The US Department of Homeland Security's Cybersecurity and Infrastructure Security Agency (CISA) has published a security alert warning of cybercriminals using phishing emails to deploy KONNI malware on target machines.
KONNI is a remote administration tool (RAT) attackers use to steal files, capture keystrokes, take screenshots, and execute malicious code on infected machines. It's often spread via phishing emails containing a Microsoft Word file with a malicious Visual Basic Application (VBA) macro code to deploy the malware.
The malicious code can change a victim machine's font color from light grey to black, to fool users into enabling content. KONNI can check if the Windows OS is a 32-bit or 64-bit version and build and execute the command line to download more files. After the VBA macro constructs the command line, it uses the certificate database tool CertUtil to download remote files from a given Uniform Resource Locator. It also uses a built-in function to decode base64-encoded files.
The Command Prompt silently copies certutil[.]exe into a temp directory and renames it to evade detection, officials explain in an advisory.
Attackers then download a text file from a remote resource containing a base64-encoded string that is decoded by CertUtil and saved as a .BAT file. They then delete the text file from the temp directory and execute the .BAT file.
CISA provides several recommendations to businesses for avoiding this kind of attack. Among these are maintaining up-to-date antivirus signatures and engines; keeping operating systems updated, disabling file and printer sharing servers, and restricting users' ability to install and run unwanted software applications. Officials advise against adding users to the local admins group.
Read the full advisory here.