CVSS 3.1
6.1 mediumCVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:U/C:H/I:H/A:NCVE-2026-27656
Medium (6.1) in mattermost mattermost server. EPSS 0.2% chance of exploitation in the next 30 days. Fixed in github.com/mattermost/mattermost-server 10.11.12, 10.11.12+incompatible, 11.2.4.
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
Severity
- EPSS
- 0.2% chance of exploitation in the next 30 days; higher than 7% of CVEs (scores of 2026-10-31)
- CWE
- CWE-303
- Published
- Modified
- Record from
- the NVD, synced
Affected
- mattermost:mattermost_server
Packages (OSV)
| Advisory | Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|---|
| GHSA-fg35-5rf6-qg3g | 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/v8 | >=8.0.0-20260105080200-d27a2195068d <8.0.0-20260217110922-b7d4a1f1f59b | 8.0.0-20260217110922-b7d4a1f1f59b |
| GO-2026-5360 | 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 | no fix published |
| GO-2026-5360 | Go | github.com/mattermost/mattermost-server/v6 | all versions | no fix published |
| 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 |