Aspnet_Compiler.exe

ASP.NET Compilation Tool

Paths

• c:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_compiler.exe
• c:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet_compiler.exe

Commands

AWL Bypass

Execute C# code with the Build Provider and proper folder structure in place.

Use case: Execute proxied payload with Microsoft signed binary to bypass application control solutions

Privileges: User

C:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet_compiler.exe -v none -p C:\users\cpl.internal\desktop\asptest\ -f C:\users\cpl.internal\desktop\asptest\none -u

Detection

• BlockRule: https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-application-control/microsoft-recommended-block-rules
• Sigma: https://github.com/SigmaHQ/sigma/blob/62d4fd26b05f4d81973e7c8e80d7c1a0c6a29d0e/rules/windows/process_creation/proc_creation_win_lolbin_aspnet_compiler.yml

Resources

• https://ijustwannared.team/2020/08/01/the-curious-case-of-aspnet_compiler-exe/
• https://docs.microsoft.com/en-us/dotnet/api/system.web.compilation.buildprovider.generatecode?view=netframework-4.8