If you are using any Apple product (phone, tablet, laptop, computer), upgrade it ASAP.
A severe bug was discovered in the WebKit engine (used by the Safari browser), which could lead to a complete take over the affected device simply by visiting a bad website.
More details can be found in this threatpost article. Here’s an excerpt.
The vulnerability stems from a memory-corruption issue in WebKit; this type of bug occurs when the contents of a memory location are modified in a way that exceeds the intention of the original program/language constructs – allowing attackers to execute arbitrary code.