iOS and iPadOS versions 15.3 were pushed in order to patch a slew of vulnerabilities on iPhones or iPads. However, it left one, which may have allowed the execution of "arbitrary" code. Never fear, though, v15.3.1 is here to address this potential security issue. There are also now new corresponding macOS and watchOS versions available to install.
https://www.notebookcheck.net/Apple-releases-the-incremental-iOS-and-iPadOS-15-3-1-update-to-fix-a-crucial-security-discrepancy.598856.0.html