So you want to be a SOC Analyst? Part 6

Let's get a little more advanced with our detection rules by leveraging automated YARA scanning.

If you just landed here, be sure to check out the Intro to this series for backstory.

You will need to complete the following posts in this series before beginning this one:

• Part 1

• Part 2

Automated Yara Scanning

The goal of this post is to take advantage of a more advanced capability of any good EDR sensor, to automatically scan files or processes for the presence of malware based on a YARA signature.

What is YARA?

YARA is a tool primarily used for identifying and classifying malware based on textual or binary patterns. It allows researchers and security professionals to craft rules that describe unique characteristics of specific malware families or malicious behaviors. These rules can then be applied to files, processes, or even network traffic to detect potential threats. When analyzing a compromised system, YARA helps in sifting through large amounts of data to find malicious artifacts by matching them against a set of predefined rules. This ability to create customized detection signatures is particularly useful in threat hunting and incident response, enabling swift identification of known and even previously unknown malicious elements.

YARA originated from the efforts of Victor M. Alvarez, a malware researcher. It was initially developed as an internal tool at his former employer, Hispasec Sistemas, which is behind the well-known VirusTotal service. The tool was designed to assist researchers in identifying and classifying malware samples based on textual or binary patterns. Due to its effectiveness and utility, YARA was later released as an open-source tool, and it quickly gained popularity in the cybersecurity community. Over the years, it has become a standard tool for malware research, threat hunting, and incident response, thanks to its flexibility and the active community that has grown around it.

There are many free and open source YARA scanners and rulesets. Read more about YARA from VirusTotal or explore one of the many open source YARA rulesets. A solid premium ruleset is the one maintained by Nextron systems.

Let’s put it to work!

We are going to prepare our LimaCharlie instance for detecting certain file system and process activities in order to trigger YARA scans.

Add a YARA signature for the Sliver C2 payload

Since we already know we’re dealing with the Sliver C2 payload, we can be more targeted in our exercise by using a signature specifically looking for Sliver. Lucky for us, the UK National Cyber Security Centre published some fantastic intel on Sliver, including YARA signatures and other useful detections. The only downside is they crammed it all into a PDF making it a little difficult to use without extracting it manually — which I’ve already done for you.

1. Within LimaCharlie, browse to “Automation” > “YARA Rules”

   

   1. NOTE: If you encounter a permission error on this page, no worries — easy fix.

      1. On the left side menu, click “Access Management” > “Users & Roles

      2. Click your username (likely your email address)

         1. Toggle “ON” the “Select All” at the top right of the permissions list to give yourself all permissions

         2. Scroll down and click “Update Permissions & Close”

         3. Proceed back to “Automations” > “Yara Rules” and continue to the next step.

2. Click Add Yara Rule

   

   1. Name the rule sliver

   2. Copy and paste the contents of this gist into the Rule block

      

   3. Click “Save Rule”

3. Now create one more YARA rule that we’ll use later on for a more specific purpose. I’ll explain this later on in the post.

   1. Name the rule sliver-process

   2. Copy and paste the contents of this gist into the Rule block.

4. Now, before we use these YARA rules, we want to setup a few generic D&R rules that will generate alerts whenever a YARA detection occurs.

   1. Browse to “Automation” > “D&R Rules”

   2. Create a new rule

      1. In the Detect block, paste the following

         event: YARA_DETECTION
         op: and
         rules:
           - not: true
             op: exists
             path: event/PROCESS/*
           - op: exists
             path: event/RULE_NAME

         1. Notice that we’re detecting on YARA detections not involving a PROCESS object, that’ll be its own rule shortly.

      2. In the Respond block, paste the following

         - action: report
           name: YARA Detection {{ .event.RULE_NAME }}
         - action: add tag
           tag: yara_detection
           ttl: 80000

         

      3. Save the rule and title it YARA Detection

   3. Create another rule

      1. In the Detect block, paste the following

         event: YARA_DETECTION
         op: and
         rules:
           - op: exists
             path: event/RULE_NAME
           - op: exists
             path: event/PROCESS/*

         1. Notice that this detection is looking for YARA Detections specifically involving a PROCESS object.

      2. In the Respond block, paste the following

         - action: report
           name: YARA Detection in Memory {{ .event.RULE_NAME }}
         - action: add tag
           tag: yara_detection_memory
           ttl: 80000

         

      3. Save the rule and title it YARA Detection in Memory

Let’s test our new YARA signature

Since we already know we have a Sliver implant sitting in the Downloads folder of our Windows VM, we can easily test our signature by initiating a manual YARA scan using the EDR sensor. This will give us a sanity check that all things are working up to this point.

1. In LimaCharlie, browse to the “Sensors List” and click on our Windows VM sensor

   

2. Access the EDR Sensor Console which allows us to run sensor commands against this endpoint

   

3. Run the following command to kick off a manual YARA scan of our Sliver payload. You will need to know the name of your Sliver executable created in Part 2 - Step 3.

