Detecting and preventing privilege escalation attacks leveraging Kerberos relaying (KrbRelayUp)

 Post from Microsoft

On April 24, 2022, a privilege escalation hacking tool, KrbRelayUp, was publicly disclosed on GitHub by security researcher Mor Davidovich. KrbRelayUp is a wrapper that can streamline the use of some features in Rubeus, KrbRelay, SCMUACBypass, PowerMad/SharpMad, Whisker, and ADCSPwn tools in attacks.

Although this attack won’t function for Azure Active Directory (Azure AD) joined devices, hybrid joined devices with on-premises domain controllers remain vulnerable. Microsoft Defender for Identity detects activity from the early stages of the attack chain by monitoring anomalous behavior as seen by the domain controller. In addition, signals from Defender for Identity also feed into Microsoft 365 Defender, providing organizations with a comprehensive solution that detects and blocks suspicious network activities, malicious files, and other related components of this attack. Microsoft Defender Antivirus detects this attack tool as the malware family HackTool:MSIL/KrbUpRly.

Microsoft encourages customers to update Domain Controller: LDAP server signing requirements to Require signing as detailed in this advisory and enable Extended Protection for Authentication (EPA) as detailed in this blog.

Understanding the attack: What is resource-based constrained delegation?

Resource-based constrained delegation (RBCD) represents the key to this attack method, enabling the tool to impersonate an administrator and eventually run a code as the SYSTEM account of a compromised device.

Authentication protocol basics

An authentication protocol verifies the legitimacy of a resource or identity. When a user signs into a website, that website uses a methodology to confirm the authenticity of the resource requesting access. In simpler terms, the authentication process involves signing in with a password—made possible by the user knowing the password anticipated by the website. The Kerberos protocol serves as the main authentication framework for this process in on-premises Active Directory.

Delegation

Sometimes, however, a resource needs to request access to another resource on behalf of a different identity. A common example of this is mail delegation, wherein executives often give delegation rights to their executive assistants to send and receive emails on their behalf without providing the assistant with the executive’s password. The executive assistant isn’t authenticating as the executive; the executive has just allowed the assistant’s account to “pretend” that they are.

Resource-based constrained delegation

Initially, only users with the SeEnableDelegation role could configure delegation, typically domain admins. These domain admins can manage resources and dictate which identities can act on behalf of a different resource. They achieve this by updating the msDS-AllowedToDelegateTo property of a user account or device. This property contains a list of all the unique identifiers (service principal names, or SPNs) to which this object can delegate or act on behalf of.

However, as organizations expanded, administrators struggled to manage all the delegation requirements, raising the need for a new type of delegation: resource-based. For instance, in an organization with several file servers that all trust a web server for delegation, an admin would have to change the msDS-AllowedToDelegateTo priority in all of the different file servers to introduce a second web server. With resource-based delegation, the list of trusted computers is held on the receiving end. Thus, in our example, only the newly created server would require a change of settings.

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