<#
.SYNOPSIS
    Reports on Active Directory service accounts: gMSAs, standalone MSAs, and plain user accounts used as service accounts.
.DESCRIPTION
    Looks up the accounts you name, or finds them for you with -Discover, and returns one row
    per account: type, enabled state, password age, whether the password ever expires, SPNs,
    delegation settings, and for gMSAs which principals are allowed to retrieve the password.
    Credentials are never stored in the script. Pass -Credential if you need another account.
.PARAMETER Identity
    One or more account names (sAMAccountName, with or without the trailing $ for gMSAs).
.PARAMETER Discover
    Find service accounts instead of naming them: every gMSA/MSA, every user with an SPN, and
    every user whose sAMAccountName matches -NamePattern.
.PARAMETER NamePattern
    Naming convention for user-based service accounts, used with -Discover. Default: svc*.
.PARAMETER Server
    Domain or domain controller to query.
.PARAMETER Credential
    Alternate credentials for the AD queries. Leave off to use your current session.
.EXAMPLE
    .\Get-ADServiceAccountReport.ps1 -Identity gmsa-sqlprod, svc-backup
.EXAMPLE
    .\Get-ADServiceAccountReport.ps1 -Discover | Export-Csv .\service-accounts.csv -NoTypeInformation
#>
[CmdletBinding(DefaultParameterSetName = 'Identity')]
param(
    [Parameter(Mandatory, ParameterSetName = 'Identity', Position = 0, ValueFromPipeline)][string[]]$Identity,
    [Parameter(Mandatory, ParameterSetName = 'Discover')][switch]$Discover,
    [Parameter(ParameterSetName = 'Discover')][string]$NamePattern = 'svc*',
    [string]$Server,
    [pscredential]$Credential
)

begin {
    Import-Module ActiveDirectory -ErrorAction Stop
    $ad = @{ ErrorAction = 'Stop' }
    if ($Server)     { $ad.Server = $Server }
    if ($Credential) { $ad.Credential = $Credential }

    $msaProps  = 'Description', 'Enabled', 'LastLogonDate', 'PasswordLastSet', 'servicePrincipalName', 'TrustedForDelegation', 'msDS-AllowedToDelegateTo', 'PrincipalsAllowedToRetrieveManagedPassword', 'msDS-ManagedPasswordInterval'
    $userProps = 'Description', 'Enabled', 'LastLogonDate', 'PasswordLastSet', 'PasswordNeverExpires', 'servicePrincipalName', 'TrustedForDelegation', 'msDS-AllowedToDelegateTo', 'MemberOf'
    $now = Get-Date

    function ConvertTo-Row($Account, [string]$Type) {
        $isManaged = $Type -ne 'User account'
        [pscustomobject]@{
            Name                 = $Account.SamAccountName
            Type                 = $Type
            Enabled              = $Account.Enabled
            PasswordLastSet      = $Account.PasswordLastSet
            PasswordAgeDays      = if ($Account.PasswordLastSet) { [int]($now - $Account.PasswordLastSet).TotalDays } else { $null }
            PasswordNeverExpires = if ($isManaged) { 'n/a (managed by AD)' } else { $Account.PasswordNeverExpires }
            RotationDays         = if ($isManaged) { $Account.'msDS-ManagedPasswordInterval' } else { $null }
            LastLogonDate        = $Account.LastLogonDate
            SPNs                 = @($Account.servicePrincipalName) -join '; '
            UnconstrainedDelegation = [bool]$Account.TrustedForDelegation
            ConstrainedDelegationTo = @($Account.'msDS-AllowedToDelegateTo') -join '; '
            PasswordRetrievableBy   = if ($isManaged) { (@($Account.PrincipalsAllowedToRetrieveManagedPassword) | ForEach-Object { ($_ -split '(?<!\\),')[0] -replace '^CN=' }) -join '; ' } else { $null }
            GroupCount           = if ($isManaged) { $null } else { @($Account.MemberOf).Count }
            Description          = $Account.Description
            Domain               = (($Account.DistinguishedName -split ',') -match '^DC=' -replace '^DC=') -join '.'
            DistinguishedName    = $Account.DistinguishedName
        }
    }

    function Get-MsaType($Account) {
        if ($Account.ObjectClass -eq 'msDS-GroupManagedServiceAccount') { 'gMSA' }
        elseif ($Account.ObjectClass -eq 'msDS-DelegatedManagedServiceAccount') { 'dMSA' }
        else { 'Standalone MSA' }
    }
}

process {
    if ($Discover) {
        foreach ($msa in Get-ADServiceAccount -Filter * -Properties $msaProps @ad) { ConvertTo-Row $msa (Get-MsaType $msa) }
        $ldap = "(&(objectCategory=person)(objectClass=user)(!(sAMAccountName=krbtgt))(|(servicePrincipalName=*)(sAMAccountName=$NamePattern)))"
        foreach ($user in Get-ADUser -LDAPFilter $ldap -Properties $userProps @ad) { ConvertTo-Row $user 'User account' }
        return
    }

    foreach ($name in $Identity) {
        # Try it as a managed service account first, then as a regular user.
        try {
            $msa = Get-ADServiceAccount -Identity ($name.TrimEnd('$')) -Properties $msaProps @ad
            ConvertTo-Row $msa (Get-MsaType $msa)
            continue
        }
        catch [Microsoft.ActiveDirectory.Management.ADIdentityNotFoundException] { Write-Verbose "$name isn't a managed service account; trying it as a user." }
        catch { Write-Warning "${name}: $($_.Exception.Message)"; continue }

        try {
            $user = Get-ADUser -Identity $name -Properties $userProps @ad
            ConvertTo-Row $user 'User account'
        }
        catch { Write-Warning "${name}: not found as a service account or user. $($_.Exception.Message)" }
    }
}