--- name: reverse-engineering-dotnet-malware-with-dnspy-4510707a6b53 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: ```bash # 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": print("[*] .NET assembly detected (mscoree.dll import)") break else: print("[!] Not a .NET assembly") # Check section names for .NET indicators for section in pe.sections: name = section.Name.decode().rstrip('\x00') if name in ['.text', '.rsrc', '.reloc']: print(f" Section: {name} (typical .NET)") PYEOF ``` ### Step 2: Deobfuscate with de4dot Remove common .NET obfuscation before manual analysis: ```bash # Run de4dot to identify and remove obfuscation de4dot suspect.exe -o suspect_cleaned.exe # Force specific deobfuscator de4dot suspect.exe -p cf # ConfuserEx de4dot suspect.exe -p sa # SmartAssembly de4dot suspect.exe -p dr # Dotfuscator de4dot suspect.exe -p rv # Reactor de4dot suspect.exe -p bl # Babel.NET # Verbose output for debugging de4dot -v suspect.exe -o suspect_cleaned.exe # Handle multi-file assemblies de4dot suspect.exe suspect_helper.dll -o cleaned/ ``` ``` Common .NET Obfuscators: ━━━━━━━━━━━━━━━━━━━━━━━ ConfuserEx: String encryption, control flow, anti-debug, anti-tamper SmartAssembly: String encoding, flow obfuscation, pruning Dotfuscator: Renaming, string encryption, control flow .NET Reactor: Native code generation, necrobit, anti-debug Babel.NET: String encryption, resource encryption, code virtualization Crypto Obfuscator: String encryption, anti-debug, watermarking Custom: Malware-specific obfuscation (manual de4dot configuration needed) ``` ### Step 3: Open in dnSpy and Analyze Code Load the deobfuscated assembly in dnSpy for source-level analysis: ``` dnSpy Analysis Workflow: ━━━━━━━━━━━━━━━━━━━━━━━ 1. File -> Open -> Select cleaned assembly 2. Navigate to the entry point: - Assembly Explorer -> -> Program class -> Main method - Or: Right-click assembly -> Go to Entry Point 3. Key areas to examine: - Entry point (Main) for initialization and execution flow - Form classes for UI-based malware (RATs, stealers) - Network/HTTP classes for C2 communication - Crypto/encryption classes for data protection - Resource access for embedded payloads - Timer/Thread classes for persistence and scheduling 4. Navigation shortcuts: Ctrl+G - Go to token/address Ctrl+Shift+K - Search assemblies F12 - Go to definition Ctrl+R - Analyze (find usages) F5 - Start debugging F9 - Toggle breakpoint ``` ### Step 4: Extract Configuration and C2 Data Locate hardcoded configuration in the decompiled source: ```csharp // Common .NET malware configuration patterns: // Pattern 1: Static class with hardcoded values public static class Config { public static string Host = "185.220.101.42"; public static int Port = 4782; public static string Key = "GhOsT_RaT_2025"; public static string Mutex = "AsyncMutex_6SI8OkPnk"; public static bool Install = true; public static string InstallFolder = "%AppData%"; } // Pattern 2: Encrypted strings decrypted at runtime public static string Decrypt(string input) { byte[] data = Convert.FromBase64String(input); byte[] key = Encoding.UTF8.GetBytes("SecretKey123"); for (int i = 0; i < data.Length; i++) { data[i] ^= key[i % key.Length]; } return Encoding.UTF8.GetString(data); } // Pattern 3: Resource-embedded configuration byte[] configData = Properties.Resources.config; string config = AES.Decrypt(configData, derivedKey); ``` ```python # Python script to extract .NET resource strings import subprocess import re import base64 # Use monodis (Mono) or ildasm (.NET SDK) to dump IL result = subprocess.run( ["monodis", "--output=il_dump.il", "suspect_cleaned.exe"], capture_output=True, text=True ) # Search for string literals in IL dump with open("il_dump.il", errors="ignore") as f: il_code = f.read() # Find ldstr (load string) instructions strings = re.findall(r'ldstr\s+"([^"]+)"', il_code) for s in strings: # Check for Base64 encoded strings try: decoded = base64.b64decode(s).decode('utf-8', errors='ignore') if len(decoded) > 3 and decoded.isprintable(): print(f" Base64: {s[:40]}... -> {decoded[:100]}") except: pass # Check for URLs/IPs if re.match(r'https?://', s) or re.match(r'\d+\.