Wikipedia describes ransomware as “a type of malicious software that carries out the cryptoviral extortion attack from cryptovirology that blocks access to data until a ransom is paid and displays a message requesting payment to unlock it.”
In Layman's terms, it denies you access to your own files until you pay the ransomware creators to unlock your files. This extortion method has proven to be very profitable, with CryptoLocker and CryptoWall netting their creators U$3 million and US$17 million respectively.
Ransomware attacks are typically carried out by a Trojan, where the user is tricked into opening a seemingly legitimate attachment or document. Once activated, the ransomware will spread by exploiting vulnerabilities on the target systems. Once a system has been compromised and the files encrypted the victim will be issued with instructions on how to unlock the files - unlocking methods invariably involves paying the ransomware creators via hard-to-trace channels such as Bitcoin.
Whilst the result of a ransomware attack may be devastating, there are a lot of measures that one can take to protect your organisation. I will give a brief overview of these methods below. Do note that attacks, and indeed defense mechanisms, are constantly evolving. Thus the measures below are in no way exhaustive, but should be seen as a very strong baseline to work from.
Secure your perimeter
Your first line of defense is crucial. As such you need to make sure that a Unified Threat Management (UTM) firewall is part of your perimeter security solution. This will allow you to activate measures such as Anti-Virus and Malware scanning as well as Intrusion Detection and Prevention Systems (IDS / IPS). A UTM firewall will also allow you to configure outbound filtering, which will prevent any "phone-home" communication between a compromised system and its command-and-control servers.
Secure your E-Mail
E-Mail is one of the most prevalent attack vectors, so it just makes sense to invest significant effort to secure it. Start off by enable strong spam filters to prevent phishing and other similar forms of attack from reaching your users. You should furthermore look at implementing methods to authenticate inbound mail, these include technologies such as Domain Message Authentication Reporting and Conformance (DMARC), DomainKeys Identified Mail (DKIM) and Sender Policy Framework (SPF). All these technologies complement each other and works together to detect and prevent mail spoofing. Of course you also need to make sure you are scanning all mails for viruses and, last but not least, filter all executable and "bad" file types.
Most ransomware propagates by exploiting Operating System Vulnerabilities. Point in case is the WannaCry variant which utilizes the EternalBlue exploit, which was actually patched by Microsoft two months before the first attacks hit. This clearly demonstrates the value of patching. Ensure you have a proper patch management strategy in place and that systems are patched timeously.
This goes without saying - always ensure that your anti-virus (AV) solution is up to date and that real-time scanning is enabled. Apart from that you'll also need to configure it to perform regular scheduled scans. Most AV solutions now also include heuristic technologies, which allows it to detect and remove threats for which there are no signatures yet, make sure you utilize this. Lastly, ensure that the endpoint's firewall is activated if provided as part of your AV solution.
Implement Least Privilege
Limit the use of administrative privileges to a minimum and also make sure that User Access Control (UAC) is activated. The principle of Least Privilege should also be applied when granting users access to network resources. Only give write permissions to users that absolutely need those permissions. Default to read only permissions when possible. You should also regularly audit your file shares and review the needed permissions for each share. Ransomware will typically enumerate all network resources an infected system has access to and then encrypt those resources. You can significantly reduce the impact of Ransomware by limiting user access to network resources and just granting them access to what they actually need.
Block file execution from certain paths
Ransomware generally executes from temporary folders located in Windows AppData folders. To protect against this you should configure a Software Restriction Policy to prevent executable files from running out of the %APPDATA% location. A much more exhaustive list can be obtained here.
Implement Geo-Blocking and blocking of known bad IP addresses
If possible for your organisation you should configure your perimeter firewall to block traffic to "at-risk" countries if you do not do business in these countries. For example a large percentage of Ransomware variants rely on communication with Russian IP blocks to function and spread. Protection mechanisms such as these are admittedly crude, but can be surprisingly effective. Furthermore if your perimeter solution allows you to do so, you should explicitly block all traffic to known bad IP addresses.
Application whitelisting is perhaps the most effective method of protecting against ransomware attacks. The flip side is that it also typically takes the most effort and resources in terms of testing, implementing and maintaining. There are various solutions available to implement application whitelisting, but as a start you should consider the AppLocker and Software Restriction Policies native to Windows.
Disable MS Office Macros
Many variants of Ransomware leverage Macros to propagate. If possible you should disable this functionality within Office documents, or at the very minimum for documents received via mail and downloaded from the Internet. Alternatively you can make use of the Office File Viewer to view these untrusted documents.
Windows FileScreen was a popular method to detect ransomware infections, however its functionality has been greatly reduced because of new ransomware variants randomizing file extensions. It might still add value in your organisation and assist with alerting and tracing the origin of a ransomware outbreak. A fairly comprehensive list of file extensions can be found here.
Enable Logging (SIEM)
A proper logging solution will be invaluable in tracing the origin of a ransomware outbreak. The value in a SIEM solution is that you will be able to correlate the logs of a multitude of different devices to find out how the attack happened. This in turn will allow you to take measure to prevent a re-occurrence. A properly configured SIEM solution will also act as an efficient early-warning system, which will allow you to limit the spread and fallout of a ransomware infection should you be compromised.
If all else fails your backups will be what stands between you and disaster. Back up your data regularly, as per your organisation's Recovery Point Objective's (RPO). Verify the integrity of those backups and test the restoration process regularly to ensure it is working. You also need to secure your backups, or at the very least take them offline so that it cannot be affected by the same ransomware you are trying to protect against.