Hackers have hit on a new technique for invading desktop computers via compromised Web sites, while avoiding anti-virus detectors, according to the SANS Institute.

SANS' Internet Storm Center (ISC) said on Thursday it has come across the attack on a compromised Web site, where an iframe was used to deploy various pieces of malicious code via JavaScript; iframes allow content from one Web site to be embedded in another Web site.

This technique itself isn't new, but researchers found that the server deploying the malicious JavaScript was heavily modifying it -- "obfuscating" it -- so as to be undetectable by anti-virus detectors, the ISC said. Moreover, the obfuscations were generated randomly and on the fly, according to ISC handler Bojan Zdrnja.

"What makes this new is that the hosting Web site generates this code dynamically," he wrote in an analysis. "Every time you request this Web page, it will use completely random names for all variables and functions ... changing variable and function names even causes the payload information to change."

The technique makes the script code effectively undetectable by common types of malware scanners, Zdrnja said.

"Such heavy obfuscation makes signature-based detection much more difficult, if not impossible," he wrote. None of the anti-virus programs Zdrnja tested were able to detect the modified code.

The code contains what Zdrnja called a "typical" set of exploits, making use of known vulnerabilities in ADODB, QuickTime, WinZip, and other software.

The code also included a less well-known, but highly pernicious exploit for the NCTAudioFile2 ActiveX control, Zdrnja said.

"A fully working exploit was publicly released in April, and what's worse is that the affected ActiveX control is delivered with dozens of popular audio/video applications," Zdrnja wrote.