Detect PoWERsheLL mixed case obfuscation using Splunk

Mixed case obfuscation is a good technique if security appliances are case sensitive and a wash if they aren't. But looking at "PoWERsheLL -e"  you know its bad so lets do a quick signature for the case obfuscation to add to our other powershell detections. Of course this ONLY detects the mixed case but this is a valid TTP as it is seen in the wild.


First you will need to profile the existing common case occurrences for your network. Turned out I see just a couple of common ones which will become our filter.

Profile search


So (?i) to do a case insensitive match for powershell and stats does its normal case sensitive aggregation.
Hopefully at this point you have a small filter list.

Now for the real search



Detect the powershell command line, let regex in case sensitive mode be the filter for matches that are common on your network and away you go.
In practice I had only one tune at the top level search and see only a few false positives. The search is light weight enough to do an all-time hunt and then set it however you do reoccurring searches or dashboards.

Good hunting.

References:

 https://twitter.com/cyb3rops/status/1225334343838597120

https://twitter.com/anyrun_app/status/1225323429399793666

 https://attack.mitre.org/techniques/T1036/



Comments

Post a Comment

Popular posts from this blog

Misleading extensions Xls.exe Doc.exe Pdf.exe

I get something out of twitter almost every day and it is not uncommon to see examples a few times before the realization sinks in that you are looking at a technique that needs a rule. These should fall under the ATT&CK framework as masquerading. I saw a tweet the other day that reminded me of a couple of signatures worth talking about. These misleading double extensions are not new but they never seem to go out of style. With modern EDRs it is an easy win. The malware filename ended in .xls.exe but lets expand that to include other office file types. index=edr ( doc OR docx OR xls OR xlsx OR pdf ) exe ( process_path=*.doc.exe OR process_path=*.docx.exe OR process_path=*.xls.exe OR process_path=*.xlsx.exe OR process_path=*.pdf.exe ) The sig is largely self explanatory, the tokenization allows for keyword search by breaking up the extension, and the process path stuff just anchors it all to the process name since these are pretty generic terms. Another tweet by blackor
Read more

Netconn from suspicious directories

Suspicious directories are an interesting topic and I am always on the lookout for them as a TTP.  The premise may be that directories like temp and downloads are already under review so some other directory should be used. That is fine for a while but once known it is easy to detect. The problem is noticing the unusual directory and adding it to your detection. I have no idea where I got most of these so if you have a post similar to this I will be happy to reference it. Number one is the recycle bin, nothing should run from here, be referenced from here and certainly not netconn. I also do a signature for $recycle.bin in the command line just to catch anything non-netconn related. These are locations to look for processes doing netconn.  Additional filtering is possible looking for activity from the root of these locations if you have a noisy one or it is a legit higher level path like C:\. A search query for only the root would look something like this: index=edr process_pa
Read more

Powershell DNS C2 Notes

I recently took a look at Powershell DNS C2 and found a couple of interesting things. The special case of DNS requests from powershell should be easy enough to identify using an EDR. Using splunk and stats just look for multiple remote port 53 occurrances from powershell. There will be a few but DNS c2 is noisy so a large limit can be used for filtering. Next I took a look at DNSCat https://github.com/lukebaggett/dnscat2-powershell Interestingly powershell does not make the dns request directly but spawns nslookup to do it. Easy enough to make a signature for that. Again, powershell calling nslookup will occur legitimately, but a large filter for occurrences will filter those out. index=edr powershell.exe nslookup.exe parent_path=*\\powershell.exe | stats values(command_line) count by computer_name parent_process_guid | where count>10 Next I went back to some old Oilrig samples which used DNS C2. Nothing new here, just multiple DNS requests directly from powershell. B
Read more