# CheckSum, Kerberos S4U missing cryptographic step

Source: https://startwithidentity.com/cves/cve-2025-60704/
Last updated: 2026-08-29
License: content by Start with Identity. Cite the source URL.

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## 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](https://startwithidentity.com/vendors/iam/microsoft-entra/) app proxy, or a resource-based grant) will do.
- [Silverfort](https://startwithidentity.com/vendors/itdr/silverfort/) and [Semperis](https://startwithidentity.com/vendors/itdr/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](https://startwithidentity.com/glossary/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](https://startwithidentity.com/glossary/privilege-escalation/) and [lateral movement](https://startwithidentity.com/glossary/lateral-movement/).

## Sources

- [NVD: CVE-2025-60704](https://nvd.nist.gov/vuln/detail/CVE-2025-60704)
- Silverfort "CheckSum" / Black Hat EU December 2025