New variants of the Haxdoor Trojan have been discovered in the wild, security firm Panda Software has reported. 

The mutants are particularly dangerous because they use a rootkit to hide their actions and avoid detection.

Several new versions of the Haxdoor family of Trojans have emerged over the past few days. The malware tries to steal confidential user details in order to commit online fraud and identity theft.

The latest strains have several common characteristics, including a capacity to install a rookit designed to hide objects, such as processes, files or entries.

Haxdoor uses this rootkit to hide itself on the computer from the user and the majority of traditional security systems.

All the new variants are designed to steal passwords for popular internet services, such as eBay, ICQ, PayPal and WebMoney, and for many email clients, including Outlook Express and The Bat.

The malware also makes modifications to any firewalls installed on the compromised computer in order to authorise its own malicious processes. By doing this, it removes any obstacles designed to prevent stolen data being transmitted.

"It seems that the authors of these malicious codes are mass-mailing these Trojans as attachments to spam messages," said Luis Corrons, director of PandaLabs.

"For this reason, it is recommended to delete any suspicious or unwanted email messages. These Trojans are very dangerous, above all due to their capacity to use a rootkit to hide their actions.

"It is highly advisable to complement traditional antivirus solutions with proactive technologies that can detect suspicious processes based on behavioural analysis."

Panda is offering its free, online ActiveScan anti-malware check to clean up affected PCs.