Vulnerability · Published May 22, 2026 · Assigned by Mattermost
CVE-2026-28735GitHub OAuth Scope Validation
Medium 5.4
In plain English
A medium-severity incorrect authorization 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 0.1% (higher than 3% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the OAuth token scope on the callback which allows an authenticated Mattermost user to gain access to private repositories via modifying the scope parameter in the GitHub authorization URL.. Mattermost Advisory ID: MMSA-2026-00628
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-plugin-github | 1.0.1-0.20260318132218-6e6b740c4852 | GO-2026-5839 |
| Go | github.com/mattermost/mattermost-plugin-github | 1.0.1-0.20260318132218-6e6b740c4852 | GHSA-r5vf-grcx-5vqp |
| Go | github.com/mattermost/mattermost-server | 11.4.5 | GHSA-r5vf-grcx-5vqp |
| Go | github.com/mattermost/mattermost-server | 11.6.1 | GHSA-r5vf-grcx-5vqp |
| Go | github.com/mattermost/mattermost-server | 10.11.15+incompatible, 11.4.5+incompatible, 11.5.4+incompatible, 11.6.1+incompatible | GO-2026-5839 |
| Go | github.com/mattermost/mattermost-server | 10.11.15 | GHSA-r5vf-grcx-5vqp |
| Go | github.com/mattermost/mattermost-server | 11.5.4 | GHSA-r5vf-grcx-5vqp |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.6.0 – through 11.6.011.5.0 – through 11.5.311.4.0 – through 11.4.410.11.0 – through 10.11.14 |
Weakness: CWE-863 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
References
- MMSA-2026-00628 vendor-advisory