# Windows Kerberos PAC validation spoof, KB5020805 bypass

Source: https://startwithidentity.com/cves/cve-2024-26248/
Last updated: 2026-08-29
License: content by Start with Identity. Cite the source URL.

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## What broke

The Kerberos Privilege Attribute Certificate (PAC) carries the user's groups and privileges. After [CVE-2022-37967](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2022-37967), Microsoft added PAC signature checks (KB5020805). CVE-2024-26248 lets a local attacker spoof that signature and skip the check, elevating on the workstation. [CVE-2024-29056](https://startwithidentity.com/cves/cve-2024-29056/) is the cross-forest sibling. Patched 9 April 2024. Enforcement is staged: both DCs and clients need the update or authentication can fail.

## Why it matters

PAC is how a service knows who you are after the TGT. Bypassing PAC validation is forging group membership. Identity teams who treated KB5020805 as "done in 2023" had to reopen the change in April 2024, then again for the 2025 PKINIT and S4U bugs.

## What to do

- Deploy April 2024 updates on DCs first, then members, then flip the PAC-validation enforcement mode Microsoft documented for 26248 / 29056.
- Do not skip the client-side update. A patched DC and an unpatched workstation is how you get "Kerberos is broken" tickets.
- Inventory service accounts that accept unconstrained delegation. PAC spoofing loves those.

## 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-2024-26248](https://nvd.nist.gov/vuln/detail/CVE-2024-26248)
- Microsoft, "How to manage PAC Validation changes related to CVE-2024-26248 and CVE-2024-29056"