Cybercriminals are utilizing hidden “well-known” directories of HTTPS sites to store and serve malicious payloads, Zscaler security researchers have discovered.
Compromised WordPress and Joomla websites were observed serving Shade/Troldesh ransomware, coin miners, backdoors, redirectors, phishing pages, and other threats. The compromised WordPress sites were running versions 4.8.9 to 5.1.1 of the content management system (CMS).
The attackers supposedly managed to gain access to the installations via outdated CMS plugins/themes or server-side software, Zscaler notes.
Common to the compromised websites, the researchers say, was the use of SSL certificates issued by Automatic Certificate Management Environment (ACME)-driven certificate authorities, such as Let’s Encrypt, GlobalSign, cPanel, and DigiCert, among others.
The attackers apparently abused a well-known hidden directory in the HTTPS sites for storing their malicious payloads. The directory is a URI prefix for well-known locations defined by IETF and used to demonstrate ownership of a domain.
For HTTPS websites that use ACME to manage SSL certificates, the administrators place a unique token inside the folder, to show the certificate authority (CA) that they control the domain. The CA sends a specific code they place in the particular directory, and the CA scans it to validate the domain.
“The attackers use these locations to hide malware and phishing pages from the administrators. The tactic is effective because this directory is already present on most HTTPS sites and is hidden, which increases the life of the malicious/phishing content on the compromised site,” Zscaler notes.
The security researchers discovered a variety of threat types in the hidden directory in the past month, with the Shade/Troldesh ransomware being the most prevalent, followed by phishing pages.
In compromises involving the Shade/Troldesh ransomware, three types of files were found in each compromised website, namely HTML, ZIP, and EXE files masquerading as .jpg images.
Spam is commonly used to spread the ransomware, either using attached ZIP files or links to HTML files that redirect to the ZIP files. The archives contain a JavaScript file that downloads the payload and executes it.
The payload is a new variant of Shade/Troldesh ransomware, which has been around for half a decade. The malware uses a TOR client to connect to the command and control (C&C) server and encrypts both the content and name of targeted files.
The phishing pages observed in this campaign are related to Office 365, Microsoft, DHL, Dropbox, Bank of America, Yahoo, Gmail, and other brands, the security researchers say.
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