com/mattermost/mattermost-server | >= 10.11.0-rc1+incompatible, < 10.11.12+incompatible · >= 11.2.0-rc1+incompatible, < 11.2.4+incompatible · >= 11.3.0-rc1+incompatible, < 11.3.2+incompatible · >= 11.4.0-rc1+incompatible, < 11.4.1+incompatible | 10.11.12+incompatible, 11.2.4+incompatible, 11.3.2+incompatible, 11.4.1+incompatible | GO-2026-5360 |
| Go | github.com/mattermost/mattermost-server/v5 | all versions | — | GO-2026-5360 |
| Go | github.com/mattermost/mattermost-server/v6 | all versions | — | GO-2026-5360 |
| Go | github.com/mattermost/mattermost/server/v8 | >= 8.0.0-20260105080200-d27a2195068d, < 8.0.0-20260217110922-b7d4a1f1f59b | 8.0.0-20260217110922-b7d4a1f1f59b | GHSA-fg35-5rf6-qg3g |
Advisories 1
- openSUSE-SU-2026:21483-1 Security update for govulncheck-vulndb SUSE / openSUSE Security Updates · Jul 31, 2026
References
- MMSA-2026-00590 vendor-advisory