Vulnerability · Published May 18, 2026 · Assigned by Mattermost
CVE-2026-5163Missing authorization check in AI message rewrite endpoint allows access to private thread content
Medium 6.5
In plain English
A medium-severity missing 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 0.2% (higher than 11% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
Mattermost versions 11.5.x <= 11.5.1 fail to verify channel membership when processing AI-assisted message rewrites which allows an authenticated attacker to read the content of threads in private channels and direct messages they do not have access to via a crafted request to the post rewrite endpoint.. Mattermost Advisory ID: MMSA-2026-00645
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 11.5.2+incompatible | GO-2026-5263 |
| Go | github.com/mattermost/mattermost-server | 5.3.2-0.20260401090745-f4d1abe7e8f5 | GHSA-8r89-8w26-cq32 |
| Go | github.com/mattermost/mattermost-server/v5 | no fix listed | GO-2026-5263 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-5263 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260401090745-f4d1abe7e8f5 | GO-2026-5263 |
| Go | github.com/mattermost/mattermost/server/v8 | 11.5.2 | GHSA-8r89-8w26-cq32 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260401090745-f4d1abe7e8f5 | GHSA-8r89-8w26-cq32 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.5.0 – through 11.5.1 |
Weakness: CWE-862 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
References
- MMSA-2026-00645 vendor-advisory