If, for some reason, you're still using Microsoft's Internet Explorer, you really should read this.
In a statement Microsoft said: "The vulnerability exists in the way that Internet Explorer accesses an object in memory that has been deleted or has not been properly allocated.
"The vulnerability may corrupt memory in a way that could allow an attacker to execute arbitrary code in the context of the current user within Internet Explorer.
"An attacker could host a specially crafted website that is designed to exploit this vulnerability through Internet Explorer and then convince a user to view the website."
It's a big deal - around 26% of all computers run IE.
Hackers are currently reported to be targeting financial and defence organisation in the US.
Microsoft are working on a fix for the bug.