Researchers from security firm Zscaler's ThreatLabZ found themselves lately looking at WordPress and Joomla sites that were serving Shade/Troldesh ransomware (which has been known since 2014), backdoors, redirectors and a variety of phishing pages. Both of these content management systems (CMS) have recently had a rough month from a security standpoint, with discovered threats originating from plugins, themes and extensions.
ThreatLabZ observed vulnerable WordPress versions from 4.8.9 to 5.1.1.
The researchers found that the attackers were using a "well-known" hidden directory present on HTTPS websites for storing and distributing Shade ransomware and phishing pages. They were hiding attack components right under the noses of sysadmins.
The hidden /.well-known/ directory mechanism is a URI prefix for locations defined by the Internet Engineering Task Force (IETF). The administrators of HTTPS websites that use Automatic Certificate Management Environment (ACME) for managing SSL certificates will place a unique token inside the /.well-known/acme-challenge/ or /.well-known/pki-validation/ directories to validate their claim of ownership of that domain.
The SSL's CA will give a specific code to the site's admins which they will put in this particular directory. The CA then looks for it to be present there, establishing that the site belongs to whom they think it does. It's like a multi-factor authorization scheme for HTTPS sites.
But attackers have been putting their own stuff in there as well. This works well for the attacker since the directory is already present on most HTTPS sites. It is also hidden, increasing the survivability of the malicious content since it may just be ignored by routine scans.
The activated malware uses a TOR client to connect to its command and control (C&C) server and will encrypt (with AES-256 in CBC mode with two different keys) both the content and name of targeted files. Phishing pages in the SSL-validated hidden directories were also observed. They are spoofing Office 365, Microsoft, DHL, Dropbox, Bank of America, Yahoo, Gmail, along with some others.
The obvious mitigation here is to upgrade/patch WordPress so an attacker can not get in and place their files. But if a site has already been compromised, paying attention to these hidden directories may be the only way to clean out an established malware infestation.
— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.