IDG News Service — Web sites such as Twitter are becoming increasingly favored by hackers as places to plant malicious software in order to infect computers, according to a new study covering Web application security vulnerabilities.

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Social-networking sites were the most commonly targeted vertical market according to a study of hacking episodes in the first half of the year. The study is part of the latest Web Hacking Incidents Database (WHID) report, released on Monday. In 2008, government and law enforcement sites were the most hit vertical.

Social networks are "a target-rich environment if you count the number of users there," said Ryan Barnett, director of application security research for Breach Security, one of the report's sponsors, which also includes the Web Application Security Consortium.

Twitter has been attacked by several worms, and other social-networking platforms such as MySpace and Facebook have also been used to distribute malware. That's often done when an infected computer begins posting links on social-networking sites to other Web sites rigged with malicious software. Users click on the links since they trust their friends who posted the links, not knowing their friend has been hacked.

The WHID sample set is small, encompassing 44 hacking incidents. The report only looks at attacks that are publicly reported and those with which have a measurable impact on an organization. The WHID's data set is "statistically insignificant" compared to the actually number of hacking incidents, but shows overall attacker trends, Barnett said.

Other data showed how Web sites were attacked. The most common attack was SQL injection, where hackers try to input code into Web-based forms or URLs (Uniform Resource Locators) in order to get back-end systems such as databases to execute it. If the input is not properly validated -- and malicious code ignored -- it can result in a data breach.

Other methods used include cross-site scripting attacks, where malicious code gets push to on a client machine, and cross-site request forgery, in which a malicious command is executed while the victim is logged into a Web site.

The WHID found that defacing Web sites is still the most common motivation for hackers. However, the WHID includes the planting of malware on a Web site as defacement, which also points to a financial motivation. Hacked computers can be used to send spam, conduct distributed denial-of-service attacks and for stealing data.

"Ultimately they [the hackers] want to make money," Barnett said.