Skeleton Key Malware Linked to Backdoor Trojan: Symantec

Researchers at Symantec say a recently discovered piece of malware aimed at Active Directory may be linked to a separate malware family used in attacks against targets in the U.S. and Vietnam.

Earlier this month, researchers from Dell SecureWorks identified malware they called ‘Skeleton Key.’ The malware was discovered on a client network that used single-factor authentication for access to webmail and VPN – giving the threat actor total access to remote access services. According to Dell SecureWorks, the malware is deployed as an in-memory patch on a victim’s Active Directory domain controllers. In the cases they found, the attackers used the PsExec tool to run the Skeleton Key DLL remotely on the target domain controllers using the rundll32 command. 

“Symantec has analyzed Trojan.Skelky (Skeleton Key) and found that it may be linked to the Backdoor.Winnti malware family,” blogged Symantec researcher Gavin O’Gorman. “The attackers behind the Trojan.Skelky campaign appear to have been using the malware in conjunction with this back door threat. It’s unclear if the malware family Backdoor.Winnti is used by one attack group or many groups.”

According to Symantec, Backdoor.Winnti has been used in the past in a number of different campaigns, in particular against Asian gaming companies. It is not known to Symantec if the malware is being used by one set of attackers or many.

When Dell SecureWorks researchers revealed their data on Skeleton Key, they noted that the Skeleton Key samples they were of lacked persistence and had to be redeployed when a domain controller is restarted. Between eight hours and eight days of a restart, the threat actors used other remote access malware already deployed on the victim’s network to redeploy Skeleton Key on the domain controllers, the researchers said.

According to Symantec’s telemetry, the Skeleton Key malware was identified on compromised computers in five organizations with offices in the United States and Vietnam, he explained. The exact nature and names of the affected organizations are unknown to Symantec; however the first activity was seen in January 2013 and lasted November 2013.

“In November 2013, the attackers increased their usage of the tool and have been active ever since,” O’Gorman noted. “Four more variants of Trojan.Skelky were discovered as well as additional file names used by the attackers.”

Since the first observed use of the tool to the present, the attackers have consistently used the same password. This is the case with three different variants of the tool, O’Gorman explained. The regular use of the same password across multiple variants means it is likely that only one group of attackers had been using the tool until at least January 2015, he added.

“There were almost no signs of other malware active at the same time as Skelky in most of the organizations investigated,” O’Gorman blogged. “However, two compromised computers had other malware present, active, and in the same directory, at the same time as Trojan.Skelky. Two files were discovered on one of the victim’s computers. One file is a variant of Backdoor.Winnti (jqs.exe) and the other is a dropper for Backdoor.Winnti (tmp8296.tmp), which is responsible for creating the Backdoor.Winnti sample.”

“Symantec is continuing its investigation into this malware family Backdoor.Winnti and the specific actors behind the combined use of Backdoor.Winnti and Trojan.Skelky,” he blogged.