Detecting malware using Yara integration

You can use the YARA integration with Wazuh to scan files added or modified on an endpoint for malware. YARA is a tool to detect and classify malware artifacts.

In this use case, we demonstrate how to configure YARA with Wazuh to detect malware on Linux and Windows endpoints.

To learn more about this integration with Wazuh, see the How to integrate Wazuh with YARA section of the documentation.

Infrastructure

Endpoint

Description

Ubuntu 22.04 / RHEL 9.0

The YARA active response module scans new or modified files whenever the Wazuh FIM module triggers an alert.

Windows 11

The YARA active response module scans new or modified files whenever the Wazuh FIM module triggers an alert.

Configuration for Linux

Linux endpoint

Perform the following steps to install YARA, and configure the active response and FIM modules.

  1. Download, compile and install YARA:

    Ubuntu

    $ sudo apt update
    $ sudo apt install -y make gcc autoconf libtool libssl-dev pkg-config jq
    $ sudo curl -LO https://github.com/VirusTotal/yara/archive/v4.2.3.tar.gz
    $ sudo tar -xvzf v4.2.3.tar.gz -C /usr/local/bin/ && rm -f v4.2.3.tar.gz
    $ cd /usr/local/bin/yara-4.2.3/
    $ sudo ./bootstrap.sh && sudo ./configure && sudo make && sudo make install && sudo make check
    

    RHEL

    $ sudo yum makecache
    $ sudo yum install epel-release
    $ sudo yum update
    $ sudo yum install -y make automake gcc autoconf libtool openssl-devel pkg-config jq
    $ sudo curl -LO https://github.com/VirusTotal/yara/archive/v4.2.3.tar.gz
    $ sudo tar -xvzf v4.2.3.tar.gz -C /usr/local/bin/ && rm -f v4.2.3.tar.gz
    $ cd /usr/local/bin/yara-4.2.3/
    $ sudo ./bootstrap.sh && sudo ./configure && sudo make && sudo make install && sudo make check
    
  2. Test that YARA is running properly:

    $ yara
    
    yara: wrong number of arguments
    Usage: yara [OPTION]... [NAMESPACE:]RULES_FILE... FILE | DIR | PID
    
    Try `--help` for more options
    

    If the error message below is displayed:

    /usr/local/bin/yara: error while loading shared libraries: libyara.so.9: cannot open shared object file: No such file or directory.
    

    This means that the loader doesn’t find the libyara library usually located in /usr/local/lib. Add the /usr/local/lib path to the /etc/ld.so.conf loader configuration file to solve this:

    $ sudo su
    # echo "/usr/local/lib" >> /etc/ld.so.conf
    # ldconfig
    

    Switch back to the previous user.

  3. Download YARA detection rules:

    $ sudo mkdir -p /tmp/yara/rules
    $ sudo curl 'https://valhalla.nextron-systems.com/api/v1/get' \
    -H 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' \
    -H 'Accept-Language: en-US,en;q=0.5' \
    --compressed \
    -H 'Referer: https://valhalla.nextron-systems.com/' \
    -H 'Content-Type: application/x-www-form-urlencoded' \
    -H 'DNT: 1' -H 'Connection: keep-alive' -H 'Upgrade-Insecure-Requests: 1' \
    --data 'demo=demo&apikey=1111111111111111111111111111111111111111111111111111111111111111&format=text' \
    -o /tmp/yara/rules/yara_rules.yar
    
  4. Create a yara.sh script in the /var/ossec/active-response/bin/ directory. This is necessary for the Wazuh-YARA active response scans:

    #!/bin/bash
    # Wazuh - Yara active response
    # Copyright (C) 2015-2022, Wazuh Inc.
    #
    # This program is free software; you can redistribute it
    # and/or modify it under the terms of the GNU General Public
    # License (version 2) as published by the FSF - Free Software
    # Foundation.
    
    
    #------------------------- Gather parameters -------------------------#
    
    # Extra arguments
    read INPUT_JSON
    YARA_PATH=$(echo $INPUT_JSON | jq -r .parameters.extra_args[1])
    YARA_RULES=$(echo $INPUT_JSON | jq -r .parameters.extra_args[3])
    FILENAME=$(echo $INPUT_JSON | jq -r .parameters.alert.syscheck.path)
    
    # Set LOG_FILE path
    LOG_FILE="logs/active-responses.log"
    
    size=0
    actual_size=$(stat -c %s ${FILENAME})
    while [ ${size} -ne ${actual_size} ]; do
        sleep 1
        size=${actual_size}
        actual_size=$(stat -c %s ${FILENAME})
    done
    
    #----------------------- Analyze parameters -----------------------#
    
    if [[ ! $YARA_PATH ]] || [[ ! $YARA_RULES ]]
    then
        echo "wazuh-yara: ERROR - Yara active response error. Yara path and rules parameters are mandatory." >> ${LOG_FILE}
        exit 1
    fi
    
