A Trojan that exploits an Internet Explorer vulnerability is capable of allowing attackers to hijack browser behaviour, anti-virus firms warn.

The QHosts (Delude) Trojan can't spread by itself. Users only become infected if they visited a maliciously constructed website containing code which allows the malware to run.

More information about this vulnerability, including a (partial) fix, can be found in an advisory from Microsoft, issued back in August.

Some anti-virus vendors reckon that this patch will protect against the exploit. However, McAfee warns that the patch fails to protect against the automatic execution of VBScript contained in an HTML file, the infection mechanism used by QHosts.

AV firms are united in saying the latest Windows menace is low spreading, which is just as well. As usual Mac, Linux, OS/2 and Unix users are immune from infection.

According to McAfee, the purpose of this Trojan is to hijack browser use. When page requests are made, they are rerouted to specified Domain Name Servers. This allows a remote 'administrator' to direct users to the pages of their choosing.

This Trojan is responsible for recent reports of strange DNS changes on systems as recently reported on NTBUGTRAQ, McAfee believes.

Finnish AV firm F-Secure has noted two variants of the Trojan. An advisory by Symantec provides technical detail on the changes the Trojan makes to infected PCs.

Users are advised to update AV signature definitions so that security tools can block the Trojan in case a user is tricked (using spam or via other mechanisms) into visiting an infected Web site. ®