According to Apple's security notes for the updates, The software fixes a WebKit issue that could allow maliciously crafted web content to result in arbitrary code execution. Apple says that it is "aware of a report that this issue may have been actively exploited."
The vulnerability was a type confusion issue that Apple says has been addressed with improved checks.
While there are no other notable fixes in macOS Ventura 13.2.1, the iOS 16.3.1 update adds a Siri Find My fix, addresses a problem with iCloud settings, and introduces additional Crash Detection optimizations for iPhone 14 models.