This they can do either directly, with a single virus carrying out both types of action or, more commonly, indirectly using more than one virus. The Opaserv worm is an example of the former, while Lovgate, which installs itself on the victim's computer before downloading other files (Trojans) from the Internet, is a typical example of the latter.
One of the pioneers of this type of attack is Bugbear, which reached epidemic proportions at the end of last year. This worm spreads across the Internet via e-mail, and when it infects a computer, it drops a Trojan which makes external connections through a communication port. More recently, cases such as Randon or Deloder, which spreads via IRC and can carry out numerous malicious actions, further highlight the need to take effective preventive action.
Detecting this kind of virus after it has infected is simply not enough. Effective protection involves blocking these worm/Trojans at the communication port level, which can be done simply by using a personal firewall. These devices can detect and block any kind of unusual activity that tries to enter the computer.
With this in mind, some antivirus developers have included this kind of protection in their solutions, as is the case with Panda Antivirus Platinum 7.0, which includes a latest generation personal firewall combined with innovative antivirus technology to help turn PCs into a fortress against all types of viruses including worm/Trojans.
According to Luis Corrons, head of Panda Software's Virus Laboratory, "All indications are that in the near future worm/Trojans will be responsible for a large number of attacks, as the creators of these viruses have now seen how effective they can be. The only way of avoiding them is to be fully aware of the threat that they represent, and ensuring that all systems are adequately protected."