An authentication issue was addressed with improved state management. This issue is fixed in macOS Monterey 12.0.1, macOS Big Sur 11.6.1. A local attacker may be able to view the previous logged-in user’s desktop from the fast user switching screen.
Apple Security Advisory 2021-10-26-4 - macOS Big Sur 11.6.1 addresses code execution, integer overflow, out of bounds read, and out of bounds write vulnerabilities.