Last week, several websites carried reports about a variant of KillDisk and described it as being ransomware for Linux. The reports cited a blog post by ESET staffers Robert Lipovsky and Peter Kalnai as their source.
The KillDisk variant does not do what ransomware does. While it does encrypt files and display a ransom notice, it does not send any decryption key back to its source, so that the attacker can profit from the exercise. The decryption key is not stored locally either. These characteristics were mentioned by iTWire's Stephen Withers in a report on Tuesday.
Additionally, the KillDisk executable, for both Linux and Windows, has to be run on an already compromised system, making it just one component in an attack.
"Personally, and for much the reasons Robert and Peter pointed out in the 'Conclusion' section of their blog post, I doubt that the ransomware element of these new variants is anything more than a 'skin' to obfuscate their real purpose, and delay (or even avoid) a proper forensic analysis of the whole network that Killdisk’ed machines are connected to."
KillDisk in its earlier form was disk-wiping malware and was used in an attack on a power station in the Ukraine in 2015.
FitzGerald described KillDisk, be it the Linux variant or the older Windows variant, as "the nuclear option".
"So far, KillDisk has been a standalone malicious payload (until now, a 'system killer' achieved through systematic file scrubbing of certain kinds of file, sub-directories, etc) deployed when the cybercriminals or cyber-spies behind any given campaign have felt that their work was done, or that the victim needed to be 'punished' or perhaps if they felt their presence was suspected and they would prefer to trash the victim's machine rather than have their tool-chain, etc compromised," he said.
"The latest KillDisk variants for Windows that have a similar, apparent ransomware function, rather than the earlier disk-scrubbing payload, are still such payloads, just as the new Linux KillDisk (with its apparent ransomware function) is."
FitzGerald said that due to a flaw in the KillDisk executable that worked on Linux systems, files that were encrypted could be decrypted using brute-force methods "within a reasonable amount of computing time".
He said he would not go into details of the flaw as it would benefit the people behind KillDisk.
"Only the Linux version has this flaw. The Windows version with the same ransom demands, etc includes an apparently functional ransomware capability, though we would still recommend that anyone affected by the Windows version not stump up the requested ransom, as the grievous flaws in the Linux version suggest that these are probably not the most scrupulous or reliable of ransomware campaign operators," he added.