Interlock Ransomware Explained: Understanding the Attack Chain and CISA's Defense Strategy

Ransomware has evolved far beyond simple file encryption. Modern ransomware groups now combine sophisticated intrusion techniques, credential theft, data exfiltration, and psychological pressure to maximize financial gain. One such emerging threat is Interlock Ransomware, a financially motivated ransomware operation first observed in September 2024. According to the joint #StopRansomware advisory issued by the Cybersecurity and Infrastructure Security Agency (CISA), Federal Bureau of Investigation (FBI), Department of Health and Human Services (HHS), and MS-ISAC, Interlock has targeted businesses and critical infrastructure across North America and Europe using advanced social engineering techniques, malware, and a double-extortion strategy.

Unlike traditional ransomware groups that primarily rely on phishing emails, Interlock employs drive-by downloads and ClickFix social engineering to gain initial access. These techniques demonstrate how attackers are shifting from exploiting software vulnerabilities to exploiting human trust, making user awareness as important as technical security controls.

Every ransomware attack follows a structured lifecycle, and understanding this chain helps organizations detect and stop attackers before encryption begins.

1. Initial Access

Interlock commonly gains access through compromised legitimate websites that distribute fake software updates. Victims may unknowingly download malicious installers disguised as Google Chrome, Microsoft Edge, Cisco Secure Client, FortiClient, or other trusted applications. The advisory also highlights ClickFix, where victims are presented with a fake CAPTCHA page instructing them to open the Windows Run dialog, paste clipboard contents, and press Enter. The clipboard contains a Base64-encoded PowerShell command that downloads malware, allowing attackers to bypass traditional phishing techniques.

2. Execution and Persistence

After the malicious payload executes, Interlock deploys a Remote Access Trojan (RAT) that launches PowerShell scripts to install additional malware. To ensure continued access, attackers establish persistence by placing malicious files in the Windows Startup folder and creating Registry Run Keys disguised with names such as "Chrome Updater." Every time the victim logs in, these components automatically execute, giving attackers long-term control over the compromised system.

3. Reconnaissance

With persistence established, attackers begin gathering intelligence about the victim's environment. Using built-in PowerShell commands such as systeminfo, tasklist, Get-Service, Get-PSDrive, and arp -a, they identify operating system details, running services, mapped drives, and network configurations. This information enables attackers to locate valuable assets and prepare for lateral movement.

4. Credential Theft and Privilege Escalation

Interlock downloads credential stealers and keyloggers to harvest usernames, passwords, browser-stored credentials, and user keystrokes. The advisory also notes the use of malware such as Lumma Stealer, Berserk Stealer, and possible Kerberoasting attacks to compromise domain administrator accounts. By obtaining privileged credentials, attackers can access critical systems and expand their control throughout the enterprise.

5. Lateral Movement

Using compromised credentials, attackers move across the network through Remote Desktop Protocol (RDP) and legitimate remote administration tools such as AnyDesk, PuTTY, and ScreenConnect. Since these tools are widely used by IT administrators, malicious activity can blend into legitimate administrative operations, making detection significantly more difficult.

6. Data Exfiltration

Before encrypting systems, Interlock steals sensitive information using legitimate tools like Azure Storage Explorer, AzCopy, and WinSCP. This enables attackers to copy confidential business data to attacker-controlled cloud storage, creating additional leverage during ransom negotiations.

7. Encryption and Double Extortion

The final stage involves deploying ransomware encryptors for both Windows and Linux, including virtual machines. Interlock uses a combination of AES and RSA encryption algorithms to lock victim files. Encrypted files receive extensions such as .interlock or .1nt3rlock, while victims receive a ransom note directing them to contact the attackers through a Tor (.onion) website. Unlike many ransomware groups, Interlock does not immediately specify a ransom amount. Instead, negotiations begin only after victims establish contact. Since attackers have already stolen sensitive information, victims face a double-extortion dilemma: recover encrypted files and prevent confidential data from being publicly leaked.

The advisory maps Interlock's activities across the MITRE ATT&CK framework, demonstrating that this ransomware operation spans the entire attack lifecycle. Techniques include Drive-by Compromise (T1189), PowerShell Execution (T1059.001), Registry Run Keys (T1547.001), Credential Dumping, Kerberoasting, Remote Desktop Protocol (T1021.001), Exfiltration to Cloud Storage (T1567.002), and Data Encrypted for Impact (T1486). This mapping helps security teams align detection rules, SIEM alerts, and threat hunting activities with known attacker behaviors.

The advisory emphasizes that preventing ransomware requires a defense-in-depth approach rather than relying on a single security product. Organizations should implement DNS filtering and web access firewalls to block malicious websites, regularly patch operating systems and internet-facing applications, enforce Multi-Factor Authentication (MFA), and implement strong Identity, Credential, and Access Management (ICAM) policies. CISA also recommends deploying Endpoint Detection and Response (EDR) solutions, segmenting networks to limit lateral movement, maintaining encrypted and immutable offline backups, monitoring network activity for abnormal behavior, applying the principle of least privilege, and conducting continuous security validation using the MITRE ATT&CK framework. Employee awareness training remains equally important because attacks such as ClickFix rely heavily on social engineering rather than technical exploits.

Conclusion.

Interlock ransomware demonstrates how modern cybercriminals have transformed ransomware into a multi-stage enterprise attack. Instead of simply encrypting files, attackers patiently establish persistence, perform reconnaissance, steal credentials, move laterally across networks, exfiltrate sensitive information, and finally encrypt systems while threatening public data disclosure. This evolution means organizations must focus on detecting early-stage attacker behavior rather than waiting until encryption begins. By implementing CISA's recommended security controls, maintaining strong cyber hygiene, and continuously validating defenses against the MITRE ATT&CK framework, organizations can significantly reduce their exposure to ransomware and improve their ability to detect, contain, and recover from attacks before they become business-critical incidents.