Reading Event Logs Smart (Part 1)

When you query an event log with PowerShell, by default you get back a text message with the logged information. For example, if you’d like to know who logged on to your machine, you could use code like this (Administrator privileges required):

Get-EventLog -LogName Security -InstanceId 4624 |
  Select-Object -Property TimeGenerated, Message

The result could look like this:

 
...
25.04.2018 07:48:41 An account was successfully logged on....                                     
25.04.2018 07:48:40 An account was successfully logged on....                                     
24.04.2018 18:18:17 An account was successfully logged on.... 
...

That’s not very meaningful because PowerShell shortens the output. In situations like this, you might want to pipe the result to Format-List:

Get-EventLog -LogName Security -InstanceId 4624 |
  Select-Object -Property TimeGenerated, Message |
  Format-List

It now produces verbose data like this:

 
PS> Get-EventLog -LogName Security -InstanceId 4624 |
  Select-Object -Property TimeGenerated, Message -first 1 |
  Format-List




TimeGenerated : 25.05.2018 11:39:29
Message       : An account was successfully logged on.
                
                Subject:
                	Security ID:		S-1-5-18
                	Account Name:		DESKTOP-7AAMJLF$
                	Account Domain:		WORKGROUP
                	Logon ID:		0x3e7
                
                Logon Information:
                	Logon Type:		5
                	Restricted Admin Mode:	-
                	Virtual Account:		%%1843
                	Elevated Token:		%%1842
                
                Impersonation Level:		%%1833
                
                New Logon:
                	Security ID:		S-1-5-18
                	Account Name:		SYSTEM
                	Account Domain:		NT-AUTORITÄT
                	Logon ID:		0x3e7
                	Linked Logon ID:		0x0
                	Network Account Name:	-
                	Network Account Domain:	-
                	Logon GUID:		{00000000-0000-0000-0000-000000000000}
                
                Process Information:
                	Process ID:		0x328
                	Process Name:		C:WindowsSystem32services.exe
                
                Network Information:
                	Workstation Name:	-
                	Source Network Address:	-
                	Source Port:		-
                
                Detailed Authentication Information:
                	Logon Process:		Advapi  
                	Authentication Package:	Negotiate
                	Transited Services:	-
                	Package Name (NTLM only):	-
                	Key Length:		0
                
                This event is generated when a logon session is created. It is 
                generated on the computer that was accessed.
                
                The subject fields indicate the account on the local system 
                which requested the logon. This is most commonly a service 
                such as the Server service, or a local process such as 
                Winlogon.exe or Services.exe.
                
                The logon type field indicates the kind of logon that 
                occurred. The most common types are 2 (interactive) and 3 
                (network).
                
                The New Logon fields indicate the account for whom the new 
                logon was created, i.e. the account that was logged on.
                
                The network fields indicate where a remote logon request 
                originated. Workstation name is not always available and may 
                be left blank in some cases.
                
                The impersonation level field indicates the extent to which a 
                process in the logon session can impersonate.
                
                The authentication information fields provide detailed 
                information about this specific logon request.
                	- Logon GUID is a unique identifier that can be used to 
                correlate this event with a KDC event.
                	- Transited services indicate which intermediate services 
                have participated in this logon request.
                	- Package name indicates which sub-protocol was used among 
                the NTLM protocols.
                	- Key length indicates the length of the generated session 
                key. This will be 0 if no session key was requested.

That’s hard to handle. If you wanted to automate something based on this text, or create a report, you’d have to parse the text.

There is a much easier way: the message you see is just a text template, and Windows inserts the relevant information as “Replacement Strings”. They are part of the event data you receive from Get-EventLog. The data is stored as an array which always contains the same pieces of information per event ID.

When you figure out which information is stored in which array element, it is very easy to extract the information you are after:

Get-EventLog -LogName Security -InstanceId 4624 |
  ForEach-Object {
    # translate the raw data into a new object
    [PSCustomObject]@{
        Time = $_.TimeGenerated
        User = "{0}{1}" -f $_.ReplacementStrings[5], $_.ReplacementStrings[6]
        Type = $_.ReplacementStrings[10]
        Path = $_.ReplacementStrings[17]
    }
  }

When you run this simple code, it returns beautiful authentication info with just the pieces of information you required:

 
12.05.2018 17:38:58 SYSTEMNT-AUTORITÄT                     Negotiate C:WindowsSystem32services.exe
12.05.2018 17:38:58 [email protected] Negotiate C:WindowsSystem32svchost.exe 
12.05.2018 17:38:58 SYSTEMNT-AUTORITÄT                     Negotiate C:WindowsSystem32services.exe
12.05.2018 17:38:58 SYSTEMNT-AUTORITÄT                     Negotiate C:WindowsSystem32services.exe
12.05.2018 17:38:53 SYSTEMNT-AUTORITÄT                     Negotiate C:WindowsSystem32services.exe 

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