Pierluigi Paganini October 20, 2019

Threat actors leverage malicious plugins that hide in plain sight to backdoor WordPress websites and to use them for brute-forcing other sites.

The use of fake WordPress plugins installed by hackers is not a novelty, recently at Sucuri observed multiple infections aimed at installing fake plugins with backdoor capabilities.

Attackers use automated tools to create malicious WordPress plugins or by and include in their code malicious payloads such as web shells.

The researchers spotted some fake plugins with backdoor functionality, two of them named initiatorseo or updrat123 were based on the structure of the popular backup/restore WordPress plugin UpdraftPlus.

The UpdraftPlus WordPress plugin has more than 2 million active installations and its contributors regularly update it.

“While their code differs in terms of variable names, the malicious plugins do share a few things in common: they possess a similar structure along with header comments from the popular backup/restore plugin UpdraftPlus.” reads the post published by Sucuri.

“The metadata comments within these fake plugins include copies from version 1.16.16 of UpdraftPlus, which was released on July 23rd, 2019,” found researchers at web security and protection company Sucuri”

The malicious WordPress plugins hide in the WordPress dashboard and are visible only by anyone who use browsers with specific User-Agent strings that vary from plugin to plugin

The attacker could verify the presence of the malicious plugin using a GET request with custom parameters such as initiationactivity or testingkey.

The fake WordPress plugins allow attackers to establish a backdoor on the compromised sites and to provide them with access to the servers even after the original infection vector was removed.

The backdoors are used to upload arbitrary files for malicious purposes to the compromised servers using POST requests.

“Malicious requests come in the form of POST parameters, which specify a remote URL for the file download locations, along with the path and name of the file to be created on the compromised server.” continues the post.

“So far, the names of these POST parameters have been unique for each plugin that we’ve analyzed.”

Post requests contain parameters such as the URL where are located the payloads to download, or the path where the files should be written on the compromised servers.

Sucuri researchers also observed attackers using fake plugins to upload files with random names (i.e. 5d9196744f88d5d9196744f893.php) to site root directories. These files contain a script that threat actors use to carry out brute force attacks on other sites.

“Hackers want to maintain access to websites as long as they can. To accomplish this, they upload various backdoors into random files scattered across the whole site. Sometimes backdoors come in the form of WordPress plugins that might not even be visible from the admin interface.” concludes Sucuri.

“Additionally, compromised websites may be used for malicious activity that is completely invisible from outside, including DDoS and brute-force attacks, mailing tons of spam, or cryptomining.“

Pierluigi Paganini

(SecurityAffairs – WordPress plugins, backdoor)

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