Upgrading domain controllers to Windows Server 2025 is a major milestone, but the work doesn’t end at promotion and replication. After the OS upgrade, administrators must re-evaluate Active Directory configuration, harden authentication, and complete new feature enablement to realize Server 2025’s scalability and security improvements. This article walks through the post-upgrade checklist for Windows Server 2025 domain controllers, explains the technical trade‑offs of new features such as the 32K page Active Directory database, delegated Managed Service Accounts (dMSA), and the latest Windows LAPS improvements, and provides actionable configuration and monitoring guidance to reduce risk while maximizing benefit.
Windows Server 2025 brings several substantial AD DS changes intended to scale large environments and harden authentication surfaces. Notable items administrators should evaluate after an upgrade include:
The 32K option is a forest‑wide feature and cannot be partially applied. New Active Directory forests created on Server 2025 default to a 32K‑capable database but simulate 8K behavior for compatibility until you explicitly enable the 32K optional feature.
Key benefits:
Windows Server 2025 introduces practical and powerful capabilities for Active Directory, but those capabilities come with operational constraints that demand careful planning. A disciplined, audit-first approach eliminates surprises, preserves availability, and lets your organization realize the scalability and security benefits without paying the price of avoidable outages.
Source: TechTarget Configure domain controllers after Server 2025 upgrade | TechTarget
Background and overview
Windows Server 2025 brings several substantial AD DS changes intended to scale large environments and harden authentication surfaces. Notable items administrators should evaluate after an upgrade include:- Enabling the Database 32K pages optional feature (formerly an 8 KB JET database) to increase AD object payload capacity and multi-valued attribute support.
- Revisiting account lockout settings and response processes to balance brute-force protection against denial-of-service risk.
- Enforcing LDAP signing and channel binding to reduce replay and man-in-the-middle attack vectors.
- Deploying delegated Managed Service Accounts (dMSA) to replace legacy service accounts with machine-bound identities.
- Upgrading Windows LAPS policies to take advantage of passphrases, image rollback detection, and automated account management.
- Auditing and planning the NTLM → NTLMv2/Kerberos transition using the enhanced NTLM auditing available in the latest client and Server 2025 platform updates.
Database 32K pages for Active Directory
What changed and why it matters
Since Windows 2000, Active Directory’s Extensible Storage Engine (ESE/JET) used an 8 KB page size. Windows Server 2025 introduces a 32 KB page database format that permits larger single-object payloads and significantly increases the number of values a multi-valued attribute can hold. The practical effect is better scalability for very large directories, fewer replication fragmentation issues for large attribute records, and improved headroom for modern workloads that attach many values to a single object.The 32K option is a forest‑wide feature and cannot be partially applied. New Active Directory forests created on Server 2025 default to a 32K‑capable database but simulate 8K behavior for compatibility until you explicitly enable the 32K optional feature.
Requirements and irreversible change
Before enabling 32K pages:- All domain controllers in the forest must be running Windows Server 2025 (or later) and be running an AD database that is 32K capable.
- The domain and forest functional levels must be raised to Windows Server 2025 (or later).
- If any domain controller was brought to Server 2025 via an in-place feature update from an earlier OS, it will continue to use the legacy 8K database format and will not be 32K capable. Those DCs require a demotion, clean OS install, and re‑promotion to adopt 32K pages.
- Once enabled, the change is one‑way — you cannot revert to 8K simulation mode. Backups taken in the 8K format will not be restorable to DCs that have been converted to 32K.
Practical roadmap to enable 32K pages
- Inventory DCs and identify any that were upgraded in-place. Mark those for re‑installation where necessary.
- Validate replication health and resolve any lingering replication errors.
- Confirm all DCs are on Windows Server 2025 and the forest/domain functional levels are set to Server 2025.
- Validate backup software supports 32K page backup images and test a recovery drill using 32K backup media.
- Stage enabling the optional feature in a lab or pilot domain, then enable forest‑wide in production during a defined maintenance window.
- After enabling, update runbooks that reference backup/restore procedures and document the irreversible change.
Risks and mitigation
- Risk: Restore failure because older 8K backup images cannot be used after 32K is enabled. Mitigation: perform full, verified 32K backups before enabling and document rollback routes.
- Risk: Performance and memory pressure since larger pages will use more RAM per database page. Mitigation: review and, if necessary, increase memory allocation on DCs that host large AD databases.
Account lockout policy — rethink, don’t just reapply defaults
The trade-off
Account lockout policies are a classic security vs. availability trade‑off. Locking accounts after repeated failed attempts thwarts credential brute‑force attacks, but a malicious actor can weaponize the same mechanism to lock out many accounts (a denial-of-service against authentication).Recommended approach
- Start by auditing login failures and attack patterns to understand your threat profile.
- Use account lockout thresholds that reflect your operational environment — the recommended starting point in modern Windows baselines is often a threshold around 5–10 failed attempts with a modest auto‑unlock window (for example, 15 minutes), but this must be tuned.
- Implement compensating controls:
- Strong password complexity and minimum length policies.
- Multifactor Authentication (MFA) for high‑value accounts and interactive logons.
