Vulnerability · Published Mar 26, 2026 · Assigned by Mattermost
CVE-2026-4274Insufficient authorization in shared channel membership sync grants team-level access instead of channel-level access
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 4% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to restrict team-level access when processing membership sync from a remote cluster, which allows a malicious remote cluster to grant a user access to an entire private team instead of only the shared channel via sending crafted membership sync messages that trigger team membership assignment. Mattermost Advisory ID: MMSA-2026-00574
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 10.11.11+incompatible, 11.2.3+incompatible, 11.3.2+incompatible, 11.4.1+incompatible | GO-2026-5393 |
| Go | github.com/mattermost/mattermost-server/v5 | no fix listed | GO-2026-5393 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-5393 |
| Go | github.com/mattermost/mattermost/server/v8 | 10.11.11 | GHSA-g7fp-cqj5-x8hf |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260217110922-b7d4a1f1f59b | GHSA-g7fp-cqj5-x8hf |
| Go | github.com/mattermost/mattermost/server/v8 | 11.4.1 | GHSA-g7fp-cqj5-x8hf |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260217110922-b7d4a1f1f59b | GO-2026-5393 |
| Go | github.com/mattermost/mattermost/server/v8 | 11.3.2 | GHSA-g7fp-cqj5-x8hf |
| Go | github.com/mattermost/mattermost/server/v8 | 11.2.3 | GHSA-g7fp-cqj5-x8hf |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.2.0 – through 11.2.210.11.0 – through 10.11.1011.4.0 – through 11.4.011.3.0 – through 11.3.1 |
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-2026-00574 vendor-advisory