![]() If the token is restricted query for the logon session token instead. An access check token is captured and duplicated.It's basically very similar to the verification done for process creation I described in detail in part 2 and 3: Looking at SrpVerifyDll itself there's not much to really note. #define IOCTL_SRP_VERIFY_DLL CTL_CODE(FILE_DEVICE_UNKNOWN, 1537, \ The control code and input/output structures are as follows: I'll save you the effort of reverse engineering, as it's pretty routine. ![]() There's a good chance that's our target to investigate.īy chasing references you'll find the SrpVerifyDll function being called via a Device IO control code to an device object exposed by the APPID driver (\Device\SrpDevice). Unsurprisingly if you look at the names in APPID you'll find a function called SrpVerifyDll. However it gives us a hint that perhaps some of the enforcement is being done inside the kernel driver. Nothing shocking here, just our rules written out in a security descriptor. Name : APPLICATION PACKAGE AUTHORITY\ALL RESTRICTED APPLICATION PACKAGES Name : APPLICATION PACKAGE AUTHORITY\ALL APPLICATION PACKAGES Condition: APPID://PATH Contains "%PROGRAMFILES%\*" Condition: APPID://PATH Contains "%WINDIR%\*" Access: Execute|ReadAttributes|ReadControl|Synchronize We might as well start with dumping the Security Descriptor from the file using the Format-AppLockerSecurityDescriptor function from part 3, to check it matches our expectations. We know from the part 1 that there's a policy for DLLs in the DLL.Applocker file. It seems MS doesn't necessarily recommend enabling DLL blocking rules, but we'll dig in anyway as I can't find any official documentation on how it works and it's always interesting to better understand how something works before relying on it. We can try and create the class with DLL enforcement to convince ourselves that's the problem: On a default installation of Windows 10 you should find a single class, "Windows Defender IOfficeAntiVirus implementation" registered which is implemented in the MPOAV DLL. Get-ComCategory -CatId '56FFCC30-D398-11d0-B2AE-00A0C908FA49' | Select -ExpandProperty ClassEntries If you have OleViewDotNet setup (note there are other tools) you can dump all registered classes using the following PowerShell command: You might wonder how is this COM class is registered? An implementor needs to register their COM object with a Category ID of "". And a failure to create the object causes the Save method to fail and the Attachment Services code to automatically delete the file so the browser can't even do anything about it such as ask the user. The implementation for that COM class is in MPOAV.DLL, which as we saw is blocked so the COM object creation fails. When the IAttachmentExecute::Save method is called the file is checked for viruses using the currently registered anti-virus COM object which implements the IOfficeAntiVirus interface. ![]() Which is a common interface to verify downloaded files and attachments, apply MOTW and check for viruses. The code was using the Attachment Services API. Tracking down the resource string for the error lead me to this code. As Chrome is open source it made more sense to look there. As the same failure occurred in both Edge (I didn't test IE) and Chrome it was clearly some common API they were calling. I thought it'd at least be interesting to see why it fails and what MPOAV is doing. of course this is known about (I'm not suggesting otherwise), AaronLocker should allow this DLL by default. This is intentional as you don't want to grant access to locations a normal user could write to, so generally allowing all of %ProgramData% would be asking for trouble. This makes sense, the default rules only permit %WINDOWS% and %PROGRAMFILES% for normal users, however %OSDRIVE%\ProgramData is not allowed. The following table lists the default rules that are available for the executable rule collection.The failing DLL load was for "%OSDRIVE%\PROGRAMDATA\MICROSOFT\WINDOWS DEFENDER\PLATFORM\.4-0\MPOAV.DLL". Because all of the default rules for the executable rule collection are based on folder paths, all files under those paths will be allowed. com extensions that are associated with an app. This topic describes the file formats and available default rules for the executable rule collection.ĪppLocker defines executable rules as any files with the. Learn more about the Windows Defender Application Control feature availability. Some capabilities of Windows Defender Application Control are only available on specific Windows versions.
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