Mastering YARA Rules: Malware Detection and Analysis

Jackie Ostlie

7 months ago

Introduction

One of the most important ways to safeguard your business is to ensure strong protection against malware attacks. If malware gains a foothold, it can be used to extort money, gather sensitive information, monitor keystrokes or even destroy your network or machines.

YARA, which stands for, Yet Another Recursive Acronym, is one of the most effective ways to help identify and classify malware like ransomware. YARA is an open sourced resource that enables cybersecurity teams to search for, detect and alert you to the presence of malware before it can do any damage.

This article presents an overview of YARA and discusses how to create and test a YARA rule, why it can be so effective against malware and how to use YARA rules to safeguard your organization.

Understanding YARA Rules

What are YARA Rules?

YARA rules check files and networks for patterns, scripts and signatures that indicate the presence of malicious software, which is often written in a simple but unique format. When a rule finds a characteristic or pattern that indicates a piece of malware, it alerts the appropriate person, who can then isolate or delete it.

Use Cases for YARA

YARA Rules for Malware Detection

The first step in creating a YARA rule is identifying the malware or malware family you want to detect. Using indicators of compromise like filenames, registry keys or hashtags, you can write a YARA rule based on them to identify malicious software. Test your YARA rule before you use it to ensure accuracy, then run it on your network with your other security products.

YARA rules are also flexible, meaning you can test small batches of files or scale up to hunt for malicious software on entire networks.

YARA Rules and Threat Intelligence

If you use a threat intelligence platform, check if it works with YARA rules. If it does, you can build rules based on the information your TIP has gathered about the different kinds of cybersecurity threats your company has previously faced.

YARA rules can be used for routine detection of simple attempts by inexperienced hackers or for more dangerous attempts by corporate spies, cybercriminals or disgruntled employees using malware, Trojans or ransomware.

Ransomware Detection with YARA Rules

According to Veeam’s 2023 Ransomware Trends Report, 85% of organizations surveyed suffered at least one ransomware attack in 2022. Top data protection companies like Veeam integrate YARA or Signature-based malware detection tools to scan backups for health and recoverability.

However, if you want a healthy backup to recover your business data as quickly as possible in the face of an attack, use a YARA rule to look beyond anomalies. If the YARA rule finds malicious software, it can alert the appropriate cybersecurity team member of the issue. Behavior- and pattern-based tools can be combined with endpoint protection, intrusion detection systems and intrusion protection systems to provide robust ransomware defense as well.

YARA Syntax

Rule

The rule’s name identifies the file. The identifier after the word rule can never be a number or an underscore. No identifier can be included in the condition section either, since their only purpose is to provide information about the rule.

Metadata

Metadata provides information about the rule itself like the author, date it was created, version of the YARA rule for tracking updates or amendments, description, etc

Strings

The string section is where you define the patterns, signatures or strings the YARA rule will hunt in your files or network. Your YARA rule can look for several types of strings, including those that use hexadecimal in combination with jumps, alternatives and wildcards. Text strings can also use modifiers like nocase, full word and wide or regular expressions that use similar modifiers to text strings. You can learn more about advanced conditions in the YARA documentation as well.

Conditions

This is the only required section, and it specifies when the YARA rule is valid for the file you’re scanning. It uses a Boolean expression (i.e., and, or, all, any, not) to help determine the result and can also check file size as a condition.

Creating YARA Rules

Defining Objectives

First, identify the malware you want to detect. For instance, if you’re hunting for ransomware such as WannaCry, there are particular strings, signatures and patterns that this ransomware family uses that you can hunt for.

Don’t write one YARA rule to find all types of malware. If you need to find other types of malicious software, then write more YARA rules to fit each type.

Writing YARA Rules

First, if you don’t have the YARA program, download it.
Next, determine the objective for your YARA rule. By identifying the malware you want to detect, you can identify the specific characteristics for your YARA rule; this can include hashes, registry keys, file names, and other indicators of compromise Additionally, you can use other YARA rules that other security analysts have created and shared across the security community.
Write your YARA rule. Using the YARA specific syntax, your rule should include descriptions of the malware based on its characteristics. Typically this would include a header, condition, and tags that categorize the rule
Test your YARA rule. Navigate to the directory where your test file is located and run the YARA file (or use one of the free services mentioned below).
If you write the YARA rule correctly, it will identify the malicious software and send you a message when it detects something.
Integrate the YARA rule into your cybersecurity infrastructure.

Simple YARA Rule

rule Detect_Malicious_String {
    strings:
        $malicious_string = “malicious_string”
    condition:
        $malicious_string

Complex YARA rule

rule Detect_Complex_Executable {
   meta:
       description = “Detects a complex Windows executable with specific characteristics”
       author = “Your Name”
       date = “2023-09-20”
   strings:
       $magic_bytes = { 4D 5A } // Check for the “MZ” DOS header
       $has_pe_signature = “PE\0\0” wide // Check for the PE signature
       $specific_section = /ThisIsMySection/ // Look for a custom section name
   condition:
       $magic_bytes at 0 and
       $has_pe_signature at $magic_bytes + 0x3C and
       $specific_section
}

YARA rules can be written to accomplish several cybersecurity tasks.

