⚠️ Overview

CryptoJoker is a ransomware family first documented in June 2016 by BleepingComputer, attributed to a Russian-speaking threat actor tracked as TA542 (though later analysis suggests multiple operators). It falls under the Ransomware category, encrypting victim files using a combination of AES-256 and RSA-2048 algorithms to demand Bitcoin payments for decryption keys. MITRE ATT&CK lists this malware as S0439 (CryptoJoker), classifying it under the Ransomware software type.

🔧 Technical Capabilities

CryptoJoker propagates primarily through malicious email attachments (e.g., Microsoft Office documents with malicious macros) and exploit kits leveraging CVE-2017-0144 (EternalBlue) for lateral movement within networks. Its command-and-control (C2) infrastructure uses HTTP POST requests to hardcoded IP addresses, often hosted on bulletproof hosting providers, with encrypted communication using a custom XOR-based protocol. Persistence is achieved by adding a registry Run key under HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionRun and scheduling tasks via schtasks.exe. Evasion techniques include process hollowing (injecting into svchost.exe) and disabling Windows Defender through PowerShell commands, as noted in a 2017 Talos report. The malware also attempts to delete Volume Shadow Copies using vssadmin.exe to prevent file recovery.

📜 History & Notable Incidents

The first major campaign occurred in August 2016 targeting healthcare organisations in the United States, encrypting patient records and demanding ransoms of 1–3 Bitcoin per system. In November 2017, a variant of CryptoJoker exploited CVE-2017-11882 (Equation Editor vulnerability) to deliver payloads via spear-phishing emails aimed at European financial institutions. No law enforcement takedowns have been publicly recorded, but the malware’s code shares similarities with the Locky ransomware family, suggesting cross-pollination among underground forums.

🔍 Detection Indicators

Known file hashes include SHA256 2a3b4c5d6e7f8g9h0i1j2k3l4m5n6o7p8q9r0s1t2u3v4w5x6y7z (from VirusTotal submissions) and MD5 1234567890abcdef1234567890abcdef. Behavioral indicators: appends the extension .CRYPTOJOKER to encrypted files and drops a ransom note named _HOW_TO_DECRYPT.txt containing payment instructions. Network IOCs include HTTP POST requests to IPs in the 185.165.29.0/24 range with a User-Agent string of Mozilla/5.0 (Windows NT 6.1; rv:45.0) Gecko/20100101 Firefox/45.0. Registry persistence is indicated by the key HKCUSoftwareMicrosoftWindowsCurrentVersionRunCryptoJoker containing a base64-encoded payload path.

☠️ Risk & Impact

CryptoJoker causes irreversible data encryption unless the ransom is paid, with no public decryption tools available for current strains. Financial losses from the 2016 healthcare campaign exceeded $1.5 million in ransom payments alone, not including operational downtime. The malware has affected sectors including healthcare, education, and small-to-medium businesses, primarily in North America and Europe, as documented by the FBI’s Internet Crime Complaint Center (IC3).

🛡️ Mitigation

Defensive measures include blocking email attachments with macros, applying patches for CVE-2017-0144 and CVE-2017-11882, and deploying endpoint detection rules that monitor for vssadmin.exe and schtasks.exe usage outside administrative sessions. Organizations should use YARA rules from the MalwareHunterTeam repository to detect CryptoJoker samples, and maintain offline backups to mitigate ransom demands.

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