Canary in the ransomware mine
Gunter Ollmann, CSO, Vectra Networks
Ransomware is clearly the scourge of 2016. Every week there is a new and notable enterprise-level outbreak of this insidious class of malware – crippling and extorting an ever widening array of organisations.
For a threat that is overwhelmingly not targeted, it seems to be hitting large and small businesses with great success.
The malware infection can come through the front door of a failed “defense-in-depth” strategy or the side door of a mobile device latched to the corporate network on a Monday morning.
Regardless, many security teams and network administrators are scurrying to halt the rapid encryption of documents accessible through networked shares.
There are several new technologies available to organisations to detect the presence of ransomware within the network and alert security staff to an outbreak (which is just one example of the things the Vectra product does). But it is often a considerably more difficult task to automatically halt an outbreak mid-stream.
I’m often asked what is the “best” way of mitigating this threat – i.e. how can an organisation stop ransomware from shutting down their business in the cheapest and most robust way?
The quickest “no-frills” way of mitigating the network encryption piece of ransomware is actually pretty simple and follows the canary-in-a-coal-mine principle.
Ransomware attempts to enumerate and encrypt files over network shares within the enterprise. A simple protection method is to ensure that every computer has a couple of monitored mounted shares.
If a user or computer attempts to write to or delete a file on those shares, the victim user’s access credentials are immediately suspended. And if the organisation is using some form of network access control (NAC), the host computer is similarly and immediately disconnected from the network.
Since the current generation of ransomware tends to sequentially step through mounted shares in alphabetical or reverse-alphabetical order, ensuring that the first and last mounted shares – such as A: or D: drive, and Z: or Y: drive – are monitored canary shares is likely good enough.
Ensuring that the canary shares have a large number of disposable files can be useful too, as the ransomware will take time to cycle through encrypting these files. This provides a period of time in which credential and network access revocation information can be propagated to the rest of the network.
This technique is similar to what I’ve used in the past to deal with spam-sending malware and automated credential brute-forcing attacks. By adding user accounts that appear as the first and last positions in a user directory – such as Active Directory and email contacts – and monitoring any use of those names, it is possible to rapidly and automatically detect and mitigate the threat.
For example, if anyone attempts to email the fictitious first name in the address book, the mail server automatically blocks all email send requests from the user until the IT team has investigated.
The use of ransomware canary file shares, like canary accounts in Active Directory and email, can be a cheap and effective mitigation approach to threats. Sometimes the simplest methods can be the most effective.
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About Gunter Ollmann
Gunter has almost three decades of experience in information security, working as an industry luminary in an array of cyber security consultancy and research roles. Gunter has been focused exclusively on security since 2000; initially diving head-first in to the world of attack-based consulting (penetration testing, ethical hacking, social engineering, etc). In addition to penetration testing and Web security analysis, for several years Gunter has been deeply involved in researching critical security topics and has lead advanced security research teams focused on threat identification and mitigation.
