This issue was addressed with improved checks. This issue is fixed in macOS Big Sur 11.2, Security Update 2021-001 Catalina, Security Update 2021-001 Mojave. A user that is removed from an iMessage group could rejoin the group.
https://support.apple.com/en-us/HT212147
Source: Mitre, NVD
Published: 2021-04-02
Updated: 2024-11-21
Base Score: 4.3
Vector: CVSS2#AV:N/AC:M/Au:N/C:N/I:P/A:N
Severity: Medium
Base Score: 3.3
Vector: CVSS:3.0/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
Severity: Low