\d+\.\d+\.\d+', s): print(f" Network: {s}") ``` ### Step 5: Debug with dnSpy Set breakpoints and debug the malware to observe runtime behavior: ``` dnSpy Debugging Workflow: ━━━━━━━━━━━━━━━━━━━━━━━ 1. Set breakpoints on key methods: - String decryption functions (to capture decrypted values) - Network connection methods (to capture C2 URLs) - File write operations (to see what is dropped) - Registry modification methods (to see persistence) 2. Debug -> Start Debugging (F5) - Select the assembly to debug - Set command-line arguments if needed - Configure exception handling (break on all CLR exceptions) 3. At each breakpoint: - Inspect local variables (Locals window) - Evaluate expressions (Immediate window) - View call stack to understand execution context - Step over (F10) / Step into (F11) / Step out (Shift+F11) 4. Capture decrypted strings: - Set breakpoint after decryption function returns - Read the return value from the Locals window - Document all decrypted configuration values ``` ### Step 6: Document Findings Compile analysis results into a structured report: ``` Analysis documentation should include: - .NET assembly metadata (CLR version, target framework, compilation info) - Obfuscator identified and deobfuscation method used - Complete C2 configuration (hosts, ports, encryption keys, mutex names) - Malware capabilities (keylogging, screen capture, file theft, etc.) - Persistence mechanisms (registry, scheduled tasks, startup folder) - Anti-analysis techniques (VM detection, debugger detection, sandbox evasion) - Extracted IOCs (C2 IPs/domains, file hashes, mutex names, registry keys) - YARA rule based on unique code patterns or strings ``` ## Key Concepts | Term | Definition | |------|------------| | **CIL/MSIL** | Common Intermediate Language; the bytecode format .NET assemblies compile to, which can be decompiled back to high-level C#/VB.NET | | **Metadata Token** | Unique identifier for .NET types, methods, and fields within the assembly metadata tables; used for navigation in dnSpy | | **de4dot** | Open-source .NET deobfuscator that identifies and removes protection from many commercial and malware-specific obfuscators | | **ConfuserEx** | Popular open-source .NET obfuscator frequently used by malware authors for string encryption and control flow obfuscation | | **String Encryption** | Obfuscation technique replacing string literals with encrypted data and runtime decryption calls to hide IOCs from static analysis | | **Resource Embedding** | Storing configuration, payloads, or additional assemblies in .NET embedded resources, often encrypted with a key derived from assembly metadata | | **Assembly.Load** | .NET method loading assemblies from byte arrays in memory, enabling fileless execution of embedded payloads | ## Tools & Systems - **dnSpy/dnSpyEx**: Open-source .NET assembly editor, decompiler, and debugger supporting C# and VB.NET decompilation - **de4dot**: Automated .NET deobfuscator supporting ConfuserEx, SmartAssembly, Dotfuscator, Reactor, and many other protectors - **ILSpy**: Open-source .NET decompiler providing C#, VB.NET, and IL views of assembly code - **dotPeek**: JetBrains' free .NET decompiler with symbol server and cross-reference navigation - **Detect It Easy (DIE)**: Multi-format file analyzer identifying .NET framework version, obfuscator, and compiler information ## Common Scenarios ### Scenario: Analyzing an AgentTesla Information Stealer **Context**: A phishing email delivers a .NET