    #------------------------- Main workflow --------------------------#
    
    # Execute Yara scan on the specified filename
    yara_output="$("${YARA_PATH}"/yara -w -r "$YARA_RULES" "$FILENAME")"
    
    if [[ $yara_output != "" ]]
    then
        # Iterate every detected rule and append it to the LOG_FILE
        while read -r line; do
            echo "wazuh-yara: INFO - Scan result: $line" >> ${LOG_FILE}
        done <<< "$yara_output"
    fi
    
    exit 0;
    
  5. Change yara.sh file owner to root:wazuh and file permissions to 0750:

    $ sudo chown root:wazuh /var/ossec/active-response/bin/yara.sh
    $ sudo chmod 750 /var/ossec/active-response/bin/yara.sh
    
  6. Add the following within the <syscheck> block of the Wazuh agent /var/ossec/etc/ossec.conf configuration file to monitor the /tmp/yara/malware directory:

    <directories realtime="yes">/tmp/yara/malware</directories>
    
  7. Restart the Wazuh agent to apply the configuration changes:

    $ sudo systemctl restart wazuh-agent
    

Wazuh server

Perform the following steps to configure Wazuh to alert for file changes in the endpoint monitored directory. The steps also configure an active response script to trigger whenever a suspicious file is detected.

  1. Add the following rules to the /var/ossec/etc/rules/local_rules.xml file. The rules detect FIM events in the monitored directory. They also alert when the YARA integration finds malware. You can modify the rules to detect events from other directories:

    <group name="syscheck,">
      <rule id="100300" level="7">
        <if_sid>550</if_sid>
        <field name="file">/tmp/yara/malware/</field>
        <description>File modified in /tmp/yara/malware/ directory.</description>
      </rule>
      <rule id="100301" level="7">
        <if_sid>554</if_sid>
        <field name="file">/tmp/yara/malware/</field>
        <description>File added to /tmp/yara/malware/ directory.</description>
      </rule>
    </group>
    
    <group name="yara,">
      <rule id="108000" level="0">
        <decoded_as>yara_decoder</decoded_as>
        <description>Yara grouping rule</description>
      </rule>
      <rule id="108001" level="12">
        <if_sid>108000</if_sid>
        <match>wazuh-yara: INFO - Scan result: </match>
        <description>File "$(yara_scanned_file)" is a positive match. Yara rule: $(yara_rule)</description>
      </rule>
    </group>
    
  2. Add the following decoders to the Wazuh server /var/ossec/etc/decoders/local_decoder.xml file. This allows extracting the information from YARA scan results:

    <decoder name="yara_decoder">
      <prematch>wazuh-yara:</prematch>
    </decoder>
    
    <decoder name="yara_decoder1">
      <parent>yara_decoder</parent>
      <regex>wazuh-yara: (\S+) - Scan result: (\S+) (\S+)</regex>
      <order>log_type, yara_rule, yara_scanned_file</order>
    </decoder>
    
  3. Add the following configuration to the Wazuh server /var/ossec/etc/ossec.conf configuration file. This configures the active response module to trigger after the rule 100300 and 100301 are fired:

    <ossec_config>
      <command>
        <name>yara_linux</name>
        <executable>yara.sh</executable>
        <extra_args>-yara_path /usr/local/bin -yara_rules /tmp/yara/rules/yara_rules.yar</extra_args>
        <timeout_allowed>no</timeout_allowed>
      </command>
    
      <active-response>
        <command>yara_linux</command>
        <location>local</location>
        <rules_id>100300,100301</rules_id>
      </active-response>
    </ossec_config>
    
  4. Restart the Wazuh manager to apply the configuration changes:

    $ sudo systemctl restart wazuh-manager
    

Attack emulation

  1. Create the script /tmp/yara/malware/malware_downloader.sh on the monitored endpoint to download malware samples:

    #!/bin/bash
    # Wazuh - Malware Downloader for test purposes
    # Copyright (C) 2015-2022, Wazuh Inc.
    #
    # This program is free software; you can redistribute it
    # and/or modify it under the terms of the GNU General Public
    # License (version 2) as published by the FSF - Free Software
    # Foundation.
    
    function fetch_sample(){
    
      curl -s -XGET "$1" -o "$2"
    
    }
    
    echo "WARNING: Downloading Malware samples, please use this script with  caution."
    read -p "  Do you want to continue? (y/n)" -n 1 -r ANSWER
    echo
    
    if [[ $ANSWER =~ ^[Yy]$ ]]
    then
        echo
        # Mirai
        echo "# Mirai: https://en.wikipedia.org/wiki/Mirai_(malware)"
        echo "Downloading malware sample..."
        fetch_sample "https://wazuh-demo.s3-us-west-1.amazonaws.com/mirai" "/tmp/yara/malware/mirai" && echo "Done!" || echo "Error while downloading."
        echo
    
