More than 100,000 WordPress-hosted websites appear to be infected with malware that redirects visitors to a malicious site, downloads rogue files on their systems or sometimes does both.

On Sunday, security firm Sucuri issued an alert on the threat and said that Google has already blacklisted more than 11,000 domains that are infected with the malware.

“We believe that number continues to increase as the payload continues to evolve,” Sucuri CEO Tony Perez said in an email. “It is possible new sites are being hit as we’re seeing constant updates to the exploit packages. As environments get owned, the attackers are spreading through the hosts like what you’d expect of traditional worms.”

Infected sites have been observed redirecting users to SoakSoak.ru, the website of the group behind the attack. The malware modifies the wp-includes/template-loader.php file of WordPress websites, causing malicious JavaScript to be loaded on every page view of the compromised website, Sucuri said in its alert.

Several owners of WordPress websites complained of the problem in an online forum.

“I’m running 4.0.1, and all my sites (6) on HostGator are not accessible anymore since this morning using Chrome,” one forum member noted. “It says they are all infected.”

Another commenter cited the same problems with an Amazon-hosted website.

“My site just got the same issue from that same attack site and around the same time,” the member said.

Vulnerable Plugin

The exact method used by the attackers to infect the websites remains unconfirmed, Perez said. However, a preliminary analysis of the malware suggests the attackers are most likely taking advantage of vulnerabilities in a WordPress plugin product. It is becoming evident that the initial vector appears to be associated with a previously known vulnerability in the WordPress Slider Revolution Premium plugin, Perez said.

“The vector seems to be via WordPress plugins, but once in, the attacker is able to own and infect any platform regardless of platform,” he said.

Earlier this year, Sucuri warned that thousands of WordPress websites were running outdated and vulnerable plugins and being compromised in a fashion similar to the SoakSoak threat. WordPress can do little to mitigate such threats because the problems have to do with the plugins that people use with the software, he added.

Widespread Issue

WordPress powers more than 70 million websites worldwide and is easily one of the most popular publishing platforms currently in use. However, the widespread use of unvetted plugins by many WordPress administrators and a failure by many to keep their sites properly updated have made the sites relatively easy cybercriminal targets.

WP White Security, a company that specializes in WordPress security, estimated last year that more than 70 percent of WordPress websites are vulnerable to cyberattacks. Researchers at the company examined the top 42,000 WordPress websites and found 74 different versions of the software being used, including 11 that were obsolete. More than 13,000 websites were discovered running a version of WordPress with known security vulnerabilities.

“It takes a malicious attacker only a couple of minutes to run automated tools that can discover such vulnerabilities and exploit them,” the report noted.

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