Windows Server 2019 – LSASS high CPU and authentication slowdowns – Fix & Prevention

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

LSASS pegs CPU on a Windows Server 2019 domain controller or member server, causing interactive logons, RDP, and Kerberos ticket issuance to stall for users and applications.

Environment & Reproduction

Observed during heavy authentication storms, NTLM bursts from legacy apps, or after enabling new auditing settings.

Get-Process lsass | Select Id,CPU,Threads
Get-Counter 'Process(lsass)% Processor Time' -SampleInterval 2 -MaxSamples 5

Root Cause Analysis

Causes include malformed clients, NTLM hammering, audit log overflow, expensive LDAP queries, or third-party security agents hooking LSASS.

Quick Triage

Capture LSASS counters and authentication events quickly.

Get-WinEvent -LogName Security -FilterXPath "*[System[(EventID=4625 or EventID=4776)]]" -MaxEvents 30
Get-Counter 'NTDSLDAP Searches/sec','Security System-Wide StatisticsNTLM Authentications' -SampleInterval 2 -MaxSamples 5

Step-by-Step Diagnosis

Use NTLM auditing and LDAP diagnostics to find the offending source.

auditpol /get /subcategory:'Credential Validation'
Get-WinEvent -LogName 'Microsoft-Windows-NTLM/Operational' -MaxEvents 50
Get-ItemProperty 'HKLM:SYSTEMCurrentControlSetServicesNTDSDiagnostics'

Solution – Primary Fix

Block runaway sources, throttle NTLM, and stabilise LSASS via targeted KBs and configuration fixes.

Set-ItemProperty 'HKLM:SYSTEMCurrentControlSetControlLsa' -Name LmCompatibilityLevel -Value 5
New-NetFirewallRule -DisplayName 'Block legacy NTLM client' -Direction Inbound -RemoteAddress 10.0.20.55 -Action Block
Restart-Service Netlogon

Solution – Alternative Approaches

Offload heavy LDAP clients to RODCs or dedicated DCs, and tune LSASS with hotfixes for known leaks.

Get-ADDomainController -Filter * | Select Name,IsReadOnly
Get-HotFix | Where-Object {$_.HotFixID -match 'KB'} | Select HotFixID,InstalledOn

Verification & Acceptance Criteria

LSASS CPU returns below 30 percent and 4768/4624 events flow without queueing.

Get-Process lsass | Select CPU,Handles,Threads
Get-WinEvent -LogName Security -MaxEvents 20 | Select TimeCreated,Id

Rollback Plan

Disable temporary NTLM block rules and revert audit overrides if helpdesk reports legacy app breakage.

Disable-NetFirewallRule -DisplayName 'Block legacy NTLM client'
Set-ItemProperty 'HKLM:SYSTEMCurrentControlSetControlLsa' -Name LmCompatibilityLevel -Value 3

Prevention & Hardening

Patch LSASS with current cumulative updates, enable LSA protection, and monitor NTLM/LDAP counters.

Set-ItemProperty 'HKLM:SYSTEMCurrentControlSetControlLsa' -Name RunAsPPL -Value 1
Get-Counter 'Process(lsass)% Processor Time'

Related Errors & Cross-Refs

Often linked to Kerberos ticket pressure, audit log overflow, and Credential Guard interactions.

Related tutorial: View the step-by-step tutorial for Windows Server 2019.

View all Windows Server 2019 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Microsoft Learn: LSA protection, NTLM auditing, and LDAP performance counters reference.