        # Xbash
        echo "# Xbash: https://unit42.paloaltonetworks.com/unit42-xbash-combines-botnet-ransomware-coinmining-worm-targets-linux-windows/"
        echo "Downloading malware sample..."
        fetch_sample "https://wazuh-demo.s3-us-west-1.amazonaws.com/xbash" "/tmp/yara/malware/xbash" && echo "Done!" || echo "Error while downloading."
        echo
    
        # VPNFilter
        echo "# VPNFilter: https://news.sophos.com/en-us/2018/05/24/vpnfilter-botnet-a-sophoslabs-analysis/"
        echo "Downloading malware sample..."
        fetch_sample "https://wazuh-demo.s3-us-west-1.amazonaws.com/vpn_filter" "/tmp/yara/malware/vpn_filter" && echo "Done!" || echo "Error while downloading."
        echo
    
        # Webshell
        echo "# WebShell: https://github.com/SecWiki/WebShell-2/blob/master/Php/Worse%20Linux%20Shell.php"
        echo "Downloading malware sample..."
        fetch_sample "https://wazuh-demo.s3-us-west-1.amazonaws.com/webshell" "/tmp/yara/malware/webshell" && echo "Done!" || echo "Error while downloading."
        echo
    fi
    
  2. Run the malware_downloader.sh script to download malware samples to the /tmp/yara/malware directory:

    $ sudo bash /tmp/yara/malware/malware_downloader.sh
    

Visualize the alerts

You can visualize the alert data in the Wazuh dashboard. To do this, go to the Security events module and add the filters in the search bar to query the alerts.

  • rule.groups:yara

Configuration for Windows

Windows endpoint

Configure Python and YARA

Perform the following steps to install Python, YARA, and download YARA rules.

  1. Download Python executable installer from the official Python website.

  2. Run the Python installer once downloaded and make sure to check the following boxes:

    • Install launcher for all users

    • Add Python 3.X to PATH. This places the interpreter in the execution path.

  3. Download and install the latest Visual C++ Redistributable package.

  4. Open PowerShell with administrator privileges to download and extract YARA:

    > Invoke-WebRequest -Uri https://github.com/VirusTotal/yara/releases/download/v4.2.3/yara-4.2.3-2029-win64.zip -OutFile v4.2.3-2029-win64.zip
    > Expand-Archive v4.2.3-2029-win64.zip; Remove-Item v4.2.3-2029-win64.zip
    
  5. Create a directory called C:\Program Files (x86)\ossec-agent\active-response\bin\yara\ and copy the YARA executable into it:

    > mkdir 'C:\Program Files (x86)\ossec-agent\active-response\bin\yara\'
    > cp .\v4.2.3-2029-win64\yara64.exe 'C:\Program Files (x86)\ossec-agent\active-response\bin\yara\'
    
  6. Install the valhallaAPI module:

    > pip install valhallaAPI
    
  7. Copy the following script and save it as download_yara_rules.py:

    from valhallaAPI.valhalla import ValhallaAPI
    
    v = ValhallaAPI(api_key="1111111111111111111111111111111111111111111111111111111111111111")
    response = v.get_rules_text()
    
    with open('yara_rules.yar', 'w') as fh:
        fh.write(response)
    
  8. Run the following commands to download the rules and place them in the C:\Program Files (x86)\ossec-agent\active-response\bin\yara\rules\ directory:

    > python.exe download_yara_rules.py
    > mkdir 'C:\Program Files (x86)\ossec-agent\active-response\bin\yara\rules\'
    > cp yara_rules.yar 'C:\Program Files (x86)\ossec-agent\active-response\bin\yara\rules\'
    

Configure active response and FIM

Perform the steps below to configure the Wazuh FIM and an active response script for the detection of malicious files on the endpoint.

  1. Create the yara.bat script in the C:\Program Files (x86)\ossec-agent\active-response\bin\ directory. This is necessary for the Wazuh-YARA active response scans:

