YARA and YARA Rules

YARA and YARA Rules

YARA is a powerful, pattern-matching tool that identifies files based on specific patterns and rules. These rules allow SOC analysts and forensic teams to detect, classify, and investigate suspicious files and malware samples. YARA rules analyze files' text or binary content and can also be applied to memory, aiding in both malware detection and proactive threat hunting.

Uses of YARA

1. Malware Detection: Identifies malware based on unique patterns or behaviors.

2. File Classification: Helps categorize files by format, version, metadata, etc.

3. IOC Detection: Searches files for indicators of compromise like registry keys or file names.

4. Threat Hunting: Proactively searches for threats across environments.

5. Incident Response: Quickly searches for artifacts in response to security incidents.

6. Custom Rules for Targeted Threats: Creates custom rules for specific organizational needs.

How YARA Works

1. Rules Set: Rules define patterns or behaviors to match against.

2. Files Set: Files or memory snapshots to scan.

3. YARA Engine: Compares file content byte-by-byte with the defined rules.

4. Detection Output: If patterns are matched, YARA flags the file as detected.

YARA Rule Structure

1. Basic Structure:

rule RuleName {
    meta:
        author = "Author Name"
        description = "Rule description"
    strings:
        $string1 = "sample_text"
        $string2 = { 4A 2D 1C }
    condition:
        all of them
}

2. Components of a YARA Rule:

• Rule Header: Begins with the keyword rule, followed by the rule name.

• Meta Section: Metadata like author, description, version, and references.

• Strings Section: Defines text, hexadecimal patterns, or regex to search for.

• Condition Section: Sets conditions for triggering the rule.

3. Example Rule – Detecting WannaCry Ransomware Strings:

rule Ransomware_WannaCry {
    meta:
        author = "Analyst Name"
        description = "Detects WannaCry-specific strings"
    strings:
        $wannacry1 = "tasksche.exe" fullword ascii
        $wannacry2 = "iuqerfsodp9ifjaposdfjhgosurijfaewrwergwea.com" ascii
        $wannacry3 = "mssecsvc.exe" fullword ascii
    condition:
        all of them
}

4. Conditions and Logic Operators:

• all of them: All specified patterns must match.

• any of them: Any one of the specified patterns can match.

• File Size Condition: Ensures the file size meets criteria:

condition:
    filesize < 100KB and uint16(0) == 0x5A4D

• uint16(0) == 0x5A4D: Checks if the first two bytes match 0x5A4D (indicating an MZ header for executables).

Advanced Features of YARA Rules

• Logical Operators: Combine conditions with and, or, not.

• External Modules: Extend rule functionality for specialized needs.

• Customizability: Allows tailoring rules to fit specific threats or indicators.