.NET Reverse Engineering

Why .NET Is Different

.NET binaries ship IL (Intermediate Language) code with full metadata — type names, method signatures, string literals, inheritance hierarchies. This makes decompilation far more effective than with native code. A clean .NET binary decompiles to near-source-quality C#. Obfuscation raises the bar but never eliminates the metadata entirely because the CLR needs it to execute.

This matters for offensive work: .NET malware leaks intent through its metadata. Even heavily obfuscated samples expose type references to System.Net.Sockets, System.Security.Cryptography, or System.Runtime.InteropServices — revealing capability before you touch the IL.

Detection — Is This .NET?

Confirm a binary is .NET before choosing your toolchain. Wrong tools waste time.

# Quick check — file magic
file <binary>
# Look for: "Mono/.Net assembly" or "PE32 executable ... Mono/.Net"

# Confirm via PE imports — .NET binaries import mscoree.dll
r2 -qc 'ii~mscoree' <binary>
# _CorExeMain = .NET EXE, _CorDllMain = .NET DLL

# CLI header presence — Data Directory entry 14
r2 -qc 'iH~CLR' <binary>

# monodis sanity check
monodis --assembly <binary>

If the binary is mixed-mode (C++/CLI), it has both native and managed code. Use monodis for the managed portion and radare2/Ghidra for native.

Core Workflow

Follow this sequence for any .NET target. Skip steps that do not apply, but never skip triage.

Phase 1 — Triage and Metadata Extraction

Goal: understand what you have before deep analysis.

# Assembly identity
monodis --assembly <binary>

# Type inventory — namespaces reveal purpose
monodis --typedef <binary>

# External dependencies — what frameworks/libraries does it use?
monodis --assemblyref <binary>
monodis --typeref <binary>

# Entry point identification
monodis --method <binary> | grep -A5 "\.entrypoint"

# Embedded resources — configs, encrypted payloads, embedded DLLs
monodis --manifest <binary>

# String extraction — URLs, IPs, registry keys, crypto constants
strings -el <binary>   # Unicode strings (common in .NET)
strings -a <binary>    # ASCII strings
floss <binary>         # FLARE obfuscated string solver

Key questions to answer during triage:

• What namespaces exist? (System.Net = networking, System.Security.Cryptography = crypto)
• Are names readable or garbled? (obfuscation indicator)
• What external assemblies does it reference?
• Are there embedded resources? (common payload delivery vector)
• Does it reference DllImport/P/Invoke? (native API access)

Phase 2 — Obfuscation Assessment

Determine if and how the binary is obfuscated. This dictates your next steps.

# de4dot auto-detection (does not modify — just reports)
de4dot --detect <binary>

# Manual checks
monodis --typedef <binary>    # Garbled names = obfuscation
monodis --customattr <binary> # Obfuscator watermark attributes

Obfuscator Fingerprints:

If obfuscated → Phase 2a (deobfuscate first). If clean → skip to Phase 3.

Phase 2a — Deobfuscation

# Automatic deobfuscation
de4dot <binary> -o <clean.exe>

# ConfuserEx specifically (use the cex fork)
de4dot-cex <binary> -o <clean.exe>

# Force a specific deobfuscator engine
de4dot <binary> -p <code> -o <clean.exe>
# Codes: an=Agile.NET, ba=Babel, co=CryptoObfuscator, cx=ConfuserEx,
#         df=Dotfuscator, dr=.NETReactor, ef=Eazfuscator, sk=SmartAssembly

# Verify deobfuscation worked
monodis --typedef <clean.exe>

If automatic deobfuscation fails, see references/deobfuscation.md for manual techniques.

Phase 3 — Decompilation

# Full C# source recovery (primary tool)
ilspycmd <binary> -o <outdir>/

# IL-level dump (when C# decompilation fails or you need exact opcodes)
monodis --method <binary> > il-dump.il

# Single type decompilation
ilspycmd <binary> -t <Namespace.TypeName>

Decompilation output goes to extracted/<analysis-name>/dotnet-source/.

Phase 4 — Deep Analysis

With readable source, focus on:

For malware:

• Entry point and initialization flow
• C2 communication (look for HttpClient, WebClient, TcpClient, Socket, WebSocket)
• Encryption/decryption routines (look for Aes, RijndaelManaged, RSA, XOR loops)
• Persistence mechanisms (Registry, ScheduledTask, WMI, Startup)
• Config extraction (hardcoded strings, resource decryption, embedded JSON/XML)
• Anti-analysis (sleep loops, VM detection, debugger checks via Debugger.IsAttached)
• Process injection (VirtualAllocEx, WriteProcessMemory, CreateRemoteThread via P/Invoke)

For application security:

• Serialization vulnerabilities (see references/serialization-attacks.md)
• Reflection abuse (Assembly.Load, Activator.CreateInstance, Type.InvokeMember)
• P/Invoke surface (see P/Invoke Analysis below)
• Hardcoded secrets (connection strings, API keys, certificates)
• Cryptographic weaknesses (ECB mode, static IVs, weak key derivation)
• SQL injection in ORM bypass patterns
• Path traversal in file operations

See references/malware-analysis.md for detailed malware analysis patterns.

Phase 5 — Patching and Instrumentation

When you need to modify behavior — bypass license checks, neutralize anti-analysis, redirect C2:

# Roundtrip: decompile IL → edit → reassemble
monodis <binary> > code.il
# Edit code.il (modify IL instructions)
ilasm code.il -output:<patched.exe>

# Verify patch
monodis --method <patched.exe> | grep -A10 "<target-method>"

See references/patching.md for IL patching patterns and runtime instrumentation.

P/Invoke Analysis

P/Invoke (DllImport) calls bridge managed code to native APIs. In malware, this is where the dangerous operations happen — process injection, privilege escalation, AV evasion.

# Extract all P/Invoke declarations
monodis --implmap <binary>

# In decompiled source, search for DllImport
grep -rn "DllImport" <outdir>/
grep -rn "extern static" <outdir>/

Critical P/Invoke patterns in malware:

Embedded Assembly Extraction

.NET malware frequently embeds additional assemblies as resources, loading them at runtime via Assembly.Load().

# List resources
monodis --manifest <binary>

# Common patterns:
# 1. Costura.Fody — look for costura.*.dll.compressed resources
# 2. Custom loader — look for AppDomain.AssemblyResolve handler
# 3. Encrypted payload — resource loaded, decrypted, then Assembly.Load()

# Extract resources with ilspycmd
ilspycmd