- Automated alerts when a high volume of lockouts or failed logon attempts are detected.
- Privileged account separation and restricted use of built-in administrative accounts.
Operational requirements
- Ensure your Help Desk and identity admins have documented, tested procedures to quickly unlock accounts and investigate lockout patterns.
- Instrument SIEM/EDR to create actionable alerts for suspicious lockout storms and failed authentication patterns.
Enforce LDAP signing and channel binding
Why you should care
Unsigned LDAP/SASL binds and missing channel binding allow attackers to perform replay or man‑in‑the‑middle attacks against LDAP authentication. Requiring signing and enforcing channel binding reduces the risk that an adversary can capture and reuse authentication material.Implementation guidance
- Turn on diagnostic LDAP event logging on your domain controllers to detect unsigned binds and channel binding failures before enforcing rejection.
- Use staged Group Policy rollout:
- Initially configure “When Supported” for channel binding to allow clients that support channel binding to use it while continuing to log failures from unsupported clients.
- Enable LDAP signing requirement in a monitoring mode first; review Event IDs that indicate unsigned attempts and remediate the clients.
- Typical registry/GPO settings to manage:
- Domain controller: LDAP server signing requirements
- LdapEnforceChannelBinding registry value (0 = never, 1 = when supported, 2 = always)
Practical steps
- Enable detailed LDAP logging on all DCs to capture Event IDs for unsigned binds.
- Audit your environment for non‑Windows appliances or older OSs that may not support channel binding or signing.
- Remediate or replace legacy clients and appliances, upgrading firmware or software where possible.
- Move to an enforced policy (reject unsigned binds) only after a clean audit window with no problem events.
Risk note
Enforcing LDAP signing/channel binding too quickly can break legacy appliances and third‑party systems. Always use the monitoring audit phase to identify compatibility issues before enforcement.Delegated Managed Service Accounts (dMSA)
What dMSA delivers
Windows Server 2025 introduces delegated Managed Service Accounts (dMSA) — a new service account type that binds the account to device identity, provides machine‑bound randomized secrets, and allows migration of existing service accounts to a more secure, device‑scoped model. dMSAs mitigate risks from traditional service accounts that have static passwords or are reused across servers.Key benefits:
- The dMSA secret is machine-bound and not retrievable like a plain password.
- Automated rotation of the secret reduces kerberoasting and credential theft risk.
- Migration tools exist to supersede legacy accounts and disable the original password authentication path.
Deployment considerations
- dMSA functionality requires Windows Server 2025 domain controllers and device support on the machines that will use the account.
- Migration is not instantaneous: plan for ticket lifetimes and replication latency. Guidance suggests waiting multiple ticket lifetimes (for example, up to 14–28 days in some migration states) to ensure all Kerberos tickets and group membership updates propagate.
- If you enable Credential Guard (or use it as part of the dMSA trust model), be aware of features (like unconstrained delegation) that may stop working with machine-bound dMSAs.
Migration flow (high level)
- Create a dMSA and grant the specific machine(s) permission to retrieve the managed password.
- Initiate migration from the legacy service account to the dMSA, monitoring logs for events that indicate migration progress.
- Replace service start configurations on target servers to use the dMSA and restart the services.
- Complete the migration by disabling the old account only after verification that all services work with the dMSA.
Caveats and risks
- If not all machines using the service account support dMSA, migration will disrupt services when the original account is disabled. Maintain the legacy account until all dependent machines are updated.
- Careful planning is required for multi‑site and long‑replication environments because Kerberos ticket renewal and membership propagation depend on timely replication.
Windows LAPS: stronger local admin management
What’s improved in Windows Server 2025
Windows Local Administrator Password Solution (Windows LAPS) has matured: Server 2025 and recent client releases add support for passphrases, image rollback detection, automatic account management, and more granular password complexity settings. These enhancements increase the entropy and operational safety of local administrative passwords across domain‑joined hosts.Recommended LAPS configuration
- Deploy Windows LAPS domain configuration and Group Policy settings to ensure unique, randomly generated local administrator credentials for every managed device.
- Use higher complexity settings (the new entropy-aware passphrase options) and longer password/passphrase lengths where compatibility allows.
- Enable image rollback detection where applicable to detect when an image or snapshot would reintroduce old credentials.
- Ensure access to LAPS secrets is granted via narrowly scoped ACLs and audited via SIEM.
Implementation checklist
- Roll out the LAPS management policy and ensure clients are updated to the supported Windows 11 24H2 / Server 2025 clients where required for passphrase features.
- Create role‑based access control for retrieval of managed local admin passwords.
- Integrate LAPS retrieval auditing into your SIEM for forensic visibility.
Audit and transition authentication protocols (NTLM → NTLMv2/Kerberos)
The need to phase out legacy NTLMv1
NTLMv1 is weak and increasingly targeted. Windows Server 2025 and recent client releases include enhanced NTLM auditing that makes it far easier to identify devices, processes, and accounts still relying on legacy NTLM versions.How to audit before you block
- Enable the new enhanced NTLM auditing on clients, servers, and domain controllers to collect who/why/where NTLM is used.
