Vulnerability · Published Jun 12, 2026 · Assigned by Mattermost
CVE-2026-6046Plugin bot username conflict allows user account to be used as bot identity in Mattermost Server
Medium 5.3
In plain English
A medium-severity information exposure 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 9% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to validate that a username returned during bot registration belongs to a bot account, which allows an unprivileged attacker to intercept private messages sent by plugins via direct message channels by pre-registering a user account with a predictable plugin bot username.. Mattermost Advisory ID: MMSA-2026-00649
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/mattermost/mattermost-server | 11.6.1 | GHSA-3vmp-whvv-5v9v |
| Go | github.com/mattermost/mattermost-server | 11.5.5 | GHSA-3vmp-whvv-5v9v |
| Go | github.com/mattermost/mattermost-server | 10.11.17 | GHSA-3vmp-whvv-5v9v |
| Go | github.com/mattermost/mattermost-server | 10.11.17+incompatible, 11.5.5+incompatible, 11.6.1+incompatible | GO-2026-6279 |
| Go | github.com/mattermost/mattermost-server/v5 | no fix listed | GO-2026-6279 |
| Go | github.com/mattermost/mattermost-server/v6 | no fix listed | GO-2026-6279 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260428151657-c79c3831061a | GO-2026-6279 |
| Go | github.com/mattermost/mattermost/server/v8 | 8.0.0-20260428151657-c79c3831061a | GHSA-3vmp-whvv-5v9v |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Mattermost | Mattermost | 11.6.0 – through 11.6.111.5.0 – through 11.5.410.11.0 – through 10.11.1510.11.0 – through 10.11.16 |
Weakness: CWE-200 · Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
References
- MMSA-2026-00649 vendor-advisory