Ransomware, or software that takes control of a computer and demands money in exchange for access to files, is one type of Trojan Horse program growing in popularity, according to Sophos' mid-year security report issued on Wednesday.

The existence of Trojans, or malware masquerading as legitimate computer programs, now outnumbers viruses and worms four to one, according to the Sophos report, which is a mid-year update to the company's annual security report released in December of 2005. Ransomware is a new and particularly vexing threat, since it confiscates a user's files and forces payment in exchange for access.

One example of ransomware is Zippo, which was released in March and demands US$300 for users to regain access to their own encrypted files. Another example is Ransom-A, which demands US$10.99 paid through Western Union and threatens to delete a file every 30 minutes until payment is made.

Malware that is targeted at a small group of Internet users and often leads to financial gain is becoming more popular than the mass-mailed worms and viruses of the last few years, says the report. Cyber criminals today are more interested in gaining financially that being able to brag about a virus' proliferation, and avoid using tactics that are sent out to large numbers of users for fear of getting caught, the report says.

E-mail viruses are down to one in every 91 pieces of mail for the first six months of this year, vs. one in every 35 reported for the same period last year.

In the spam world, unsolicited messages that pump up a certain stock were popular in the first half of the year, says the report. In this scam, spammers buy penny stocks and tout the value of the stock through spam, which inflates the stock price so that spammers can sell the stock at a profit. One example occurred in mid-June with a company called Southern Cosmetics, which a spammer touted in a spam blast until the stock's price went up by a factor of six.

In general, the number of threats plaguing Internet users continues to grow; Sophos identified 180,292 different viruses, spyware, worms, Trojans and other malware during the first half of 2006, up from 140,118 during the same period last year, according to the report.