Get-O365UserEvents.ps1

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function Get-Token {
  [CmdletBinding()]
  param
  (
    [Parameter(Mandatory = $true)][string]$Tenant,
    [Parameter(Mandatory = $true)][System.Guid]$ClientID,
    [Parameter(Mandatory = $true)][string]$ClientSecret,
    [Parameter(Mandatory = $true)][string]$Resource
  )
  
  begin {
  }

  process {
    $ReqBody = @{
      Grant_Type    = "client_credentials"
      Scope         = "$Resource.default"
      client_Id     = $ClientID
      Client_Secret = $ClientSecret
    } 
    
    $RequestToken = @{
      ContentType = 'application/x-www-form-urlencoded'
      Method      = 'POST'
      Body        = $ReqBody
      Uri         = "https://login.microsoftonline.com/$Tenant/oauth2/v2.0/token"
    }

    try {
      $response = Invoke-RestMethod @RequestToken
    }
    catch {
      Write-Error $_.Exception
    }
  }
  end {
    $response
  }
}

function Get-AuthenticationHeaders {
  [CmdletBinding()]
  param
  (
    [Parameter(Mandatory = $true)][string]$AccessToken
  )

  begin {
  }
  process {
    @{
      'Content-Type'  = 'application/json'
      'Authorization' = "Bearer $($AccessToken)"
    }    
  }
  end {
  }
}

function Get-Users {
  [CmdletBinding()]
  param
  (
    [Parameter(Mandatory = $true)][string]$AccessToken
  )

  begin {
    $headers = Get-AuthenticationHeaders -AccessToken $AccessToken
    $uri = "https://graph.microsoft.com/v1.0/users?`$filter=userType eq 'Member'"
    $users = @()
  }
  process {
    try {
      $response = Invoke-RestMethod -Uri $uri -Headers $headers -Body $Payload -Method Get
      $users = $response.value
    }
    catch {
      Write-Error "Error sending batch request. Error: $($_.Exception)"
    }
  }
  end {
    $users
  }
}

function Get-Events {
  [CmdletBinding()]
  param
  (
    [Parameter(Mandatory = $true)][string]$UserPrincipalName,
    [Parameter(Mandatory = $true)][string]$AccessToken
  )

  begin {
    $headers = Get-AuthenticationHeaders -AccessToken $AccessToken
    $uri = "https://graph.microsoft.com/v1.0/users/$UserPrincipalName/events?`$top=3"
  }
  process {
    try {
      $response = Invoke-RestMethod -Uri $uri -Headers $headers -Body $Payload -Method Get
      return @($response.value)
    }
    catch {
      if ($_.Exception.Message.Contains("(403) Forbidden")) {
        Write-Host "  [ACCESS DENIED TO EVENTS]" -ForegroundColor Yellow
      }
      else {
        Write-Error "Request Error: $($_.Exception)"        
      }
      throw $_
    }
  }
}

# script requries Users.Read.All, Calendars.ReadWrite
$tenant = "contoso.onmicrosoft.com"
$clientId = "3617fd1b-xxxx-xxxx-xxxx-057668e52db0"                # aka "Application ID" in Azure Portal > Azure Active Directory > App Registrations
$clientSecret = "abc123secret"              # aka "Keys" in Azure Portal > Azure Active Directory > App Registrations

$token = Get-Token -Tenant $tenant -ClientID $clientId -ClientSecret $clientSecret -Resource "https://graph.microsoft.com/"

if ($token.access_token) {
  
  $users = Get-Users -AccessToken $token.access_token

  foreach ($user in $users | ? { $_.mail -ne $null }) {
    Write-Host "[USER] $($user.userPrincipalName)"
    try {
      $events = Get-Events -AccessToken $token.access_token -UserPrincipalName $user.userPrincipalName
      
      if ($null -ne $events -and $events.Count -gt 0) {
        foreach ($event in $events) {
          Write-Host "  [EVENT] $($event.subject)" -ForegroundColor Green
        }
      }
      else {
        Write-Host "  [NO EVENTS FOUND]" -ForegroundColor Blue
      }
    }
    catch {}
    Write-Host
  }
}