executable identified as AgentTesla. The sample needs analysis to determine what credentials it steals, how it exfiltrates data, and its C2 configuration. **Approach**: 1. Run Detect It Easy to identify the obfuscator (commonly ConfuserEx or custom) 2. Deobfuscate with de4dot to restore readable class/method names and decrypt strings 3. Open in dnSpy and navigate to the entry point to understand initialization 4. Locate the credential harvesting modules (browser, email, FTP, VPN password theft classes) 5. Find the exfiltration method (SMTP email, FTP upload, HTTP POST, Telegram bot API) 6. Extract C2 configuration (SMTP server, credentials, recipient email, or HTTP URL) 7. Set debugger breakpoints on the decryption function to capture all decrypted strings at once **Pitfalls**: - Analyzing without de4dot first (ConfuserEx makes manual analysis extremely difficult) - Not checking for multi-stage loading (initial .NET executable may load additional assemblies from resources) - Missing configuration stored in .NET resources rather than hardcoded strings - Running the debugger without network isolation (AgentTesla will attempt to exfiltrate immediately) ## Output Format ``` .NET MALWARE ANALYSIS REPORT ================================ Sample: invoice_scanner.exe SHA-256: e3b0c44298fc1c149afbf4c8996fb924... Type: .NET Assembly (C#) Framework: .NET Framework 4.8 Obfuscator: ConfuserEx v1.6 Deobfuscated: Yes (de4dot -p cf) CLASSIFICATION Family: AgentTesla v3 Type: Information Stealer / Keylogger Compile Date: 2025-09-10 C2 CONFIGURATION Exfil Method: SMTP (Email) SMTP Server: smtp.yandex[.]com:587 SMTP User: exfil.account@yandex[.]com SMTP Pass: Str0ngP@ssw0rd2025 Recipient: operator@protonmail[.]com Interval: 30 minutes Encryption: AES-256 with key "AgentTesla_2025_key" CAPABILITIES [*] Browser credential theft (Chrome, Firefox, Edge, Opera) [*] Email client passwords (Outlook, Thunderbird) [*] FTP client credentials (FileZilla, WinSCP) [*] VPN credentials (NordVPN, OpenVPN) [*] Keylogging (SetWindowsHookEx) [*] Screenshot capture (every 30 seconds) [*] Clipboard monitoring PERSISTENCE Method: Registry Run key + Scheduled Task Registry: HKCU\Software\Microsoft\Windows\CurrentVersion\Run\WindowsUpdate Task: \Microsoft\Windows\WindowsUpdate\Updater EXTRACTED IOCs SMTP Server: smtp.yandex[.]com Exfil Email: exfil.account@yandex[.]com Recipient: operator@protonmail[.]com Mutex: AgentTesla_2025_Q3_MUTEX Install Path: %AppData%\Microsoft\Windows\svchost.exe ``` --- Source attribution: costrict-plugins-repo Original project: https://github.com/costrict-plugins-repo/mukul975-anthropic-cybersecurity-skills-cybersecurity-skills/blob/1b344d155f00bae5cf27d8eba9508e8f5050d904/skills/reverse-engineering-dotnet-malware-with-dnspy/SKILL.md License: Apache-2.0 License information: https://www.apache.org/licenses/LICENSE-2.0 Original upstream instructions; downloaded from Mega AI Hub. ## Upstream license notice Apache License Version 2.0, January 2004 http://www.apache.org/licenses/ TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION 1. 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See the License for the specific language governing permissions and limitations under the License. # Mega AI Hub resource metadata Creator / publisher: costrict-plugins-repo Original project: https://github.com/costrict-plugins-repo/mukul975-anthropic-cybersecurity-skills-cybersecurity-skills/blob/1b344d155f00bae5cf27d8eba9508e8f5050d904/skills/reverse-engineering-dotnet-malware-with-dnspy/SKILL.md Attribution basis: Source repository record Fields of work: Development & Engineering Roles: Security Engineer Specialties: Backend Tools: De4Dot, Detect It Easy, Dnspy, Ilspy Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: ca9568a9ae7fc64e5d68fd09e7a586f97363f3a7e7c787e43fd3420859472f9a