Mattermost versions 10.11.x <= 10.11.3, 10.5.x <= 10.5.11 fail to properly validate team membership permissions in the Add Channel Member API which allows users from one team to access user metadata and channel membership information from other teams via the API endpoint
Exploitability
AV:NAC:HPR:LUI:NScope
S:UImpact
C:LI:NA:N3.1/CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:NAccess Control