What are Parasites?
'Parasite' is a shorthand term for "unsolicited commercial software" - that is, a program that gets installed on your computer which you never asked for, and which does something you probably don't want it to, for someone else's profit.

The parasite problem has grown enormously recently, and many millions of computers are affected. Unsolicited commercial software can typically:

• plague you with unwanted advertising ('adware');


• watch everything you do on-line and send information back to marketing companies ('spyware');


• add advertising links to web pages, for which the author does not get paid, and redirect the payments from affiliate-fee schemes to the makers of the software (such software is sometimes called 'scumware');


• set browser home page and search settings to point to the makers' sites (generally loaded with advertising), and prevent you changing it back ('homepage hijackers');


• make your modem (analogue or ISDN) call premium-rate phone numbers ('dialers');


• leave security holes allowing the makers of the software - or, in particularly bad cases, anyone at all - to download and run software on your machine;


• degrade system performance and cause errors thanks to being badly-written;


• provide no uninstall feature, and put its code in unexpected and hidden places to make it difficult to remove.

• Some freeware programs are 'bundled' with parasites, which are installed at the same time. The P2P file-sharing programs are notorious for this.
  Often if you are careful to read the small print when you install the software it will warn you about this, and it is sometimes possible to opt out. So always skim the licence agreement when you install and don't just click Next-Next-Next... but you still can't be sure they'll tell you.



• Many parasites load using Internet Explorer's ActiveX installation option. When a web page includes a link to an ActiveX program, a window will appear asking the user wishes to execute it. If 'Yes' is clicked (or if IE security settings are set lower than normal so that it never even asks*), the software is allowed to run and can do anything at all it likes on our computer, including installing parasites.
  For this reason, you should never click 'Yes' to a "Do you wish to download and install..." prompt unless you are 100% sure you trust the publisher of the software, which might not be the publisher of the web site you are viewed - read the dialogue box very carefully.
  Sometimes sites (or pop-up ads) try to fool you into clicking 'Yes' by stating that the software is necessary to view the site, or opening endless error windows if you click 'No', or claiming that the digital certificate on the code means it is safe. It means no such thing. 'Microsoft Authenticode', signed by companies like Verisign, means only that the company that wrote the software is the same as the company whose name appears on the download prompt - nothing more.



• Some of the really sleazy parasites, particularly homepage-hijackers and dialers, execute by exploiting security holes in Internet Explorer, ways of getting code to run that are not supposed to be possible, but are due to mistakes in the browser code.
  You can do your best to guard against this by ensuring you have the latest updates and patches from Microsoft. Still, there are usually a handful of security holes that have not yet been corrected, so you can never be 100% sure you are safe.
  One way of reducing your risk of exploitation is to go to Tools->Internet Options->Security and set the security level for the Internet Zone to 'High'. (If no slider is visible, click 'Default level to make it appear first.) Then set the security level for the Trusted Zone to 'Medium' and add the sites you use and trust to this zone; you may need to do this quite often as many badly-designed sites just won't work in high-security mode.