CVE-2021-1771: 
macOS vulnerability analysis and mitigation

CVE-2021-1771 is a security vulnerability affecting Apple's iMessage group chat functionality in macOS operating systems. The vulnerability was discovered by Shreyas Ranganatha and was fixed in macOS Big Sur 11.2, Security Update 2021-001 Catalina, and Security Update 2021-001 Mojave, released on February 1, 2021. The affected systems include macOS Big Sur 11.0.1, macOS Catalina 10.15.7, and macOS Mojave 10.14.6 (Apple Advisory).

The vulnerability is characterized by insufficient checks in the iMessage group chat system. According to the National Vulnerability Database, it has been assigned a CVSS v3.1 base score of 3.3 (LOW) with the vector string CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N, indicating local access, low attack complexity, no privileges required, and user interaction required (NVD).

The primary impact of this vulnerability is that a user who has been removed from an iMessage group chat could potentially rejoin the group without authorization. This creates a privacy and security concern as it allows previously removed users to regain access to group conversations (Apple Advisory).

Apple addressed this vulnerability by implementing improved checks in the affected systems. The fix was released in macOS Big Sur 11.2, Security Update 2021-001 Catalina, and Security Update 2021-001 Mojave. Users are advised to update their systems to these versions or later to mitigate the vulnerability (Apple Advisory).