MacRumors published a story that Apple has released macOS Big Sur 11.3.1 with fixes for WebKit security issues.

Apple Releases macOS Big Sur 11.3.1 With Fixes for WebKit Security Issues

Alongside iOS 14.5.1 and watchOS 7.4.1, Apple today also released macOS Big Sur 11.3.1, which the company says "provides important security updates". According to the full security notes for the release, it addresses a memory corruption issue and an integer overflow in WebKit that could both be exploited using maliciously crafted web content.

Apple Releases macOS Big Sur 11.3.1 With Fixes for WebKit Security Issues