Best Practices for YARA Rule Development

Some best practices to make your YARA rules stronger and more effective include:

Building templates to stay consistent and more organized.
Considering using the standard YARA rule convention and contributing to the rule responsitors to help the broader cybersecurity community. These conventions typically include the use of a header to identify the rule, a condition that describes the malware characteristics, and tags to help categorize the rule you’ve created, to help other cybersecurity exerts
Creating mulitple rules for each file type instead of just using one rule for all kinds of malware.
Using a nocase modifier selectively, since it uses a lot of memory and can produce false positives.

Check out the National Institute of Standards and Technology for more cybersecurity guidelines and best practices.

Testing and Validating YARA Rules

Testing YARA Rules

Test your YARA rule with the YARA Scan Service and its database of over 77,000 malicious software examples for free! The more you refine your YARA rules with test results, the better it can detect malicious software.

Validation in Real-World Scenarios

To detect malware (in this example we’ll use the name “BadStuff”), create a text file, name it “badstuff_rule.yara,” and define the YARA rule. Call your rule “rule Detect_BadStuff” and write the YARA rule using the appropriate metadata, strings and conditions. Save the file and test it. Once you have the test results, clarify your objective, if necessary.

The YARA rule can then be integrated into your cybersecurity software to scan files in real time. Your YARA rule can also be integrated into email security software and web filtering tools to hunt for attachments and downloads containing malicious content, including “BadStuff.”

Valuing YARA rules in live environments is important for using these rules effectively, since you want to achieve accuracy and precision. Since YARA rules are based on patterns or signatures, testing your YARA rule in a live environment allows you to verify that these patterns have been identified accurately.

YARA Rule Sharing and Collaboration

Community YARA Rules

A large YARA rules community exists with several public repositories for sharing YARA rules. The benefits of sharing are apparent, and result in more robust and effective YARA rules.

Collaborative Rule Development

When security professionals and organizations share YARA rules, the community can develop more focused malware-fighting tools. When these professionals share theirrules, they’re not only sharing vital knowledge but looking for others to improve their work as well. When threat intelligence is shared, identifying dangerous attacks becomes easier for the entire community.

Integrating YARA Rules in Your Security Stack

Once your cybersecurity team has written and tested a YARA rule, it’s ready to be integrated into your organization’s SIEM, Security Information and Event Management platform.

Many cybersecurity applications currently allow for the integration of YARA rules. This means you can set up automated scanning processes to enable YARA rules to monitor your network for malicious software and alert you when necessary.

Don’t forget to monitor your YARA rules and check online repositories for updates. YARA rules are explicitly written to search for malware families, so if cybercriminals or hackers change their malware code, your YARA rule becomes useless.

Incorporating YARA into Security Workflows

Look for instances where a YARA rule would be the most beneficial and create rules designed for that purpose. Test them and incorporate them to work with the appropriate security software solution, like email, antivirus, web filtering, etc.

Automation and Alerting

Bash/shell scripts (on Linux or UNIX-based systems) will alert you via email, log entry or system notification. Another method is to use Python scripting to generate alerts when your YARA rule finds malicious software. Use a system-appropriate scheduler to run these automated scripts.

You can streamline responses to these alerts in several ways:

Ensuring your YARA rule is well-designed and specific to the task.
Assigning importance to different YARA rules (some alerts you may want to know immediately; others can wait).
Developing workflow charts that outline the actions taken in response to different types of YARA rule alerts.
Speeding up the response to a YARA rule match by notifying the various security teams in your organization via email, Microsoft Teams, SMS or Slack.
Regularly training your cybersecurity team to respond to YARA rule alerts.

YARA Rules, Documentation and Resources

There are several YARA rule repositories and communities where you can share your YARA rules and collaborate with others.

Official YARA GitHub Repository: This is the primary source for all things YARA. You can find the latest releases, YARA documentation and the YARA source code here.
YARA Documentation: Hosted on ReadTheDocs, the official YARA documentation provides comprehensive information on how to use YARA and its syntax, what the rules do and how its capabilities detect malicious software.
YARA Rules and Signatures Repository: This is a great resource where you can find a collection of community-based YARA rules and signatures. You can also contribute your YARA rules so that others can use them.
YARA Google Group (Mail List): This is a discussion forum where users and developers can talk, help each other, share experiences, and discuss everything there is to know about YARA. It’s a great way to meet other members of the community.

Conclusion

YARA rules are an effective way to enhance cybersecurity and detect malicious software. Many programs have integrated YARA rules. If you’re looking for a way to enhance your company’s cybersecurity efforts, YARA rules are one of the best ways to do it.