TrickBot Drops an Anchor

Boston-based security firm Cybereason's Nocturnus Research Group has found a threatthat comes from a new malware that it calls Anchor. Anchor has been used in campaigns against financial, manufacturing and retail businesses across the US and Europe. These new attacks focus on stealing information from POS systems and other sensitive resources in the victims' network by compromising critical assets.

Cybereason says that the dropping anchor campaign started with a TrickBot infection and progressed into an operation targeting sensitive financial systems. The threat actors seems to selectively use a new variant of the rare Anchor_DNS tool. Anchor_DNS is a backdoor that uses the DNS protocol to stealthily communicate with C2 servers. In addition to the new Anchor_DNS variant, the attackers use a completely new and previously undocumented malware dubbed Anchor.

Cybereason notes that the Anchor malware is "a backdoor used very selectively on high-profile targets, and appears to be tightly connected to TrickBot, potentially even authored by the same individuals who created TrickBot."

It found that some of the tools and techniques detailed in its report resemble past attacks that were linked to the financially motivated FIN6 threat actor, a group that is known to target POS systems and has been linked to TrickBot infections in the past.

The attack starts with a phishing email that contains a malicious link to a file hosted on Google Docs named "Annual Bonus Report.doc". This differs from the usual email attachment. After TrickBot establishes Internet access and sends information about the location of the target machine, it starts its malicious activity.

The attacker also uses PowerShell to test DNS entry settings. They use the command -q=srv_kerberos_tcp on the process nslookup.exe to open an interactive shell. They use the shell to expand their search to other machines on the network by searching for things like a list of the domain controllers.

If they find they are on a high-value target, they escalate their efforts by switching to interactive hacking: reconnaissance, credential dumping, lateral movement and in some cases the mass deployment of ransomware across endpoints that are connected to the domain controller.

Anchor and Anchor_DNS are both directly linked to TrickBot infections, since they are downloaded by TrickBot as secondary payloads after the primary one has been installed.

The Anchor payload is delivered by AnchorInstaller which unpacks the Anchor DLL and drops it in the %SYSTEMROOT% or %SYSTEMROOT%\System32 folder. The dropped DLL is loaded by the service netTcpSvc, which is created by the malware.

The malware's binary is signed, which is meant to provide a level of credibility and integrity to a binary from the developer, and to guarantee that the binary has not been tampered with. It can also bypass some security solutions that grant trust to signed binaries.

Anchor as well as older versions of Anchor_DNS implement the exact same self deletion routine using two sets of commands to ensure that the dropper is deleted once the malware was successfully deployed.

Anchor is part of the evolution of TrickBot into a multi-phasic threat that can not only do ransomware, but steal information as part of its operation.

— 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.