CS0-003 Security Operations Practice Question
An analyst is creating a YARA rule to detect a specific malware family that uses the string 'evil' in its PE file. Which of the following rule structures is correct?
⚠ Common exam trap
CS0-004 often tests the exact YARA rule skeleton — candidates pick options that omit the $variable assignment in the strings section or use imperative pseudocode instead of the rule/strings/condition block.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
rule detect_malware { strings: $a = "evil" condition: $a }
A valid YARA rule requires the structure: rule <name> { strings: $var = "pattern" condition: $var }. Option A correctly declares a string variable $a assigned to 'evil' and then references $a in the condition, which is the canonical YARA syntax.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
rule detect_malware { strings: $a = "evil" condition: $a }
Why this is correct
This is the correct YARA rule syntax. It defines a rule named detect_malware, declares a string identifier $a assigned to the literal byte sequence "evil" inside the strings section, and then uses that identifier as the condition. The condition $a evaluates to true if the string 'evil' is found anywhere in the scanned file. YARA requires a dollar-sign prefix for string identifiers, an equals sign to bind the literal value, and a condition that references the identifier without quotes or further assignment. This rule compiles and will trigger a match when the file contains the specified string.
- ✗
rule detect_malware { strings: "evil" condition: $a }
Why it's wrong here
This rule is invalid because in YARA, every string entry in the strings section must be bound to a named identifier that begins with a dollar sign, such as $a. Here, the literal "evil" is written without any identifier, so the parser has no way to associate it with the $a referenced later in the condition. As a result, $a in the condition is an undeclared identifier, causing a compilation error. The correct syntax is $a = "evil" inside the strings section, not a bare literal.
- ✗
rule detect_malware { condition: $a = "evil" }
Why it's wrong here
This rule fails because it tries to define a string assignment directly inside the condition block. YARA's condition section accepts only boolean expressions, such as references to string identifiers or logical operators; it does not allow equals-sign assignments of literals. Additionally, there is no strings section at all, so $a is never defined anywhere in the rule. For a valid rule, you must declare $a in a separate strings: section and then simply reference it in the condition, e.g., condition: $a.
- ✗
if "evil" in file then alert
Why it's wrong here
This is not proper YARA syntax—it resembles an imperative scripting language like Perl or Python, but YARA is a declarative rule language. A YARA rule does not contain an 'if ... in file then alert' construct; instead, it consists of a rule name, optional metadata, a strings section, and a condition that is a boolean expression. The phrase 'in file' is not a valid YARA operator, and YARA does not include imperative actions like 'then alert'—the rule either matches or does not based solely on the condition. A valid equivalent would be a rule with a condition like $a, not an alert statement.
Go deeper
Related to this question
Learn chapter
SIGMA and YARA Detection Rules
Key term
Malware
Malware is any software intentionally designed to cause damage, disrupt operations, steal data, or gain unauthorized access to computer systems.
Key term
YARA
YARA is a pattern-matching tool used by cybersecurity professionals to identify and classify malware based on textual or binary patterns.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official CompTIA exam blueprint
This CS0-004 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CS0-004 exam.