Over 23,000 Web servers were infected with a backdoor called CryptoPHP that's bundled with pirated themes and plug-ins for popular content management systems.
CryptoPHP is a malicious script that provides remote attackers with the ability to execute rogue code on Web servers and to inject malicious content into websites that are hosted on them.
According to Dutch security firm Fox-IT, which published a report about the threat last week, the backdoor is used primarily for black hat search engine optimization (BHSEO), an operation that involves injecting rogue keywords and pages into compromised sites to hijack their search engine rankings and push malicious content higher up in search results.
Unlike most website backdoors, CryptoPHP is not installed by exploiting vulnerabilities. Instead attackers distribute pirated versions of commercial plug-ins and themes for Joomla, WordPress and Drupal through several sites and wait for webmasters to download and install them on their own websites. Those pirated plug-ins and themes have the CryptoPHP backdoor embedded into them.
Web servers infected with CryptoPHP act as a botnet. They connect to command-and-control servers operated by the attackers using an encrypted communication channel and listen for commands.
With help from the Dutch government's National Cyber Security Center and the Abuse.ch, Shadowserver Foundation and Spamhaus cybercrime fighting organizations, Fox-IT took control of the CryptoPHP command-and-control domains and directed them to servers under its control to gather statistics -- an operation known as sinkholing.
"In total 23,693 unique IP addresses connected to the sinkholes," Fox-IT's researchers said Wednesday in a blog post. However, the number of affected websites is likely higher, because some of those IP addresses correspond to shared Web hosting servers that have more than one infected sites.
The top five countries for CryptoPHP infections were the U.S. (8,657 IP addresses), Germany (2,877 IP addresses), France (1,231 IP addresses), the Netherlands (1,008 IP addresses) and Turkey (749 IP addresses).
Since Fox-IT published its CryptoPHP report last week, attackers took down the websites that hosted the rogue plug-ins and themes and set up new ones. They also pushed out a new version of the backdoor, possibly in an attempt to evade detection.
The Fox-IT researchers released two Python scripts on GitHub that webmasters can use to scan servers and sites for CryptoPHP infections. They also provided removal instructions in their blog post, but noted that ultimately it's best to reinstall an affected content management system completely since its integrity may have been compromised.