Dialers are necessary to connect to the internet (at least for non-broadband connections), but some dialers are designed to connect to premium-rate numbers. The providers of such dialers often search for security holes (usually in Microsoft Windows) on the user's computer and use them to change the computer to dial up through their number, pocketing the additional money for themselves. Alternatively, some dialers inform the user what it is that they are doing, with the promise of special content, accessible only via the special number. Examples of this content include software for download, (usually illegal) MP3s, pornography, and in the case of at least one website, 'underground' hacking materials such as viruses.

The cost of setting up such a service is relatively low, amounting to a few thousand dollars for telecommunications equipment, whereupon the unscrupulous operator will typically take 90% of the cost of a premium rate call, with very few overheads of their own.

Premium rate numbers, e.g. 900 numbers in the U.S., are an increasingly popular method for easy electronic payment for services. But if they are not careful, users could be charged up to $4.00 or more per minute through a dialer using one of these numbers. Typically, users will stay connected for at least 10 minutes, as the internet and modems in particular are inherently slow, and so the costs can really rack up.

Users with DSL lines (or similar broadband connections) are usually not affected. A dialer can be downloaded and installed, but dialing in is not possible as there are no regular phone numbers in the DSL network and users will not typically have their dialup modem, if any, connected to a phone line. However, if an ISDN adapter or additional analog modem is installed, the dialer might still be able to get a connection.

Malicious dialers can be identified by the following characteristics:

 

• A download popup opens when opening a website.
• On the website there is only a small hint, if any, about the price.
• The download starts even if the cancel button has been clicked.
• The dialer installs as default connection without any notice.
• The dialer creates unwanted connections by itself and without user interaction.
• The dialer does not show any notice about the price (only few do) before dialing in.
• The high price of the connection is not being shown while connected
• The dialer cannot be uninstalled, or only with serious effort.