PR Summary

Ensures the filename value used in WDAC ETW events is set to an empty string if the input value is null. This ensures the value matches up with the manifest defined which has the following properties for this task allowing ETW consumers to be able to consume this event.

PR Context

The manifest defines the properties for this event as

<template tid="WDACQueryTobeusedwhenoperationisjustexecutingamethodArgs_V1">
    <data name="QueryName" inType="win:UnicodeString" />
    <data name="FileName" inType="win:UnicodeString" />
    <data name="QuerySuccess" inType="win:Int32" />
    <data name="QuerySResult" inType="win:Int32" />
</template>

When running as it is now and the FileName is null we get the following event in the event log

The raw XML for the event is

<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
  <Provider Name="PowerShellCore" Guid="{f90714a8-5509-434a-bf6d-b1624c8a19a2}" /> 
  <EventID>16386</EventID> 
  <Version>1</Version> 
  <Level>4</Level> 
  <Task>131</Task> 
  <Opcode>20</Opcode> 
  <Keywords>0x0</Keywords> 
  <TimeCreated SystemTime="2023-12-13T08:27:51.4381401Z" /> 
  <EventRecordID>3</EventRecordID> 
  <Correlation ActivityID="{b8f2cbfe-2d7f-0001-0ea6-f4b87f2dda01}" /> 
  <Execution ProcessID="8876" ThreadID="7492" /> 
  <Channel /> 
  <Computer>SERVER2022.domain.test</Computer> 
  <Security /> 
  </System>
<ProcessingErrorData>
  <ErrorCode>15005</ErrorCode> 
  <DataItemName>QuerySResult</DataItemName> 
  <EventPayload>57006C00640070004700650074004C006F0063006B0064006F0077006E0050006F006C0069006300790000000000001000000080</EventPayload> 
  </ProcessingErrorData>
  </Event>

The ErrorCode represents ERROR_EVT_INVALID_EVENT_DATA and the EventPayload shows the raw value which when formatted with Format-Hex shows

   Label: Byte[] (System.Byte[]) <318C4389>

          Offset Bytes                                           Ascii
                 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
          ------ ----------------------------------------------- -----
0000000000000000 57 00 6C 00 64 00 70 00 47 00 65 00 74 00 4C 00 W l d p G e t L
0000000000000010 6F 00 63 00 6B 00 64 00 6F 00 77 00 6E 00 50 00 o c k d o w n P
0000000000000020 6F 00 6C 00 69 00 63 00 79 00 00 00 00 00 00 10 o l i c y      �
0000000000000030 00 00 00 80                                        �

We can see the first value for QueryName is the Unicode NULL terminated string WldpGetLockdownPolicy. The next string value is going to consume the 00 00 as the empty string NULL terminator for FileName. This only leaves 6 bytes left for the QuerySuccess and QuerySResult which is invalid causing the error. The problem here is there is actually no value for FileName, there should be an extra 2 0 bytes between the first and third value.

With this PR we can see that the event is now decoded properly

<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
  <Provider Name="PowerShellCore" Guid="{f90714a8-5509-434a-bf6d-b1624c8a19a2}" /> 
  <EventID>16386</EventID> 
  <Version>1</Version> 
  <Level>4</Level> 
  <Task>131</Task> 
  <Opcode>20</Opcode> 
  <Keywords>0x0</Keywords> 
  <TimeCreated SystemTime="2023-12-13T08:37:17.8895239Z" /> 
  <EventRecordID>373</EventRecordID> 
  <Correlation ActivityID="{b8f2cbfe-2d7f-0001-c7b4-f4b87f2dda01}" /> 
  <Execution ProcessID="5352" ThreadID="14300" /> 
  <Channel /> 
  <Computer>SERVER2022.domain.test</Computer> 
  <Security /> 
  </System>
<EventData>
  <Data Name="QueryName">WldpGetLockdownPolicy</Data> 
  <Data Name="FileName" /> 
  <Data Name="QuerySuccess">268435456</Data> 
  <Data Name="QuerySResult">-2147483648</Data> 
  </EventData>
  </Event>

There's probably other events affected but this is just what I came across when testing things out and only needed a simple fix.

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  • N/A or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed
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