# Windows Kerberos information disclosure

Source: https://startwithidentity.com/cves/cve-2025-21242/
Last updated: 2026-08-29
License: content by Start with Identity. Cite the source URL.

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## What broke

Windows Kerberos leaked information a low-privileged caller should not have seen. Medium severity. Microsoft patched it in the January 2025 updates.

## Why it matters

Info-disclosure on the KDC is how you seed the next escalation: usernames, etypes, or ticket metadata that make password spraying and delegation abuse cheaper. It is not [CheckSum](https://startwithidentity.com/cves/cve-2025-60704/), and you should not brief it that way. You should still take the patch, because Kerberos updates ship as a train.

## What to do

- Keep DCs current. Do not skip "medium" Kerberos CVEs to save a change window.
- Monitor for unusual TGS requests after you patch. Disclosure bugs sometimes show up in tooling before they show up in tickets.

## 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-2025-21242](https://nvd.nist.gov/vuln/detail/CVE-2025-21242)