Security researchers are seeing a resurgence of Gumblar, the name for a piece of malicious code that is spread by compromising legitimate but unsecure Web sites.
In May, thousands of Web sites were hacked to serve up an iframe, which is a method of bringing content from one Web site into another. The iframe led to the "gumblar.cn" domain. Gumblar would then try to exploit the user’s PC via software vulnerabilities in Adobe Systems products such as Flash or Reader and then deliver malicious code.
Gumblar also changed its tactics. Rather than hosting the malicious payload on a remote server, the hackers are now putting that code on compromised Web sites, vendors IBM and ScanSafe say. It also appears Gumblar updated to use one of the more recent vulnerabilities in Adobe’s Reader and Acrobat programs, according to IBM’s Internet Security Systems Frequency X blog.
The hackers know that it’s only a matter of time before a malicious domain is shut down by an ISP. The new tactic, however, "gives them a decentralized and redundant attack vector, spread across thousands of legitimate Web sites around the world," IBM said.
To help avoid detection, the bad code that is uploaded to the legitimate Web sites is molded to match "existing file structures," IBM said. It is also scrambled or obfuscated to try to avoid detection.
"Gumblar is a force to be reckoned with, and this latest push of theirs is a true testament to that fact," IBM said.
The hackers behind Gumblar have also taken to forcibly injecting a malicious iframe into forums, according to a blog post from ScanSafe. This means people become victim to a so-called drive-by attack, where they are instantly exposed to malicious content from elsewhere when visiting a legitimate site.
Once a PC is infected, Gumblar also looks for other FTP credentials that it can use to compromise other Web sites. It hijacks the Internet Explorer browser, replacing Google search results with those of other Web sites, which IBM thinks may be connected with "pay-per-click" fraud.
In May, thousands of Web sites were hacked to serve up an iframe, which is a method of bringing content from one Web site into another.