Quick Answer
The answer is to disable DTD processing and use a JSON or other less complex data format instead of XML. Disabling Document Type Definition (DTD) processing is the most direct mitigation because XXE injection attacks rely on the XML parser’s ability to resolve external entities defined within a DTD; when DTD processing is turned off, the parser cannot fetch or execute those external resources, effectively neutralizing the attack vector. On the CompTIA SecurityX CAS-004 exam, this concept tests your understanding of secure XML parsing configurations, often appearing in scenario-based questions where a web service accepts XML input—common traps include confusing input validation with parser hardening or overlooking that JSON inherently lacks entity expansion features. Remember the mnemonic “No DTD, No XXE” to recall that disabling DTD processing is the foundational defense, while switching to JSON eliminates the attack surface entirely.
This CAS-004 practice question tests your understanding of application environment, configuration and security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which two of the following are effective mitigations against XML External Entity (XXE) injection attacks? (Select the two best options.)
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Use a blacklist to filter out dangerous XML tags
Why wrong: Blacklists are easily bypassed; disabling DTD is more robust.
Validate all XML input against a schema
Why wrong: Schema validation does not prevent XXE if DTDs are still enabled.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Disable Document Type Definition (DTD) processing in the XML parser
Option A is correct because XXE attacks exploit the XML parser's ability to process external entities defined in a DTD. By disabling DTD processing entirely, the parser cannot resolve or fetch external resources, which neutralizes the primary vector for XXE injection. This is a standard security hardening step for XML parsers like libxml2, Xerces, or .NET's XmlReader, often achieved by setting properties such as `LIBXML_NOENT` to false or `XmlReaderSettings.DtdProcessing` to `Prohibit`.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Use a blacklist to filter out dangerous XML tags
Why it's wrong here
Blacklists are easily bypassed; disabling DTD is more robust.
Validate all XML input against a schema
Why it's wrong here
Schema validation does not prevent XXE if DTDs are still enabled.
Option-by-option analysis
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The CAS-004 exam frequently reuses these exact scenarios with slightly different constraints.
Why this is wrong here
Blacklists are easily bypassed; disabling DTD is more robust.
Why this is wrong here
Schema validation does not prevent XXE if DTDs are still enabled.
Analysis generated from the official CAS-004blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
CompTIA often tests the misconception that input validation or schema validation alone can prevent injection attacks, but the trap here is that XXE exploits parser-level features (DTD processing) that occur before any schema validation or content filtering takes place.
Detailed technical explanation
Under the hood, XXE attacks leverage the XML specification's support for external entities (RFC 2119), where a DTD can define an entity that references a file URL (e.g., `file:///etc/passwd`) or a network resource. