Extracting Config from Agent Tesla RAT

Overview

Agent Tesla is a .NET-based Remote Access Trojan (RAT) and keylogger that ranked among the top 10 malware variants in 2024, impacting 6.3% of corporate networks globally. It exfiltrates stolen credentials via SMTP email, FTP upload, Telegram bot API, or Discord webhooks. The malware configuration is embedded in the .NET assembly, typically obfuscated using string encryption, resource encryption, or custom loaders that decrypt and execute Agent Tesla in memory via .NET Reflection (fileless). Configuration extraction involves decompiling the .NET assembly with dnSpy or ILSpy, identifying the decryption routine for configuration strings, and extracting SMTP server addresses, credentials, FTP endpoints, Telegram bot tokens, and targeted applications.

Prerequisites

dnSpy or ILSpy for .NET decompilation
Python 3.9+ with dnlib or pythonnet for automated extraction
de4dot for .NET deobfuscation
Understanding of .NET IL code and Reflection
Sandbox for dynamic analysis (ANY.RUN, CAPE)

Practical Steps

Step 1: Deobfuscate and Extract Configuration

#!/usr/bin/env python3
"""Extract Agent Tesla RAT configuration from .NET assemblies."""
import re
import sys
import json
import base64
import hashlib
from pathlib import Path


def extract_strings_from_dotnet(filepath):
    """Extract readable strings from .NET binary for config analysis."""
    with open(filepath, 'rb') as f:
        data = f.read()

    # Extract US (User Strings) heap from .NET metadata
    strings = []

    # Look for common Agent Tesla config patterns
    patterns = {
        "smtp_server": re.compile(rb'smtp[\.\-][\w\.\-]+\.\w{2,}', re.I),
        "email": re.compile(rb'[\w\.\-]+@[\w\.\-]+\.\w{2,}'),
        "ftp_url": re.compile(rb'ftp://[\w\.\-:/]+', re.I),
        "telegram_token": re.compile(rb'\d{8,10}:[A-Za-z0-9_-]{35}'),
        "telegram_chat": re.compile(rb'(?:chat_id=|chatid[=:])[\-]?\d{5,15}', re.I),
        "discord_webhook": re.compile(rb'https://discord\.com/api/webhooks/\d+/[\w-]+'),
        "password": re.compile(rb'(?:pass(?:word)?|pwd)[=:]\s*[\w!@#$%^&*]{4,}', re.I),
        "port": re.compile(rb'(?:port|smtp_port)[=:]\s*\d{2,5}', re.I),
    }

    results = {}
    for name, pattern in patterns.items():
        matches = pattern.findall(data)
        if matches:
            results[name] = [m.decode('utf-8', errors='replace') for m in matches]

    # Extract Base64-encoded strings (common obfuscation)
    b64_pattern = re.compile(rb'[A-Za-z0-9+/]{20,}={0,2}')
    b64_decoded = []
    for match in b64_pattern.finditer(data):
        try:
            decoded = base64.b64decode(match.group())
            text = decoded.decode('utf-8', errors='strict')
            if text.isprintable() and len(text) > 5:
                b64_decoded.append(text)
        except Exception:
            pass

    if b64_decoded:
        results["base64_decoded_strings"] = b64_decoded[:30]

    return results


def decrypt_agenttesla_strings(data, key_hex):
    """Decrypt Agent Tesla encrypted configuration strings."""
    key = bytes.fromhex(key_hex)
    # Agent Tesla V1: Simple XOR with key
    decrypted_strings = []

    # Find encrypted blobs (high-entropy byte sequences)
    blob_pattern = re.compile(rb'[\x80-\xff]{16,256}')
    for match in blob_pattern.finditer(data):
        blob = match.group()
        # Try XOR decryption
        decrypted = bytes(b ^ key[i % len(key)] for i, b in enumerate(blob))
        try:
            text = decrypted.decode('utf-8', errors='strict')
            if text.isprintable() and len(text.strip()) > 3:
                decrypted_strings.append(text.strip())
        except UnicodeDecodeError:
            pass

    # V2: SHA256-based key derivation then AES
    sha256_key = hashlib.sha256(key).digest()

    return decrypted_strings


def analyze_exfiltration_config(config):
    """Analyze extracted configuration for exfiltration methods."""
    methods = []

    if config.get("smtp_server"):
        methods.append({
            "type": "SMTP",
            "servers": config["smtp_server"],
            "emails": config.get("email", []),
        })

    if config.get("ftp_url"):
        methods.append({
            "type": "FTP",
            "urls": config["ftp_url"],
        })

    if config.get("telegram_token"):
        methods.append({
            "type": "Telegram",
            "tokens": config["telegram_token"],
            "chat_ids": config.get("telegram_chat", []),
        })

    if config.get("discord_webhook"):
        methods.append({
            "type": "Discord",
            "webhooks": config["discord_webhook"],
        })

    return methods


if __name__ == "__main__":
    if len(sys.argv) < 2:
        print(f"Usage: {sys.argv[0]} <agent_tesla_sample>")
        sys.exit(1)

    config = extract_strings_from_dotnet(sys.argv[1])
    methods = analyze_exfiltration_config(config)

    report = {"raw_config": config, "exfiltration_methods": methods}
    print(json.dumps(report, indent=2))

Validation Criteria

Exfiltration method identified (SMTP/FTP/Telegram/Discord)
Server addresses and credentials extracted from config
Targeted applications list recovered
Keylogger and screenshot capture settings documented
Persistence mechanism identified
IOCs suitable for network blocking extracted

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