The threat from Cerber ransomware has increased substantially after the gang behind the file-encrypting software have leveraged Dridex botnets to deliver a malicious payload that loads the ransomware onto users’ devices.

Cerber ransomware was first discovered in the wild in February 2016, but researchers at security firm FireEye noticed a massive increase in infections in recent weeks. Initially, Cerber ransomware infections occurred as a result of visiting malicious websites hosting the Nuclear or Magnitude exploit kits. Nuclear and Magnitude probe visitors’ browsers for a number of zero day vulnerabilities, although infections primarily occurred by exploiting a vulnerability in Adobe Flash (CVE-2016-1019). Now the ransomware is being installed via infected files sent via spam email.

Cerber differs from many ransomware strains by being able to speak to victims. The ransomware is able to use text-to-speech to tell victims they have been infected and that their files have been encrypted.

Massive Increase in Cerber Ransomware Infections Discovered in April

The number of infections remained relatively low since the discovery of the new ransomware earlier this year; however, there was a massive spike in infections around April 28 according to FireEye. The ransomware was being downloaded using Microsoft Word macro downloaders.

The attached files are usually disguised as invoices, receipts, or purchase orders, while the emails – written in English – urge the user to open the attachment. If macros are enabled on the computer a VBScript will be installed in the victim’s %appdata% folder. If macros are not enabled users will be prompted to enable them in order to view the contents of the file. Doing so will guarantee infection.

Once installed, the script performs a check to determine whether the infected computer has an Internet connection by sending an HTTP request to a website. If an Internet connection is present, the script will perform a HTTP Range Request, that will ultimately result in the final stage of the infection. FireEye reports the technique has previously been used to deliver the financial Trojans Dridex and Ursnif.

Cerber has been configured to encrypt Word documents, emails, and Steam gaming files, which are given a “.cerber” extension. To unlock the encryption, the victims are told to visit one of a number of websites with the domain “decrypttozxybarc”. Further instructions are then provided on how to unlock the encryption, although a Bitcoin ransom must first be paid. In addition to encrypting files, Cerber ransomware adds the victim’s computer to a spambot network.

The ransomware uses a number of obfuscation techniques to avoid detection by spam filters and anti-virus programs. If the emails are delivered and the macros are allowed to run, victims’ files will be encrypted. To prevent infection, it is important to have macros disabled and to be extremely cautious about opening email attachments, and never to open files deliver via email from an unknown sender. The decrypttozxybarc domain should also be added to web filter blacklists.