CVE-2025-60704CheckSum, Kerberos S4U missing cryptographic step
What broke
Windows Kerberos skipped a required cryptographic step when processing PA-S4U-X509-USER (CWE-325, missing cryptographic step). An attacker can forge the identity used in S4U2self and obtain tickets as another user, up to domain compromise. Silverfort branded the research "CheckSum." It was presented at Black Hat Europe in December 2025. Microsoft shipped a KDC patch. Microsoft rated it Important (CVSS 7.5), not Critical.
Why it matters
S4U is how services impersonate users in constrained delegation. A missing checksum on that path is not a local bug. It is a protocol-level identity forge inside Active Directory. If you run unconstrained or resource-based constrained delegation, assume this CVE is in your threat model even after the patch, and go look at the delegation graph.
What to do
- Patch every DC. KDC bugs are not workstation tickets.
- Inventory S4U / constrained-delegation use. Remove it where a modern alternative (Kerberos-only, Entra ID app proxy, or a resource-based grant) will do.
- Silverfort and Semperis both publish detection guidance for this class. Use it.
After you patch
Active Directory compromise is not contained by patching, because the artifacts an attacker creates outlive the vulnerability.
- Rotate the krbtgt account twice, with the replication interval between rotations, if there is any indication of ticket forgery. One rotation is not enough.
- Audit AD CS certificate templates for enrolment and enrollee-supplied-subject permissions, which are the most common escalation path left behind. See certificate lifecycle.
- Review privileged group membership and delegation rights (Domain Admins, DnsAdmins, constrained and resource-based constrained delegation) for changes during the window.
- Hunt for tickets with anomalous lifetimes or encryption types, and for authentications to services that identity never touches.
- Treat any issued certificate as a durable credential: revoking a user's password does not revoke a certificate that authenticates as them. See privilege escalation and lateral movement.
Sources
- NVD: CVE-2025-60704
- Silverfort "CheckSum" / Black Hat EU December 2025
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Technique
This CVE is an instance of Kerberos delegation abuse. Delegation lets a service act as the user who called it, which is necessary for multi-tier applications and dangerous the moment the service or the delegation scope is not exactly what an administrator intended.