Comprehensive and Detailed in-Depth Explanation:

Understanding the Vulnerability:

The payload presented is a classic example of anXML External Entity (XXE) attack.

In this attack, an attacker exploits improperly configured XML parsers to includeexternal entitiesthat can read sensitive files, such as/etc/passwdon Linux systems.

XXE vulnerabilities occur when:

XML input containing aDOCTYPEdeclaration is processed.

The parser is configured to resolve external entities.

Why the Correct Answer is B (Input validation):

Input validationis the most effective way to prevent XXE attacks.

Proper validation ensures thatmalicious XML entities are not accepted or processed.

Techniques to mitigate XXE include:

Disabling DTDs (Document Type Definitions)in XML parsers.

Implementingsecure parser configurationsthat do not process external entities.

Performingschema validationto restrict allowed XML elements.

Many modern XML parsers provide options todisable external entity processingentirely, significantly reducing the risk of XXE.

Why the Other Options Are Incorrect:

A. CAPTCHA:

CAPTCHAs prevent automated bots from interacting with web forms but donot mitigate XML parser vulnerabilities.

CAPTCHAs addressautomated input, notmalicious payloads.

C. Data encoding:

Encoding data can preventinjection attacks, such as XSS, but it does not specifically address the issue ofexternal entity resolutionin XML.

Encoding cannot prevent the parser from interpretingDOCTYPEor external entity references.

D. Network intrusion prevention:

AnNIPS (Network Intrusion Prevention System)may detect some XXE attempts but isnot the primary method of prevention.

Relying solely on network-level security does not address theapplication-layer vulnerabilityitself.

Additional Best Practices:

Use libraries or frameworks that do not support XML parsing oruse safer alternatives like JSON.

Regularly update XML parsers to the latest version to patch known vulnerabilities.

Conductregular security testing(like dynamic analysis) to identify XXE risks.

Extract from CompTIA SecurityX CAS-005 Study Guide:

According to theCompTIA SecurityX CAS-005 Official Study Guide, XXE vulnerabilities can be mitigated bydisabling external entity processingand using strict input validation to control what types of data are accepted. The guide emphasizes thatproperly configured XML parsersare critical for preventing such attacks, aligning with OWASP guidelines.