CISA Releases Best Practices for Reducing Risk from Ransomware Attacks

• Require multi-factor authentication for remote access to OT and IT networks.
• Enable strong spam filters to prevent phishing emails from reaching end users. Filter emails containing executable files from reaching end users.
• Implement a user training program and simulated attacks for spear phishing to discourage users from visiting malicious websites or opening malicious attachments and re-enforce the appropriate user responses to spear phishing emails.
• Filter network traffic to prohibit ingress and egress communications with known malicious IP addresses. Prevent users from accessing malicious websites by implementing URL blocklists and/or allowlists.
• Update software including operating systems, applications, and firmware on IT network assets, in a timely manner. Consider using a centralized patch management system; use a risk-based assessment strategy to determine which OT network assets and zones should participate in the patch management program.
• Limit access to resources over networks, especially by restricting RDP. After assessing risks, if RDP is deemed operationally necessary, restrict the originating sources and require multi-factor authentication.
• Set antivirus/antimalware programs to conduct regular scans of IT network assets using up-to-date signatures. Use a risk-based asset inventory strategy to determine how OT network assets are identified and evaluated for the presence of malware.
• Implement unauthorized execution prevention by:
  • Disabling macro scripts from Microsoft Office files transmitted via email. Consider using Office Viewer software to open Microsoft Office files transmitted via email instead of full Microsoft Office suite applications.
  • Implementing application allow listing, which only allows systems to execute programs known and permitted by security policy. Implement software restriction policies (SRPs) or other controls to prevent programs from executing from common ransomware locations, such as temporary folders supporting popular internet browsers or compression/decompression programs, including the AppData/LocalAppData folder.
  • Monitor and/or block inbound connections from Tor exit nodes and other anonymization services to IP addresses and ports for which external connections are not expected (i.e., other than VPN gateways, mail ports, web ports). For more guidance, refer to Joint Cybersecurity Advisory AA20-183A: Defending Against Malicious Cyber Activity Originating from Tor.
  • Deploy signatures to detect and/or block inbound connection from Cobalt Strike servers and other post-exploitation tools.

Reducing risk in your business should you fall victim in the future

• Implement and ensure robust network segmentation between IT and OT networks to limit the ability of adversaries to pivot to the OT network even if the IT network is compromised. Define a demilitarized zone that eliminates unregulated communication between the IT and OT networks.
• Organize OT assets into logical zones by taking into account criticality, consequence, and operational necessity. Define acceptable communication conduits between the zones and deploy security controls to filter network traffic and monitor communications between zones. Prohibit industrial control system (ICS) protocols from traversing the IT network.
• Identify OT and IT network inter-dependencies and develop workarounds or manual controls to ensure ICS networks can be isolated if the connections create risk to the safe and reliable operation of OT processes. Regularly test contingency plans such as manual controls so that safety-critical functions can be maintained during a cyber incident. Ensure that the OT network can operate at necessary capacity even if the IT network is compromised.
• Regularly test manual controls so that critical functions can be kept running if ICS or OT networks need to be taken offline.
• Implement regular data backup procedures on both the IT and OT networks. Backup procedures should be conducted on a frequent, regular basis. The data backup procedures should also address the following best practices:
  • Ensure that backups are regularly tested.
  • Store your backups separately. Backups should be isolated from network connections that could enable the spread of ransomware. It is important that backups be maintained offline as many ransomware variants attempt to find and encrypt or delete accessible backups. Maintaining current backups offline is critical.  If your network data is encrypted with ransomware, then your organization can restore systems to its previous state.  Best practice is to store your backups on a separate device that cannot be accessed from a network, such as on an external hard drive. (See the Software Engineering Institute’s page on ransomware.)
  • Maintain regularly updated “gold images” of critical systems in the event they need to be rebuilt. This entails maintaining image “templates” that include a preconfigured operating system (OS) and associated software applications that can be quickly deployed to rebuild a system, such as a virtual machine or server.
  • Retain backup hardware to rebuild systems in the event rebuilding the primary system is not preferred. Hardware that is newer or older than the primary system can present installation or compatibility hurdles when rebuilding from images.
  • Store source code or executables. It is more efficient to rebuild from system images, but some images will not install on different hardware or platforms correctly; having separate access to needed software will help in these cases.
• Ensure user and process accounts are limited through account use policies, user account control, and privileged account management. Organize access rights based on the principles of least privilege and separation of duties.

If your organization is impacted by a ransomware incident

• Isolate the infected system. Remove the infected system from all networks, and disable the computer’s wireless, Bluetooth, and any other potential networking capabilities. Ensure all shared and networked drives are disconnected, whether wired or wireless.
• Turn off other computers and devices. Power-off and segregate (i.e., remove from the network) the infected computer(s). Power-off and segregate any other computers or devices that shared a network with the infected computer(s) that have not been fully encrypted by ransomware. If possible, collect and secure all infected and potentially infected computers and devices in a central location, making sure to clearly label any computers that have been encrypted. Powering off and segregating infected computers and computers that have not been fully encrypted may allow for the recovery of partially encrypted files by specialists. (See Before You Connect a New Computer to the Internet for tips on how to make a computer more secure before you reconnect it to a network.)
• Secure your backups. Ensure that your backup data is offline and secure. If possible, scan your backup data with an antivirus program to check that it is free of malware.
• Refer to Joint Cybersecurity Advisory: AA20-245A: Technical Approaches to Uncovering and Remediating Malicious Activity for more best practices on incident response.

Note: CISA and the FBI do not encourage paying a ransom to criminal actors. Paying a ransom may embolden adversaries to target additional organizations, encourage other criminal actors to engage in the distribution of ransomware, and/or may fund illicit activities. Paying the ransom also does not guarantee that a victim’s files will be recovered. CISA and FBI urge you to report ransomware incidents to your local FBI field office.  If you are concerned about reducing risk in your organization, please contact us.