When working with Active Directory environments, you might encounter repetitive Kerberos authentication errors in your Windows Event Log similar to:

Event ID: XYZ
A Kerberos Error Message was received:
on logon session DOMAIN\serviceaccount
Error Code: 0x19 KDC_ERR_PREAUTH_REQUIRED
Server Name: krbtgt/DOMAIN
Target Name: krbtgt/DOMAIN@DOMAIN

Contrary to initial impressions, this isn't necessarily an error condition. Windows clients use a two-step negotiation process:

1. Initial request without pre-authentication (triggering KDC_ERR_PREAUTH_REQUIRED)
2. Immediate follow-up request with proper pre-authentication data

The complete flow looks like this in network traces:

1. Client -> KDC: AS-REQ (no preauth)
2. KDC -> Client: KRB-ERROR (KDC_ERR_PREAUTH_REQUIRED)
3. Client -> KDC: AS-REQ (with PA-ENC-TIMESTAMP)
4. KDC -> Client: AS-REP (TGT ticket)

While typically benign, these events indicate potential issues when:

• They persist beyond the initial handshake
• Accompanied by actual authentication failures
• Generated for service accounts using older encryption types

To reduce log noise while maintaining security:

# PowerShell: Check account encryption types
Get-ADUser -Identity serviceaccount -Properties msDS-SupportedEncryptionTypes

# PowerShell: Update to modern encryption
Set-ADAccountControl -Identity serviceaccount -KerberosEncryptionType AES128,AES256

For debugging or legacy system support:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Kerberos\Parameters]
"SupportedEncryptionTypes"=dword:0000001c

When building monitoring scripts, exclude these expected events:

# Sample Log Filter
Get-WinEvent -LogName System | Where-Object {
    $_.Id -eq XYZ -and 
    $_.Message -notmatch "KDC_ERR_PREAUTH_REQUIRED.*followed by successful auth"
}

While suppressing these events, ensure:

• All domain controllers support modern encryption types
• No accounts are configured with DES-only encryption
• Pre-authentication remains enabled for all user accounts

The KDC_ERR_PREAUTH_REQUIRED (0x19) error occurs when a client attempts to obtain a Kerberos Ticket-Granting Ticket (TGT) without including required pre-authentication data. This is actually part of Windows' normal negotiation process to determine supported encryption types.

// Example of Kerberos AS-REQ (Authentication Service Request) flow
1. Client sends initial request without pre-auth
2. KDC responds with KDC_ERR_PREAUTH_REQUIRED
3. Client resends request with proper pre-auth data
4. KDC issues TGT

The constant logging occurs because:

• Service accounts may be configured without AES encryption support
• Older systems might only support RC4 encryption (deprecated)
• Domain controller policy enforces strict pre-authentication

Option 1: Configure Encryption Types

Update your domain's supported encryption types through Group Policy:

Computer Configuration → Policies → Windows Settings → 
Security Settings → Local Policies → Security Options →
Network security: Configure encryption types allowed for Kerberos

Option 2: Modify Service Account Properties

For problematic service accounts, set the proper encryption flags:

Set-ADAccountControl -Identity "serviceaccount" -KerberosEncryptionType "AES256,AES128"

For legacy systems, you might need registry tweaks:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System\Kerberos\Parameters]
"SupportedEncryptionTypes"=dword:0000001c

Create a custom Event Viewer filter to exclude these non-critical events:

<QueryList>
  <Query Id="0" Path="System">
    <Select Path="System">
      *[System[Provider[@Name='Microsoft-Windows-Kerberos-Key-Distribution-Center'] 
      and (Level<3) and not (EventID=26)]]
    </Select>
  </Query>
</QueryList>

Investigate further if you see:

• Failed authentication attempts after the initial PREAUTH error
• Multiple retries from the same account
• Accompanying errors like KDC_ERR_ETYPE_NOSUPP