Vulnerability · Published Mar 25, 2026 · Assigned by Mattermost
CVE-2026-27656Account Takeover via Substring Matching in OpenID Connect Authentication
Medium 5.7
In plain English
A medium-severity security flaw in Mattermost from Mattermost. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 8% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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 packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 10.11.12 | GHSA-fg35-5rf6-qg3g |
| Go | github.com/mattermost/mattermost-server | 11.4.1 | GHSA-fg35-5rf6-qg3g |
| Go | github.com/mattermost/mattermost-server | 11.2.4 | GHSA-fg35-5rf6-qg3g |
| Go | github.com/mattermost/mattermost-server | 11.3.2 | GHSA-fg35-5rf6-qg3g |
| Go | github.com/mattermost/mattermost-server | 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 | no fix listed | GO-2026-5360 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-5360 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260217110922-b7d4a1f1f59b | GO-2026-5360 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260217110922-b7d4a1f1f59b | GHSA-fg35-5rf6-qg3g |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.4.0 – through 11.4.011.3.0 – through 11.3.111.2.0 – through 11.2.310.11.0 – through 10.11.11 |
Weakness: CWE-303 · Vector: CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:H/A:N
References
- MMSA-2026-00590 vendor-advisory