- Use the domain controller logs and client/server operational logs to map NTLM usage to specific services, devices, and processes.
- Identify interoperable patterns and third‑party apps that cannot move to Kerberos immediately.
Migration strategy
- Audit for NTLMv1 usage and NTLM overall.
- Prioritize remediation of internet‑facing services and high‑privilege accounts.
- Where possible, reconfigure or upgrade applications to use Kerberos or NTLMv2.
- Set policy to audit and then restrict NTLM selectively before domain-wide disablement.
Operational risk
Blindly disabling NTLM can break legacy apps. The enhanced NTLM auditing feature provides the necessary visibility to avoid "accidental outages," but remediation timelines must be realistic and include vendor engagement for appliances and third‑party software.Monitoring, logging, and ongoing maintenance
Key telemetry to enable and monitor
- AD replication health and latency; fix any lingering replication errors prior to enabling large changes like 32K pages.
- LDAP diagnostic logging and the Event IDs for unsigned binds to spot compatibility issues.
- NTLM enhanced logs on clients/servers/DCs to identify legacy authentication dependencies.
- Windows LAPS operational logs and retrieval activity.
- dMSA migration operational events and Kerberos service ticket activity.
Routine checks post-upgrade
- Verify backups and restore procedures for both 8K and 32K page backups where applicable; after converting to 32K, ensure only 32K restore media are used.
- Validate that account lockout thresholds are producing acceptable operational load and not being abused.
- Confirm that LDAP signing/channel binding enforcement is staged and that no critical systems are failing authentication.
- Monitor dMSA adoption and any services still relying on legacy accounts.
Recommended post-upgrade checklist (actionable)
- Inventory: catalog all domain controllers, note which were upgraded in-place vs. installed fresh.
- Replication health: verify no replication errors remain.
- Backup validation: ensure your backup solution supports Server 2025 and 32K page backups; test restore.
- Plan 32K conversion: schedule forest‑wide conversion only after steps 1–3 are complete.
- Enable LDAP logging: audit unsigned binds and channel binding failures; remediate clients.
- Configure LAPS: apply Windows LAPS policies with strong randomness, passphrase option if supported.
- Audit NTLM: enable enhanced NTLM auditing and create a remediation plan.
- Plan dMSA migrations: identify candidate service accounts and stage device upgrades to support dMSA usage.
- Review account lockout: tune threshold/duration and enable alerting for abnormal lockouts.
- Update runbooks: document changes to backup/restore, service account management, and emergency rollback procedures.
Strengths, trade-offs, and risks — critical analysis
Strengths
- The 32K page format addresses real scale limits in very large Active Directory deployments and is a forward‑looking technical change that will reduce multi-valued attribute fragmentation.
- dMSA provides a substantial security improvement for service accounts by binding secrets to machines, limiting credential misuse.
- Windows LAPS enhancements reduce the local administrator password attack surface and add practical entropy features (passphrases) that are easier to manage at scale.
- Enhanced NTLM auditing and improved LDAP signing/channel binding options give admins the tools to harden environments without blind risk.
Trade‑offs
- 32K pages require forest‑wide coordination and are irreversible. That makes them powerful but operationally costly in mixed or slow‑replicating environments.
- Enforcing LDAP signing/channel binding can break legacy systems and appliances — the operational cost to remediate non‑compliant devices can be significant.
- dMSA migration introduces complexity for services distributed across multiple machines; migration windows must accommodate Kerberos ticket lifetimes and replication delays.
Risks
- The most tangible risk is operational disruption from flipping enforcement switches without thorough auditing — account lockouts, service failures, or broken authentication flows are likely missteps when changes are rushed.
- Backup/restore incompatibility after enabling 32K pages is a high‑impact risk if not tested; make restoration tests mandatory before enabling this optional feature.
- Vendor and appliance compatibility with channel binding or enhanced LDAP signing is uneven; expect vendor coordination and possible firmware upgrades.
Final recommendations
- Treat the Server 2025 upgrade as the start of a second planning phase. Allocate time and resources for the post‑upgrade hardening and feature‑enablement work, not just the OS upgrade itself.
- Use a staged, evidence‑driven rollout model: audit → remediate → enforce. Take advantage of the improved auditing in Server 2025 and supported clients to eliminate guesswork.
- Make testing and validation non‑negotiable: backup restores, LAPS retrieval and access controls, LDAP and NTLM compatibility tests, and dMSA pilot migrations should all be performed and documented before production enforcement.
- Update operational runbooks and train support staff on new behaviors and diagnostics (LDAP event IDs, NTLM enhanced logs, dMSA event types, LAPS events).
- Prioritize risk vs. reward: enable features that deliver clear security or scalability improvements first, while deferring any change that risks critical service disruption until remediation plans are in place.
Windows Server 2025 introduces practical and powerful capabilities for Active Directory, but those capabilities come with operational constraints that demand careful planning. A disciplined, audit-first approach eliminates surprises, preserves availability, and lets your organization realize the scalability and security benefits without paying the price of avoidable outages.
Source: TechTarget Configure domain controllers after Server 2025 upgrade | TechTarget
