A large number of websites have been compromised over the past days by cybercriminals who are exploiting a recently patched security hole in a popular newsletter plugin for WordPress.
Researchers from Sucuri started spotting malware injections on WordPress websites last week, but they initially couldn’t determine the attack vector. In a blog post published on Wednesday, Sucuri CTO and founder Daniel Cid revealed that a flaw in the MailPoet WordPress plugin is responsible for the massive malware infection.
Sucuri published an advisory on the MailPoet vulnerability at the beginning of July, when the company warned users to update their installations immediately to prevent cybercriminals from taking over their websites. The flaw can be exploited to upload any PHP file to affected sites. In this case, the attackers are uploading a malicious theme that contains a backdoor.
“The Backdoor is very nasty and creates an admin user called 1001001. It also injects a backdoor code to all theme/core files. The biggest issue with this injection is that it often overwrites good files, making very hard to recover without a good backup in place,” Cid said.
The security firm estimates that around 50,000 websites have been compromised, but the number could increase considerably because the MailPoet plugin has been downloaded nearly 2 million times from the official WordPress website. Furthermore, other types of websites hosted on the same server as the compromised WordPress site could also be affected.
“Only WordPress sites can be compromised by the vulnerability. But once the attackers’ code succeeds compromising the WordPress site, it attempts to also inject malware into all neighboring sites within the same server/account. So you may see other CMS’s compromised because of that,” Cid told SecurityWeek.
MailPoet developers addressed the vulnerability with the release of version 2.6.8, so users are advised to upgrade the plugin to this or a newer version.