macOS Big Sur has been replaced, but still receives updates
Apple has released updates to its older operating systems, with users able to download updates for iOS 15.7.5, iPadOS 15.7.5, macOS Monterey 12.6.5, and macOS Big Sur 11.7.6.
Updates are sometimes offered for older operating system versions, used by iPhone, iPad, and Mac models that either haven’t been moved to the modern generation, or aren’t supported. Rather than including new features, the updates typically include security fixes that the newer operating systems also get.
On Apple’s security content page, the four operating systems all share a fix for IOSurfaceAccelerator, which can allow an app to execute arbitrary code with kernel privileges. An issue that Apple is aware is being actively exploited.
The iOS and iPadOS update also has a WebKit security fix, that closes a hole where “maliciously crafted web content” can execute code. Again, it is an issue that has been fixed, and Apple is aware it is being actively exploited.
