reverse-engineering-dotnet-malware-with-dnspy

name	reverse-engineering-dotnet-malware-with-dnspy
description	Reverse engineers .NET malware samples using the dnSpy decompiler and debugger to read C#/VB.NET source, deobfuscate code protected by tools like ConfuserEx or SmartAssembly, and extract hardcoded C2 configurations, keys, and credentials. Use when a sample is identified as a .NET assembly (e.g. AgentTesla, AsyncRAT, RedLine Stealer, Quasar RAT) and needs decompilation, deobfuscation, or config extraction.
domain	cybersecurity
subdomain	malware-analysis
tags	["malware","dotnet","reverse-engineering","dnSpy","decompilation"]
version	1.0.0
author	mahipal
license	Apache-2.0
nist_csf	["DE.AE-02","RS.AN-03","ID.RA-01","DE.CM-01"]
mitre_attack	["T1027","T1055","T1140","T1497"]

Reverse Engineering .NET Malware with dnSpy

When to Use

• A malware sample is identified as a .NET assembly (C#, VB.NET, F#) requiring decompilation
• Analyzing .NET-based malware families (AgentTesla, AsyncRAT, RedLine Stealer, Quasar RAT)
• Deobfuscating .NET code protected by ConfuserEx, SmartAssembly, or custom obfuscators
• Extracting hardcoded C2 configurations, encryption keys, and credentials from managed assemblies
• Debugging .NET malware at runtime to observe decryption routines and dynamic behavior

Do not use for native (unmanaged) PE binaries; use Ghidra or IDA for native code analysis.

Prerequisites

• dnSpy or dnSpyEx installed (https://github.com/dnSpyEx/dnSpy - community maintained fork)
• de4dot for automated .NET deobfuscation (https://github.com/de4dot/de4dot)
• ILSpy as an alternative decompiler for cross-validation
• .NET SDK installed for recompiling modified assemblies during analysis
• Isolated Windows VM for running dnSpy debugger on live malware
• Detect It Easy (DIE) for identifying the .NET obfuscator used

Workflow

Step 1: Identify .NET Assembly and Obfuscator

Verify the sample is a .NET binary and detect protection:

# Check if file is .NET assembly
file suspect.exe
# Output should contain "PE32 executable" with .NET metadata

# Detect obfuscator with Detect It Easy
diec suspect.exe

# Python-based .NET detection
python3 << 'PYEOF'
import pefile

pe = pefile.PE("suspect.exe")

# Check for .NET COM descriptor
if hasattr(pe, 'DIRECTORY_ENTRY_COM_DESCRIPTOR'):
    print("[*] .NET assembly detected")
    print(f"    Runtime version: {pe.DIRECTORY_ENTRY_COM_DESCRIPTOR}")
else:
    # Check for mscoree.dll import (alternative detection)
    for entry in pe.DIRECTORY_ENTRY_IMPORT:
        if entry.dll.decode().lower() == "mscoree.dll":
            ()
            
    :
        ()


 section  pe.sections:
    name = section.Name.decode().rstrip()
     name  [, , ]:
        (f)
PYEOF