Computer security firm Secunia is warning of a new security vulnerability affecting Mac Internet browsers Safari 1.x and Internet Explorer 5.x.

The report claims the weakness: "Potentially allows malicious Web sites to compromise a vulnerable system".

"The problem is that the "help" URI handler allows execution of arbitrary local scripts (.scpt) via the classic directory traversal character sequence using 'help:runscript'", the warning explains.

This makes it possible for malicious computer users to place "arbitrary" files (including script files) in a known location on a user's system – but only if either browser has been set-up to open safe files after they are downloaded. This is the default browser setting.

Secunia recommends users switch off the latter capability in Safari's preferences folder; that they do not go online as a "privileged user" and that they rename the help handler, though no instructions related to the latter are avaiable.