   1. Replace [payload_name] with your actual payload name

   yara_scan hive://yara/sliver -f C:\Users\User\Downloads\[payload_name].exe

   

   1. Hit enter twice to execute this command

4. Ideally, your output looks similar to mine below, indicating a positive hit on one of the signatures contained within the Sliver YARA rule

   

5. Now, also confirm that you have a new Detection on the “Detections” screen

   

6. If you’re with me so far, we’re ready to automate this process!

Automatically YARA scan downloaded EXEs

1. Browse to “Automation” > “D&R Rules”

2. Create a new rule

   1. In the Detect block, paste the following

      event: NEW_DOCUMENT
      op: and
      rules:
        - op: starts with
          path: event/FILE_PATH
          value: C:\Users\
        - op: contains
          path: event/FILE_PATH
          value: \Downloads\
        - op: ends with
          path: event/FILE_PATH
          value: .exe

      1. Notice that this detection is simply looking for NEW .exe files to appear in any users Downloads directory.

   2. In the Respond block, paste the following

      - action: report
        name: EXE dropped in Downloads directory
      - action: task
        command: >-
          yara_scan hive://yara/sliver -f "{{ .event.FILE_PATH
          }}"
        investigation: Yara Scan Exe
        suppression:
          is_global: false
          keys:
            - '{{ .event.FILE_PATH }}'
            - Yara Scan Exe
          max_count: 1
          period: 1m

      1. This response action generates an alert for the EXE creation, but more importantly, kicks off a YARA scan using the Sliver signature against the newly created EXE.

   3. Save the rule and title it YARA Scan Downloaded EXE

Automatically YARA scan processes launched from Downloads directory

1. Browse to “Automation” > “D&R Rules”

2. Create a new rule

   1. In the Detect block, paste the following

      event: NEW_PROCESS
      op: and
      rules:
        - op: starts with
          path: event/FILE_PATH
          value: C:\Users\
        - op: contains
          path: event/FILE_PATH
          value: \Downloads\

      1. This rule is matching any process that is launched from a user Downloads directory

   2. In the Respond block, paste the following

      - action: report
        name: Execution from Downloads directory
      - action: task
        command: yara_scan hive://yara/sliver-process --pid "{{ .event.PROCESS_ID }}"
        investigation: Yara Scan Process
        suppression:
          is_global: false
          keys:
            - '{{ .event.PROCESS_ID }}'
            - Yara Scan Process
          max_count: 1
          period: 1m

      1. Notice in this rule, we’re no longer scanning the FILE_PATH, but the actual running process by specifying its PROCESS_ID. We are also now using the other YARA rule we created, sliver-process

      2. If you’re curious why we’ve switched YARA rules here, it’s due to the fact that in my testing, I realized that the NCSC rules do not fire on running versions of Sliver due to the specific logic they’ve used in the match condition. So I loosened up that logic to detect strings I know should be present in a running Sliver beacon, and I verified this by using LimaCharlie to pull strings from my Sliver process.

      3. See my method in the screenshots below

         

         

         

   3. Save the rule and title it: YARA Scan Process Launched from Downloads

Let’s trigger our new rules!

Scanning New EXEs in Downloads

To make things easier, we won’t re-download our Sliver payload, but we’ll simulate this activity by moving it to another location, then putting it back into C:\Users\User\Downloads

1. Run the following PowerShell command to move your Sliver payload from Downloads to Documents

   1. Replace [payload_name] with your actual payload name

      Move-Item -Path C:\Users\User\Downloads\[payload_name].exe -Destination C:\Users\User\Documents\[payload_name].exe

2. Now, put it back to generate the NEW_DOCUMENT event for an EXE being dropped into the Downloads folder

   1. Replace [payload_name] with your actual payload name

      Move-Item -Path C:\Users\User\Documents\[payload_name].exe -Destination C:\Users\User\Downloads\[payload_name].exe

3. Head over to your Detections tab and see what happened!

   1. It may take a moment or so, maybe a refresh, to see the new detections

      

   2. We should see an initial alert for EXE dropped in Downloads directory followed shortly by a YARA detection once the scan kicked off and found Sliver inside the EXE

   3. If you do not see these detections, pop into the Timeline of your system and search for the payload name and make sure you have NEW_DOCUMENT events showing the move from Documents → Downloads

Scanning processes launched from Downloads

Now let’s test our NEW_PROCESS rule to scan running processes launched from Downloads

1. Launch an Administrative PowerShell prompt

   1. First, let’s kill any existing instances of our Sliver C2 from previous labs

      1. Replace [payload_name] with your actual payload name WITHOUT .exe - for instance, if your payload name is STRIKING_PASSION.exe, just use STRIKING_PASSION in this command

         Get-Process [payload_name] | Stop-Process

         

         If you get an error, either you didn’t have one running (which is fine), or you messed up the [payload_name] part.

   2. Execute your Sliver payload to create the NEW_PROCESS event we need to trigger the scanning of a process launched from the Downloads directory

      C:\Users\User\Downloads\[payload_name].exe

2. Head over to your Detections tab and see what happened!

   

   1. We should see an initial alert for Execution from Downloads directory followed shortly by a YARA detection in Memory once the scan kicked off and found Sliver inside the EXE

3. Crushed it :)

Now what?

Just imagine all the possibilities unlocked by this capability…

• Scan new files in web server wwwroot directories for webshells

• Scan suspicious rundll32 processes for CobaltStrike

• Automatically terminate processes or delete files matching a YARA detection🔥

The only limit is your imagination :)

Now, go find evil.

Courtesy of my new-found joy for DALL·E art