About Apple's Emergency update :
Many users choose Apple because of it's Security provided by Company which gives utmost important to user privacy and security. In this Technically Advanced Era anytime any device may effect with unknows flaws in it. Likewise Apple devices have some flaws too but every time apple acts fast and succeed in fixing the issues.
Apple Released iOS 15.3.1 for Mobile devices, iPad OS 15.3.1 for iPads and Mac OS 12.2.1 for PC devices which fixes a critical Webkit zero-day vulnerability.
Apple Stated the details about this update as :
About Issue :
Webkit is Apple's web Rendering Engine which is used not only used by Safari (Apple's Native browser) but every browser in your Apple device which gives trouble to users which allows attackers to run anything on the device.
Apple also pushed Safari 15.3 update for macOS Big Sur (build number 16612.4.9.1.8) and macOS Catalina (build number 15612.4.9.1.8). The iOS 15.3.1 update also fixes a small issue that could cause Braille displays to stop responding.
As this update is necessary update your devices accordingly to avoid Security Breaches.