Saturday, 7 November 2015

Ransomware on Linux

Dr.WEB is reporting that ransomware has come to the Linux ecosystem. Fortunately, this has only affected "tens" of users thus far. In particular, this malware is targeting those with a lot to lose: web site administrators. This gives the malware a good chance of ensnaring some business-critical data or functionality, thereby giving the victim a bit more incentive to pay the ransom.

Ransomware has been around for some time in the Windows ecosystem. Previously these programs would show a dialogue, claiming that the machine was locked and could be unlocked when a suitable payment was made. In reality, these were often just programs configured to run automatically on start-up, and did not directly endanger user data. In recent years, these have made attempts at encrypting the user's data and putting the key out of reach. A prompt payment promises to return the key, and thus the data, to the victim. These have had varying levels of success, with the "best" managing to pull in millions of dollars for their creators. They have not been without their flaws which allowed the victims to recover their data without paying; some variants stored the key locally on the machine, some eventually had the keys disclosed by security researchers, and some which have yet to to be broken. Often, organisations have no option but to pay the ransom.

Fortunately, this particular strain of malware requires extensive user interaction to run, requiring root privileges. This does not prevent future generations of this malware piggy-backing on other access vectors, such as vulnerable web browsers, email clients, web servers, and so on. I would predict that we will see this kind of malware attached to remote exploits in the moderately near future. Even using old exploits, or only encrypting a user's home directory could turn up quite the bounty for the attacker, as those who don't update their systems may well not have suitable backup processes in place to recover from the attack, and many people store their valuable files in their home directory.

There are a few options to mitigate the risk posed by this threat. However, none will be wholly effective, so a combination may be required. For some organisations, this will simply be a strengthening or verification of existing defences. For others, this threat may call for entirely new defences to be deployed.

The first and most common would be to ensure that all systems under your control have all relevant security patches applied. This should limit the likelihood of an exploit being used to launch an attack without user interaction. A backup system which stores backups offline should be used. If an on-line backup system is in use, either deploy an offline system or ensure that a previously saved backup cannot be overwritten by a corrupted copy, or easily reached by an attacker. This will reduce the impact of a breach, as it should be possible to recover from relatively recent backups in the event of a compromise. Where possible, software which consumes untrusted input, such as web browsers, email clients, web servers, and so on, should be placed into to a suitable sandbox environment. This should reduce the likelihood that the malware will be able to reach critical business data. Finally, better user education may reduce the likelihood of a breach, as they may be better able to detect social engineering attacks which might have otherwise lead them to run the malware.

It is fortunate that Linux has several sandbox mechanisms available, and an appropriate one can be selected. Such mechanisms include chroots, SELinux, AppArmor, or seccomp-bpf. Other systems, such as FreeBSD, should not be considered invulnerable, and similar mitigations applied, such as the use of jails or Capsicum. Unfortunately, restricting a complex web browser's access to the file system may have unexpected consequences, or simply be very time consuming. Ubuntu provides an AppArmor profile to do this for Chromium. However, it is not without it's issues, such as not being able to determine if it is the default browser on the system.

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