Actually Preventing Ransomware

This post is in response to a preponderance of unhelpful advice posts running around on the Internet with click-baity titles, such as “Top X ways to prevent ransomware in YOUR business!”.  This advice is intended for organizations rather than home users.

A common factor that leads to ransomware problems is failing to implement appropriate controls because of some perceived trade off or ignorance of the risks.  I contend that organizations that make such trade offs are not actually interested in preventing ransomware.  Many organizations and “thought leadership” articles approach ransomware prevention using a combination of awareness training, patching, up-to-date antivirus, and data backups.  But those of us who have been around the ransomware block a few times know that none of these things prevent modern ransomware attacks.

This list of controls is narrowly focused on ransomware – there are many other good controls I don’t mention below.

I ran a most unscientific poll on twitter, and the results are a bit disturbing.  I did not expect the majority of security people responding to state that training can stop most or almost all ransomware.  In retrospect, the question may be badly worded.  Possibly by volume of ransomware training may help, but I don’t expect that to be the case if we consider the spectrum of different attack techniques and malware samples.

In your experience, how much ransomware can be prevented through training?

— Jerry Bell (@Maliciouslink) August 26, 2017

At a high level, we need to implement controls that:

1. Block ransomware from getting to workstations.
2. Block the execution of ransomware once it gets onto a workstation.
3. Block propagation of the ransomware once it gets onto a workstation, assuming it executes.
4. Mitigate the damage cause by ransomware once it gets onto a workstation, assuming it executes.

Controls to prevent ransomware from getting to workstations:

• Block MS Office file attachments that contain macros (i.e., .docm, .xlsm) and other common executable and script file types in email.
• Implement web filtering and block access to known malicious sites, sites for which there is little value for the business, and uncategorized sites.
• Block non-company email and webmail using web filtering and blocking of common email ports at the firewall.
• Implement ad blockers on web browsers.

Controls to prevent ransomware from executing on workstations:

• Provide quality ongoing phishing training to employees, paired with phishing simulations.
• Flag incoming email from the Internet as [external] or similar in the subject line, and incorporate this into the security training program.
• Apply operating system and application patches promptly after ensuring the patch will not do more harm than good.
• Ensure that all applications on used on workstations are being managed (i.e., someone is responsible for deploying patches).
• Disable MS Office macros on workstations.
• Associate undesirable file types, such as .js, et al, with notepad.exe.
• Implement application whitelisting to block execution of unknown applications.

Controls to limit propagation of ransomware:

• Disable SMBv1.
• Block inbound SMB and RDP connections to workstations.
• Block accounts with elevated privileges from logging into workstations.
• Remove local administrator rights from general user accounts.
• Implement port isolation on the network such that workstations cannot communicate with each other.
• Use LAPS to configure a unique local administrator account password on each workstation.

Mitigating the damage caused by ramsomware:

• Monitor file servers and deny permissions to accounts that create particular file types associated with ransomware-locked files.
• Enable volume shadow copies.
• Use a backup solution that supports file versioning on workstation and servers.  Periodically test backups with a restore.  Script an alert to detect if backups stop running for some reason.
• Minimize the number of mapped drives on workstations.
• Restrict write access on network drives to those people who need to be able to save new versions of files.

I know there are many other good ideas that should be added to this list.  I would be grateful if you can add them below as a comment.  I am thinking about putting this on a publicly editable wiki, with the intention of providing an objective set of controls, and some explanation on the how and why for each.