Send Slack message on Local Admin logon

#requires -Version 3.0
<#
 .SYNOPSIS
 Send a message to Slack when a user who has Local Admin privileges logs on

 .DESCRIPTION
 This is an example of how the 'Send Slack message on Session condition' script can be used in a Trigger follow up action.
 Configure the defaults for your Slack environment and set the script as a follow up action on User Logon on a machine.
 NOTE: In Settings the following changes have been made (compared to the 'Send Slack message on Session condition'' script)
 Action Assigned to: Session
 Execution Context: User Session
 Security Context: Defualt (Session's User)

 .PARAMETER WebhookUri
 Slack webhook URI
 This parameter must be provided.

 .PARAMETER UserName
 Enter the author of your Slack message
 This parameter must be provided.

 .PARAMETER Title
 Enter a title for your Slack message
 This parameter must be provided.

 .PARAMETER Message
 Enter the main text of the Slack message
 This parameter must be provided.

 .PARAMETER Proxy
 If you are using a proxy enter the FQDN or IP number of the proxy server. IF YOU ARE NOT USING A PROXY SERVER, SET THIS TO 'DoNotUse'!
 This parameter is mandatory.

 .PARAMETER ButtonExplanation
 Enter the body of the second part of the Slack message for the optional button
 This parameter is optional, but if provided so must ButtonText and ButtonURI.

 .PARAMETER ButtonText
 Enter the text for the optional button in your Slack message
 This parameter is optional, but if provided so must ButtonExplanation and ButtonURI.

 .PARAMETER ButtonURI
 Enter the link for the optional button in your Slack message
 This parameter is optional, but if provided so must ButtonExplanation and ButtonText.

 .LINK
 https://slack.com/help/articles/115005265063-Incoming-webhooks-for-Slack

 .NOTES
 This script requires a webhook to be configured in your Slack site. See the provided link on how to do this.
 If you want to include a clickable button with your message the ButtonExplanation, ButtonText and ButtonURI must be provided. If any of these is missing the script will return an error.
 The ButtonURI can only be a valid URI. Invalid URIs are rejected.
 Valid URI : 'http://www.controlup.com' or 'controlup://MyOrganization/Machines'
 Invalid URI : 'www.controlup.com', as it does not contain either http:// or https:// to indicate this is a website.

 Author:   Samuel Legrand
 Version:  1.2
 Ton de Vreede - Refactor, added error handling, converted string JSON to hash table, comment block additions.
 Ton de Vreede - Changed to test for Local Admin logon
#>
[CmdletBinding()]
Param(
 [Parameter(Mandatory = $true, HelpMessage = 'Slack webhook URI')]
 [ValidateNotNullOrEmpty()]
 [string]$WebhookUri,
 [Parameter(Mandatory = $true, HelpMessage = 'Enter the author of your Slack message')]
 [ValidateNotNullOrEmpty()]
 [string]$UserName,
 [Parameter(Mandatory = $true, HelpMessage = 'Enter a title for your Slack message')]
 [ValidateNotNullOrEmpty()]
 [string]$Title,
 [Parameter(Mandatory = $true, HelpMessage = 'Enter the main text of the Slack message')]
 [ValidateNotNullOrEmpty()]
 [string]$Message,
 [Parameter(Mandatory = $true, HelpMessage = 'If you are using a proxy enter the FQDN or IP number of the proxy server, otherwise leave at DoNotUse')]
 [ValidateNotNullOrEmpty()]
 [string]$Proxy,
 [Parameter(Mandatory = $false, HelpMessage = '(Optional) Enter the body of the second part of the Slack message, to explain what the button does' )]
 [string]$ButtonExplanation,
 [Parameter(Mandatory = $false, HelpMessage = '(Optional) Enter the text on the button in your Slack message')]
 [string]$ButtonText,
 [Parameter(Mandatory = $false, HelpMessage = '(Optional) Enter the URI for the button in your Slack message')]
 [string]$ButtonURI
)

$ErrorActionPreference = 'Stop'

# CHANGES MADE FOR EXAMPLE USE
# Test for local admin privs
$currentPrincipal = New-Object Security.Principal.WindowsPrincipal([Security.Principal.WindowsIdentity]::GetCurrent())
If ($currentPrincipal.IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)) {
 $Details = [System.Security.Principal.WindowsIdentity]::GetCurrent().Name.Split('\')
 $Message = $Message.Replace('USERNAME',$Details[1]).Replace('COMPUTERNAME',$env:computername)
 $ButtonText = $ButtonText.Replace('USERNAME',$Details[1])
 $ButtonUri = $ButtonURI.Replace('USERNAME',$Details[1])
}
Else {
 Exit 0
}

# Set security protocol in this session to Transport Layer Security 1.2 in case TLS 1.0 is set on the machine.
[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12

# Create hashtables for body and REST parameters
[hashtable]$hshBody = @{
 'username'   = $UserName
 'icon_emoji' = ':robot_face:'
 'blocks'     = @(
  @{
   'type' = 'section'
   'text' = @{
    'type' = 'mrkdwn'
    'text' = $Title
   }
  },
  @{
   'type' = 'divider'
  },
  @{
   'type' = 'section'
   'text' = @{
    'type' = 'mrkdwn'
    'text' = $Message
   }
  }
 )
}

# If more than 5 arguments were passed, the button should be used. But if fewer than 8 were passed there is not enough information to add the button.
If ($PSBoundParameters.Count -in 6..7) {
 Throw "One of the arguments for adding a button was used, but to add a button three values must be entered. Please check you have entered a value for the button explanation text, the text on the button and the button URI. If it was not your intention to add a button, please ensure all three of these arguments are left empty."
}
ElseIf ($PSBoundParameters.Count -eq 8) {
 # Test if a valid URI was passed for the Button
 If (!($ButtonURI -as [System.URI]).IsAbsoluteUri) {
  Throw "The Button $ButtonURI is not a valid URI. Please check the syntax and try again."
 }

 # Add the 'button section'
 $hshBody.blocks += (
  @{
   'type'      = 'section'
   'text'      = @{
    'type' = 'mrkdwn'
    'text' = $ButtonExplanation
   }
   'accessory' = @{
    'type' = 'button'
    'text' = @{
     'type'  = 'plain_text'
     'text'  = $ButtonText
     'emoji' = $true
    }
    'url'  = $ButtonURI
   }
  }
 )
}

# Create parameters, convert Body to JSON already to avoid possible Depth problems
[hashtable]$hshParameters = @{
 'Uri'         = $WebhookUri
 'Method'      = 'POST'
 'Body'        = $hshBody | ConvertTo-Json -Depth 5
 'ContentType' = 'application/json'
}

# Add the proxy if passed
If (!$Proxy -eq 'DoNotUse') {
 $hshParameters.Add('Proxy', $Proxy)
}

Write-Verbose -Message $hshParameters

# Send the message to Slack
try {
 $return = Invoke-RestMethod @hshParameters
}
catch {
 Throw "Failed to send message $Title to Slack. Exception:`n$_"
}

# Test the result
If ($return -eq 'ok') {
 Write-Output -InputObject "Slack message sent."
 Exit 0
}
Else {
 Throw "There was an issue sending the Slack message. The expected return from the REST call is 'ok', but $return was received."
}