From this source.
Security research group Secunia is warning all Mac OS X (news - web sites) users about what it calls a "highly critical" vulnerability. The vulnerability, Secunia says, affects all Mac OS X users who surf the Web using either Apple Computer's Safari Web browsers or Microsoft's Internet Explorer.

According to Secunia's advisory, two security flaws can be used by malicious Web sites to remotely access at-risk systems. The security holes have been confirmed on systems running Mac OS X with either Safari 1.2.1 or Internet Explorer 5.2, Secunia says.