Mass attacks that exploit a known vulnerability in the WordPress publishing platform have continued to bear fruit for hackers, with thousands of websites claimed in the past few weeks, a researcher said.
The security bug, in a widely used image resizing utility known as TimThumb, allows attackers to seize control of WordPress websites, one of the victims warned nine weeks ago. A few days later, a security researcher found almost 4,400 WordPress sites had been commandeered in an attack that poisoned Google Image results with sites that attempted to trick users into installing counterfeit antivirus software. He speculated the cause was the same TimThumb exploit.
Although a fix for the TimThumb vulnerability has been available for more than two months, plenty of websites remain vulnerable. According to a research report published by Avast on Monday, thousands of websites have been infected by Black Hole, a hack-by-numbers toolkit available in underground forums for about $1,500 or for free for a scaled-down version. The kit installs an iframe in infected sites that silently redirects visitors to malicious sites.
“The bad guys are using a security vulnerability in non-updated TimThumb,” Avast researcher Jan Sirmer wrote. “This allows attackers to upload and execute arbitrary PHP code in the TimThumb cache directory which will download other malicious files.”
Avast alone blocked the redirection attempts from 3,500 unique websites in August and 2,515 sites last month, and Sirmer said he expects to see similar results this month. That may be only a small percentage of the total number of infected sites, since Avast is used by a small minority of people browsing the web. Sirmer said attackers may have compromised some of the websites by exploiting weak passwords.
Once a site is infected, it’s not always easy to remove all the malicious code. Denis Sinegubko, the Russian researcher who discovered the WordPress attack used to poison Google Image results, has advised webmasters of compromised sites to look for rogue rules in the .htaccess files in the site root and above the site root directory. He has more here.