A threat actor has deployed a WordPress backdoor that can hide its presence by posing as a legitimate plugin, WordPress security firm Defiant reports.
Identified during the cleanup of a compromised site, the backdoor was designed to run in the context of WordPress, thus having access to all the functions a normal plugin would have.
To avoid raising suspicion, the code also presented the user with a “professional looking opening comment implying it is a caching plugin”, Defiant’s Wordfence team explains.
Once installed, the malware adds specific filters so that it would not show in the list of activated plugins, while providing the attackers with a variety of functions, including the ability to create an admin account.
Able to operate as a standalone script and as a plugin, the backdoor also contains pinging functionality so that the threat actor can check if it is operational, and can activate and deactivate other plugins remotely.
According to the Wordfence team, the script can create an account with the username ‘superadmin’, with the role set as administrator, and with a hardcoded password. It also contains code to remove the account when it is no longer needed.
Additionally, the security firm identified a bot detection function in the code, which allows the malware to serve malicious content to users, based on specific filters.
“One common thread shared by these infection scenarios is that site owners find their site looks fine to them but their visitors have reported issues such as seeing spam or being redirected to dubious sites. Others report that their site looks and behaves completely normally but only shows spam content when visiting from a search engine,” the company explains.
Such malware typically serves the malicious content to search engines, so it can be indexed, to drive traffic to the infected website.
The backdoor also contains a hook to check if the current user is an administrator, and performs other checks as well, to serve them the unmodified site content. If these conditions are not met, the user is served malicious content instead, and other functions are invoked to insert spam on pages.
“The malware contains other cleanup functions that allow it to remove malicious content from the database in addition to the admin user deletion,” Defiant explains.
Additionally, the security firm identified specific malware functions that allow the attackers to remotely control and monetize the victim websites.