When working with Windows Server environments, many administrators assume that joining a domain automatically disables local accounts. However, the reality is more nuanced. The domain join process doesn't inherently disable local accounts - it simply changes the default authentication behavior.

The behavior you're observing stems from how different RDP clients handle authentication:

// Example PowerShell to check local account status
Get-LocalUser -Name "Administrator" | Select Enabled

Modern RDP clients (like Windows 8.1+) default to domain authentication, while older clients may still attempt local authentication first. This explains why your Mac RDP client and older Windows 7 box could still authenticate with local credentials.

Best practice recommends explicitly disabling local administrator accounts through Group Policy:

# GPO setting to disable local admin accounts
Computer Configuration -> Windows Settings -> Security Settings -> Local Policies -> Security Options
Accounts: Administrator account status -> Disabled

The authentication sequence varies by client implementation:

• Windows 8.1+/Current RDP: Prefers domain credentials
• Legacy RDP clients: May try local auth first
• Third-party clients: Implementation varies (Microsoft RDP client for Mac handles this differently)

For production environments, consider these hardening steps:

# PowerShell to rename and disable local admin account
Rename-LocalUser -Name "Administrator" -NewName "NotAdmin"
Disable-LocalUser -Name "NotAdmin"

# Or completely remove via DISM (Windows 10/Server 2016+)
DISM /Online /Disable-Feature /FeatureName:Adminless

For Azure-specific configurations, you might want to implement Azure AD-based authentication instead of relying on local accounts.

When troubleshooting, check these event logs:

# PowerShell to check relevant security events
Get-WinEvent -LogName Security -FilterXPath "*[System[(EventID=4624 or EventID=4625)]]" | 
Where-Object {$_.Message -like "*Administrator*"} | 
Format-List -Property TimeCreated,Message

This will show successful and failed authentication attempts, helping identify which authentication method was used.

When working with domain-joined Windows Servers, I recently encountered an interesting authentication scenario. While the domain administrator account worked as expected for RDP connections, I discovered that local administrator accounts remained accessible through certain RDP clients.

During domain join operations, Windows doesn't automatically disable local accounts by default. The behavior depends on several factors:

# PowerShell command to check local account status
Get-LocalUser -Name "Administrator" | Select-Object Enabled,Name

The variation stems from how different RDP clients handle authentication:

• Modern Windows RDP clients (8.0+) enforce domain authentication by default
• Older clients and third-party implementations may fall back to local authentication

This behavior creates potential security gaps. Here's how to properly secure your environment:

# Disable local Administrator account via PowerShell
Disable-LocalUser -Name "Administrator"

# Or through Group Policy:
Computer Configuration -> Windows Settings -> Security Settings -> Local Policies -> Security Options
"Accounts: Administrator account status" -> Disabled

For environments with multiple RDP client versions:

1. Implement Group Policy to restrict local admin access
2. Configure NLA (Network Level Authentication) settings
3. Standardize on modern RDP clients where possible

When debugging such scenarios, consider these diagnostic commands:

# Check effective authentication policies
gpresult /h report.html

# Verify RDP session security settings
qwinsta /server:servername

# Examine security logs for authentication events
Get-WinEvent -FilterHashtable @{LogName='Security';ID=4624,4625} | Where-Object {$_.Message -like "*Administrator*"}

For clients requiring forced domain authentication, modify the RDP file:

enablecredsspsupport:i:1
authentication level:i:2
negotiate security layer:i:1