Now when ransomware tries to take over your computer, there’s something you can do besides pay up: stop it, buy more time to deal with it or mitigate the damage it might do.
These options include both hardware and software approaches IT pros can take to defeat the malware, a group at this weekend’s Security BSides Boston conference was told.
By looking at how several variants of ransomware work - CryptoLocker, CryptoWall, Locky, SamSam - researcher Weston Hecker found characteristics of their behavior that could be turned against them.
One method goes after the droppers that first infect target machines in preparation for downloading the main malware payloads. Their purpose is to examine the machines for indications that it might be an inhospitable host and to eliminate the roadblocks if possible.
Many of the droppers kill watchdog software on the machines that is intended to restart antivirus software when the dropper shuts it down, Hecker says. He came up with a watchdog that gets detected by the dropper, but when the dropper shuts it down that triggers a blue screen. In a corporate environment that likely means the machine will be sent to IT where the attack should be discovered.
Another strategy is to make physical machines appear to be virtual machines, he says. Ransomware checks whether it’s attacking a virtual environment and will bail on its attempt and self-destruct because it interprets the virtual machine as a research environment trying to capture and analyze it.
One way to do that involves making victim machines seem like these research environments by virtue of the amount of RAM they have, he says. Malware looks for how much RAM is available to the machine, and less than 1.5GByte is indicative of a security platform trying to grab the malware so it can be studied in a safe environment, he says. If the malware detects that, it will likely halt its own deployment and destroy itself, he says.
Tying up the malware by having it waste time encrypting meaningless files can also halt ransomware attacks, he says, or at least flag them so they are discovered by IT pros. He bought inexpensive USB SSDs online that falsely claim to hold 256GByte but actually hold just 8GByte. They will keep overwriting files once they fill up, but overwritten files remain listed. The malware tries to encrypt these files that don’t exist, which causes a Windows memory leak that crashes the operating system.
Again, the device gets turned over to IT, which should discover the attack and deal with it.
Hecker says he performed his experiments on Windows 7 machines, but versions of his mitigations could be made for other versions of Windows, he says.
One characteristic of Locky ransomware is that seeks to neutralize shadow copies of files, such as incremental backups of Word files. “It erases local backup so you pay the ransom,” he says. Hiding these backups in .sys folders can prevent their being encrypted or at least delay it because .sys folders are low on the list of places the malware looks for files to encrypt, he says. Files that escape encryption this way may be sufficient so the owners of the victim machine feel they don’t need to pay ransom to salvage all the files that were encrypted, he says.
He’s also come up with a CryptoLocker ransomware simulator that runs in a production machines without actually encrypting files, but otherwise acting exactly like CryptoLocker. That reveals to security pros what would be vulnerable if the ransomware actually infected corporate machines. That way they can take steps to correct the weaknesses and reduce some of the potential damage from actual ransomware.
He says such simulators could be written for CryptoLocker, CrykptoWall, Locky and SAMSAM, which would represent more than 75% of ransomware that has actually infected machines.