Internet security specialist Secunia has raised its threat assessment of a newly-identified security vulnerability affecting Mac Web browsers.

The threat rating's been increased to 'Extremely Critical', as it can allow remote Mac System access by remote users. The security firm also warns that the "issues are very easy to exploit", adding that a "large number of working exploits are available".

Warning that there is no efficient solution to the problem, Secunia warns Mac users (using both Internet Explorer and Safari browsers) to not visit untrusted Web sites; to not surf the Internet as a privileged user; and to rename all URI (uniform resource identifier) handlers which are not required.

Apple issued a statement to MacCentral: "We take security very seriously at Apple and we are actively investigating this potential security issue. While no operating system can be completely secure from all threats, Apple has an excellent track record of identifying and rapidly correcting potential vulnerabilities."

A reader's letter received by Macworld but not yet tested recommends using a software package called Misfox (Mussing Internet Settings for X), and using this to change the protocol 'help' to use a different application. I have set mine up to use TextEdit.

"This way if TextEdit suddenly opens then I know someone tried to exploit this," the reader writes.