    @echo off
    
    setlocal enableDelayedExpansion
    
    reg Query "HKLM\Hardware\Description\System\CentralProcessor\0" | find /i "x86" > NUL && SET OS=32BIT || SET OS=64BIT
    
    
    if %OS%==32BIT (
        SET log_file_path="%programfiles%\ossec-agent\active-response\active-responses.log"
    )
    
    if %OS%==64BIT (
        SET log_file_path="%programfiles(x86)%\ossec-agent\active-response\active-responses.log"
    )
    
    set input=
    for /f "delims=" %%a in ('PowerShell -command "$logInput = Read-Host; Write-Output $logInput"') do (
        set input=%%a
    )
    
    
    set json_file_path="C:\Program Files (x86)\ossec-agent\active-response\stdin.txt"
    set syscheck_file_path=
    echo %input% > %json_file_path%
    
    for /F "tokens=* USEBACKQ" %%F in (`Powershell -Nop -C "(Get-Content 'C:\Program Files (x86)\ossec-agent\active-response\stdin.txt'|ConvertFrom-Json).parameters.alert.syscheck.path"`) do (
    set syscheck_file_path=%%F
    )
    
    del /f %json_file_path%
    set yara_exe_path="C:\Program Files (x86)\ossec-agent\active-response\bin\yara\yara64.exe"
    set yara_rules_path="C:\Program Files (x86)\ossec-agent\active-response\bin\yara\rules\yara_rules.yar"
    echo %syscheck_file_path% >> %log_file_path%
    for /f "delims=" %%a in ('powershell -command "& \"%yara_exe_path%\" \"%yara_rules_path%\" \"%syscheck_file_path%\""') do (
        echo wazuh-yara: INFO - Scan result: %%a >> %log_file_path%
    )
    
    exit /b
    
  2. Add the C:\Users\<USER_NAME>\Downloads directory for monitoring within the <syscheck> block in the Wazuh agent configuration file C:\Program Files (x86)\ossec-agent\ossec.conf. Replace <USER_NAME> with the username of the endpoint:

    <directories realtime="yes">C:\Users\<USER_NAME>\Downloads</directories>
    
  3. Restart the Wazuh agent to apply the configuration changes:

    > Restart-Service -Name wazuh
    

Wazuh server

Perform the following steps on the Wazuh server. This allows alerting for changes in the endpoint monitored directory and configuring an active response script to trigger whenever it detects a suspicious file.

  1. Add the following decoders to the Wazuh server /var/ossec/etc/decoders/local_decoder.xml file. This allows extracting the information from YARA scan results:

    <decoder name="yara_decoder">
        <prematch>wazuh-yara:</prematch>
    </decoder>
    
    <decoder name="yara_decoder1">
        <parent>yara_decoder</parent>
        <regex>wazuh-yara: (\S+) - Scan result: (\S+) (\S+)</regex>
        <order>log_type, yara_rule, yara_scanned_file</order>
    </decoder>
    
  2. Add the following rules to the Wazuh server /var/ossec/etc/rules/local_rules.xml file. The rules detect FIM events in the monitored directory. They also alert when malware is found by the YARA integration:

    <group name="syscheck,">
      <rule id="100303" level="7">
        <if_sid>550</if_sid>
        <field name="file">C:\\Users\\<USER_NAME>\\Downloads</field>
        <description>File modified in C:\Users\<USER_NAME>\Downloads directory.</description>
      </rule>
      <rule id="100304" level="7">
        <if_sid>554</if_sid>
        <field name="file">C:\\Users\\<USER_NAME>\\Downloads</field>
        <description>File added to C:\Users\<USER_NAME>\Downloads  directory.</description>
      </rule>
    </group>
    
    <group name="yara,">
      <rule id="108000" level="0">
        <decoded_as>yara_decoder</decoded_as>
        <description>Yara grouping rule</description>
      </rule>
    
      <rule id="108001" level="12">
        <if_sid>108000</if_sid>
        <match>wazuh-yara: INFO - Scan result: </match>
        <description>File "$(yara_scanned_file)" is a positive match. Yara rule: $(yara_rule)</description>
      </rule>
    </group>
    
  3. Add the following configuration to the Wazuh server /var/ossec/etc/ossec.conf file:

    <ossec_config>
      <command>
        <name>yara_windows</name>
        <executable>yara.bat</executable>
        <timeout_allowed>no</timeout_allowed>
      </command>
    
      <active-response>
        <command>yara_windows</command>
        <location>local</location>
        <rules_id>100303,100304</rules_id>
      </active-response>
    </ossec_config>
    
  4. Restart the Wazuh manager to apply the configuration changes:

    $ sudo systemctl restart wazuh-manager
    

Attack emulation

Note

For testing purposes, we download the EICAR anti-malware test file as shown below. EICAR is considered dangerous so don’t install it in production environments.

Download a malware sample on the monitored Windows endpoint:

  1. Turn off Microsoft Virus and threat protection.

  2. Download the EICAR zip file:

    Invoke-WebRequest -Uri https://secure.eicar.org/eicar_com.zip -OutFile eicar.zip
    
  3. Unzip it:

    > Expand-Archive .\eicar.zip
    
  4. Copy the EICAR file to the monitored directory:

    > cp .\eicar\eicar.com C:\Users\<USER_NAME>\Downloads
    

Visualize the alerts

You can visualize the alert data in the Wazuh dashboard. To do this, go to the Security events module and add the filters in the search bar to query the alerts.

  • rule.groups:yara