In domain environments, you'll frequently encounter "Account Unknown (S-1-5-21...)" entries in Access Control Lists (ACLs) across various Windows objects. These represent security principals (users/groups) that once existed in Active Directory but were subsequently deleted while their permissions remained.

Windows maintains security through Security Identifiers (SIDs), not names. When you see these entries, it means:

ACE Type: Allow/Deny
Principal: S-1-5-21-3623811015-3361044348-30300820-1013 (Account Unknown)
Permissions: [List of permissions]
Inheritance: [Inheritance flags]

Removing these entries is generally safe because:

• They represent deleted accounts that cannot authenticate
• The SID-to-name resolution has permanently failed
• No functional permissions are being enforced

For large-scale cleanup, consider PowerShell scripts like:

# NTFS Cleanup Example
Get-ChildItem -Path "C:\Shared" -Recurse | 
ForEach-Object {
    $acl = Get-Acl $_.FullName
    $modified = $false
    $acl.Access | Where-Object {
        $_.IdentityReference -like "*Account Unknown*"
    } | ForEach-Object {
        $acl.RemoveAccessRule($_)
        $modified = $true
    }
    if ($modified) { Set-Acl -Path $_.FullName -AclObject $acl }
}

# AD Object Cleanup
Get-ADObject -Filter * -Properties nTSecurityDescriptor | 
Where-Object {
    ($_.nTSecurityDescriptor.Access | 
    Where-Object { $_.IdentityReference -like "*Unknown*" }).Count -gt 0
} | ForEach-Object {
    $sd = $_.nTSecurityDescriptor
    $sd.Access | Where-Object {
        $_.IdentityReference -like "*Unknown*"
    } | ForEach-Object { $sd.RemoveAccessRule($_) }
    Set-ADObject -Identity $_ -Replace @{nTSecurityDescriptor=$sd}
}

Exercise caution when:

• Dealing with built-in SIDs (S-1-5-*)
• Working with cross-domain or external trust relationships
• Handling system-critical ACLs (like SYSTEM32 or registry hives)

1. Always test in non-production first
2. Maintain a backup of original ACLs
3. Document changes through change control
4. Implement group-based permissions going forward

To prevent future accumulation:

# Detection script for periodic audits
$report = @()
Get-ChildItem -Path "C:\CriticalShares" -Recurse | ForEach-Object {
    $unknown = (Get-Acl $_.FullName).Access | 
               Where-Object { $_.IdentityReference -like "*Unknown*" }
    if ($unknown) {
        $report += [PSCustomObject]@{
            Path = $_.FullName
            UnknownEntries = $unknown.Count
            LastModified = $_.LastWriteTime
        }
    }
}
$report | Export-Csv -Path "UnknownACEs_Report.csv"

When managing Windows domain environments, you'll frequently encounter Access Control Entries (ACEs) displaying as "Account Unknown" followed by a Security Identifier (SID). These represent security principals (users/groups) that previously existed in Active Directory but were deleted without proper permission cleanup.

From a technical perspective, these entries are generally harmless but can cause:

• Reduced ACL readability during audits
• Potential confusion during permission troubleshooting
• Minor performance impact when processing large ACLs

Before mass deletion, verify the SIDs truly represent deleted accounts:

# PowerShell: Verify SID status
$orphanedSid = "S-1-5-21-3623811015-3361044348-30300820-1013"
try {
    $objSID = New-Object System.Security.Principal.SecurityIdentifier($orphanedSid)
    $objUser = $objSID.Translate([System.Security.Principal.NTAccount])
    Write-Host "Account exists:" $objUser.Value
} catch {
    Write-Host "Confirmed orphaned SID:" $orphanedSid
}

For systematic cleanup across multiple resources:

# PowerShell: Remove orphaned SIDs from NTFS permissions
function Remove-OrphanedAces {
    param ([string]$Path)
    $acl = Get-Acl $Path
    $modified = $false
    
    $acl.Access | Where-Object {
        $_.IdentityReference -like "Account Unknown*"
    } | ForEach-Object {
        $acl.RemoveAccessRule($_)
        $modified = $true
    }
    
    if ($modified) {
        Set-Acl -Path $Path -AclObject $acl
        Write-Host "Cleaned ACL for:" $Path
    }
}

# Example usage for directory cleanup
Get-ChildItem "C:\Shared" -Recurse -Directory | ForEach-Object {
    Remove-OrphanedAces -Path $_.FullName
}

Exercise additional caution with:

• System directories (Windows, Program Files)
• Registry hives (especially HKLM)
• Active Directory objects with replication implications

Implement these practices to minimize future occurrences:

• Always assign permissions to AD groups rather than individual users
• Establish proper offboarding procedures that include permission revocation
• Consider implementing ACL cleanup in user deletion scripts

When performing cleanup:

• Maintain change logs of removed ACEs
• Document before/after states for critical resources
• Consider taking ACL backups before mass modifications