Dell SecureWorks Counter Threat Unit(TM) (CTU) researchers discovered malware that bypasses authentication on Active Directory (AD) systems that implement single-factor (password only) authentication. Threat actors can use a password of their choosing to authenticate as any user. This malware was given the name “Skeleton Key.”
CTU researchers discovered Skeleton Key on a client network that used single-factor authentication for access to webmail and VPN, giving the threat actor unfettered access to remote access services. Skeleton Key is deployed as an in-memory patch on a victim’s AD domain controllers to allow the threat actor to authenticate as any user, while legitimate users can continue to authenticate as normal. Skeleton Key’s authentication bypass also allows threat actors with physical access to login and unlock systems that authenticate users against the compromised AD domain controllers.
The only known Skeleton Key samples as of this publication lack persistence and must be redeployed when a domain controller is restarted. CTU researchers suspect that threat actors can only identify a restart based on their inability to successfully authenticate using the bypass, as no other malware was detected on the domain controllers. Between eight hours and eight days of a restart, threat actors used other remote access malware already deployed on the victim’s network to redeploy Skeleton Key on the domain controllers.
Skeleton Key requires domain administrator credentials for deployment. CTU researchers have observed threat actors deploying Skeleton Key using credentials stolen from critical servers, administrators’ workstations, and the targeted domain controllers.