Once upon a time surfers could stay unmolested by malware by staying away from warez and smut. Those days are well and truly over as changes in hacking tactics mean that compromised content on legitimate website has become the main conduit for so-called drive-by download attacks.

Web security firm ScanSafe reports that two in three instances of web-based malware (68 per cent) it blocked last month came from legitimate sites. ScanSafe blames the increase on attacks that have planted malicious scripts, often exploiting iFrame web browser vulnerabilities, on pukka websites. Hacked sites are commonly used to deliver password-stealing Trojans and other strains of malware onto compromised PCs.

For example, ScanSafe reported earlier this week that some pages on the Wal-Mart website were compromised in the latest phase of an ongoing series of SQL injection attacks. The attack was used to plant exploits of recent Flash vulnerabilities onto Wal-Mart's site. High-profile victims of malware attacks in May alone included Nature.com, Foofighterslive.com, Acer.co.th, Webster.edu and Photopass.com.

Large-scale SQL Injection attacks started around six months ago in October 2007 and are affecting mom and pop website operations as well as household names. Attacks based on stolen FTP are also playing a significant (albeit secondary) role, according to ScanSafe.

This evolution in tactics by black hat hackers means that miscreants are able to quickly 'colonize' thousands of legitimate sites with malware. ScanSafe reports a 220 per cent increase in the amount of Web-based malware over the last twelve months. The volume of backdoor and password-stealing malware blocked by the firm increased by an order of magnitude (855 per cent) between May 2007 to May 2008.

"Over the last year malware authors have moved away from direct attacks — attacks in which they directly interact with victims, via social engineering for example — to indirect attacks accomplished through compromised websites," said Mary Landesman, senior security researcher at ScanSafe.

"Currently, thousands of legitimate sites are being compromised daily. The net result is that you absolutely cannot assume that because you are on a brand name or well known site that it is a safe site," she added.

ScanSafe's analysis is based on the 10 billion web requests ScanSafe scans each month on behalf of its corporate customers. Its study, entitled A Comparative Look at the State of Web Security, May 2007-May 2008, can be found here. ®

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