CS0-003 Security Operations Practice Question

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.

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