As I do each month on Patch Tuesday, I look through the list of published vulnerabilities in search of things that might interest Always On VPN Administrators. Frequently there are updates for things like Routing and Remote Access Service (RRAS) or various VPN protocols. The good news is that the November 2024 security updates include NO such vulnerabilities! However, a vulnerability has been disclosed that affects Active Directory Certificate Services (AD CS) on which Always On VPN often relies on for user and device authentication.
Certificate Templates
AD CS Enterprise certificate authorities are closely integrated with Active Directory and use certificate templates that administrators can publish for users and devices to enroll. These templates control properties of the issued certificates, such as the subject name, usage, key length, enrollment policies, and much more. There are several different template versions available, versions 1 through 4. Version 1 templates are legacy templates that don’t provide many capabilities. Later versions include more features and capabilities.
CVE-2024-49019
The November 2024 Microsoft security updates include CVE-2024-49019, a privilege escalation vulnerability recently discovered in AD CS. Specifically, this vulnerability affects only legacy schema version 1 certificate templates published on a certificate authority (CA) server that include the option to supply the subject name in the certificate request. A typical example of this would be the default Web Server template.
Exploitation
The Web Server template does not include the Client Authentication Enhanced Key Usage (EKU) by default, which is required to authenticate to Active Directory. However, this vulnerability allows an attacker with enrollment privileges on this template to supply additional EKUs in the request and the certificate issued will include those capabilities. This allows a non-privileged attacker to quickly elevate to a domain or enterprise administrator by supplying a known administrator’s User Principal Name (UPN) along with the Client Authentication EKU in the certificate request. As version 1 templates cannot enforce CA manager approval for enrollment, an attacker can easily leverage this vulnerability if permissions allow, leading to complete domain compromise.
Note: This applies to any schema version 1 certificate template published with the subject name supplied in the request, not just the Web Server template.
Complications
Making matters worse, the Web Server template is one of the default certificate templates published automatically when a Windows Server CA is deployed. The best practice is to disable the publishing of any certificate templates by default when a new CA is provisioned. However, it requires additional configuration that is often overlooked. In addition, many administrators use overly broad enrollment permissions for this template, such as Domain Users, Domain Computers, or Authenticated Users, further broadening the attack surface.
Mitigation
Administrators should update their CA servers as soon as possible. If you cannot deploy this update immediately, consider replacing any schema version 1 templates with version 2 templates, which are not vulnerable. Also, as best practice, ensure that any certificate templates that allow the subject name to be supplied in the request also requires CA manager approval or additional authorized signatures for enrollment.
Investigation
Administrators should review enrollment privileges for all published certificate templates to ensure the least privileged access. In addition, administrators should audit all valid certificates issued with schema version 1 certificate templates that allow the subject name to be supplied in the request immediately to look for indicators of compromise. Review issued certificates for unauthorized EKUs or unusual subject names, especially those with a UPN.
Additional Information
Microsoft November 2024 Security Updates
