iPhone users should install iOS 14.4.1, which Apple released Monday entirely to kill a bug in the Safari web browser. Obviously, Apple considers it a significant enough threat to devote an entire software update to justify it.
There’s also an equivalent iPadOS 14.4.1 version. Plus, the newly released watchOS 7.3.2 deals with the same security problem, as does macOS Big Sur 11.2.3.