Elastic Detection Rule Development: OSX/Dok Malware

Elastic Detection Rule Development: OSX/Dok Malware

OS/Dok Malware Example

Lets take a example. If was working and got notification through Cyware social threat feeds and read the following malware research:

After reading this research about this threat, I can automatically develop a couple (in "development") rules for this specific threat [In the real world, I would sandbox the malware to trigger  the rules for real], what I'm showing is just the process of the first go around of developing a rule based on malware research.

So this bundle name Truesteer.AppStore when it is ran, it copys itself to /User/Shared/ and execute again from that location

chmod +x /User/Shared/AppStore.app
sleep 5
rm -fr "Users/_%USER%_/Downloads/Dokument.App"
"/User/Shared/AppStore.app/Contents/MacOS/AppStore" Dokument

But to make the rule more general, you need to think about the possibly of a threat actors using this format, but changing certain things. Threat actors love to reuse old malware and modify it a little bit.

Rule query - chmod +x /User/Shared/AppStore.app

process where event.type in ("start", "process_started") and 
process.name : "chmod" and process.args : "/User/Shared/*.app"

So the * in /User/Shared/*.app is now a wildcard, and this is incase a Threat Actor want reuse this setup, but change AppStore.app to something different.

Rule query - rm -fr "/Users/%USER%/Downloads/Dokument.App"

process where event.type in ("start", "process_started") and 
process.name : rm and process.args : ("/Users/*/Downloads/*.App" and  "-fr")

So the * in /Users/*/Downloads/*.App is now a wildcard, and this is incase a Threat Actor want reuse this setup, but change Dokument.App to something different.

Now I will take the following queries above and build a sequence. I need to remember that one of the commands was sleep 5, with that said, I now know I need to make the sequence that last more than 5 seconds. So I make this one 30 seconds to be on the safe side.

Yara File

I will use the following Yara Rule to help build the rules.
rule osx_retefe_w0 {
    meta:
        author = "AlienVault Labs"
        type = "malware"
        description = "OSX/Dok"
        malpedia_reference = "https://malpedia.caad.fkie.fraunhofer.de/details/osx.retefe"
        malpedia_version = "20170602"
        malpedia_license = "CC BY-NC-SA 4.0"
        malpedia_sharing = "TLP:WHITE"

    strings:
        $c1 = "/usr/local/bin/brew"
        $c2 = "/usr/local/bin/tor"
        $c3 = "/usr/local/bin/socat"
        $c4 = "killall Safari"
        $c5 = "killall \"Google Chrome\""
        $c6 = "killall firefox"
        $c7 = "security add-trusted-cert -d -r trustRoot -k /Library/Keychains/System.keychain %@"

    condition:
        all of them
}

Rule Building

NOTE: I have not tested these rules out in a sandbox environment. Sandboxing these would allow the process of improving these rules in regards of detecting the malware.

Rule 1: OSX/Dok Pre-LaunchAgents

not process.args - Anything following this command will help quiet down false positives.

Rule 2: OSX/Dok - LaunchAgents

event.type != "deletion" - That the event type is anything other than deletion.

  • You can see the other event types on a website that I created that explains them here Event.Type | ELK

Rule 3 - OSX/Dok - Post-LaunchAgents

Conclusion:

In the ideal scenario, these rules would trigger one after the other, which would verify the malware being present, but these set of rules would require extra testing.


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