Hackers are exploiting a problem with an image-resizing utility called TimThumb that is widely used in many themes for the blogging platform WordPress, although some fixes have been made to the latest version.
Mark Maunder, the CEO of Feedjit, discovered the problem when his own blog started loading ad content when previously his blog contained no ads. He blogged about the problem, tracing it to an issue with the "timthumb.php" library, which is used within the theme he purchased for his blog.
"Since it's already in the wild and I just got hacked by it, I figure it's OK to release the vulnerability to the general public," Maunder wrote.
TimThumb is "inherently insecure" because it writes files into a directory when it fetches an image and resizes it. But that directory is accessible to people visiting the website, Maunder wrote. An attacker can compromise the site by figuring out how to get TimThumb to grab a malicious PHP file and put it in the Wordpress directory. The code will be executed if an attacker then accesses the file using a Web browser.
Maunder explained how to disable TimThumb's ability to load images from external sites, but the most sure-fire way to stop the problem is to remove TimThumb or limit its access to other websites. Aside from that, users should update to the latest version of TimThumb.
The developer of TimThumb, Ben Gillbanks, was the first to comment on Maunder's post. "I can't apologize enough for this oversight in the code and hope nobody has anything too bad happen to their sites because of my error."
Gillbanks recommended that people use the latest version of TimThumb. "There have been a stack of tweaks that will make the script harder to abuse," Gillbanks wrote.