A new fileless malware has been detected that uses DNS to receive commands and send information to the attackers’ command and control server. The stealthy communication method together with the lack of files written to the hard drive makes this new malware threat almost impossible to spot.
The attack method, termed DNSMessenger, starts with a phishing email, as is the case with many of the new malware threats now being detected. The host is infected via a malicious Word document.
Opening the Word document will display a message informing the user that the document has been protected using McAfee Secure. The user is required to enable content to view the document; however, doing so will call a VBA function that defines the Powershell command and includes the malicious code. As is the case with other forms of fileless malware, since no files are written to the hard drive during the infection process, the threat is difficult to detect.
Fileless malware are nothing new, in fact they are becoming increasingly common. What makes this threat unique is the method of communication it uses. The malware is able to receive commands via the DNS – which is usually used to look up Internet Protocol addresses associated with domain names. The malware sends and received information using DNS TXT queries and responses.
DNS TXT records are commonly used as part of the controls organizations have in place to identify phishing emails and verify the sender of a message – Sender Policy Framework (SPF) and Domain Message Authentication Reporting and Conformance (DMARC).
The attackers can send commands to the malware via DNS TXT queries and the malware can send the attackers the output of the commands via the same channel. Even if an organization has blocked outbound DNS for unapproved servers, the malware will still be able to communicate with the attackers C2 infrastructure.
While many organizations inspect the contents of web traffic, relatively few inspect the content of DNS requests. The malware is therefore likely to operate unnoticed. Further, the Cisco Talos team that detected the malware reports that only 6/54 AV engines detected the threat, although ClamAV did identify the file as malicious.
Cybercriminals are constantly looking for new methods of bypassing security controls and infecting end users. However, since this threat is delivered via email, that is the point at which it is easiest to block. Infection also requires macros to be enabled. If macros are blocked, the malware will not be executed. Otherwise, since the DNS communications between the malware and the attackers differs from standard DNS communications, inspecting DNS content should enable security professionals to identify infection.