I was reading this Splunk blog post recently, and I have to say up front, I was disappointed by the fact that the promise of the title (i.e., "Detecting Cl0p Ransomware") was not delivered on by the remaining content of the post. Very early on in the blog post is the statement:
Ransomware is by nature a post-exploitation tool, so before deploying it they must infiltrate the victim's infrastructure.
Okay, so at this point, I'm looking for something juicy, some information regarding the TTPs used to "infiltrate the victim's infrastructure" and to locate files of interest for staging and exfil, but instead, the author(s) dove right into analyzing the malware itself, through reverse engineering. Early in that malware RE exercise is the statement:
This ransomware has a defense evasion feature where it tries to delete all the logs in the infected machine to avoid detection.
The embedded command is essentially a "one-liner" used to list and clear all Windows Event Logs, leveraging wevtutil.exe. However, while used for "defense evasion", it occurred to me that this command is not, in fact, intended to "avoid detection". After all, with ransomware, the threat actors want to get paid, so they want to be detected. In fact, to ensure they're detected, the actors put ransom notes on the system, with clear statements, declarations, warnings, and time limits. In this case, the ransom note says that if action is not taken in two weeks, the files will be deleted. So, yes, it's safe to say that clearing all of the Windows Event Logs is not about avoiding detection. If anything, its really nothing more than a gesture of dominance, the threat actor saying, look at what I can do to your system.
So, what is the purpose of clearing the Windows Event Logs? As a long-time #DFIR analyst, the value of the Windows Event Logs in such cases is to assist in a root-cause analysis (RCA) investigation, and clearing some Windows Event Logs (albeit not ALL of them) will hobble (but not completely remove) a responder's ability to determine aspects of the attack cycle such as lateral movement. By tracing lateral movement, the investigator can determine the original system used by the threat actor to gain access to the infrastructure, the "nexus" or "foothold" system, and from there, determine how the threat actor gained access. I say "hobble" because clearing the Windows Event Logs does not obviate the ability of the investigator to recover the event records, it simply requires a bit more effort. However, the vast majority of organizations impacted by ransomware are not conducting full investigations or RCAs, and #DFIR consulting firms are not publicly sharing ransomware trends and TTPs, anonymized through aggregation. In short, clearing the Windows Event Logs, or not, would likely have little impact either way on the response.
But why clear ALL Windows Event Logs? IMHO, it was used to ensure that the ransomware attack was, in fact, detected. Perhaps the assumption is that most organizations have some small modicum of a detection capability, and the most rudimentary SIEM or EDR framework should throw an alert of some kind in the face of the "wevtutil cl" command, or when the SIEM starts receiving events indicating that Windows Event Logs were cleared (especially if the Security Event Log was cleared!).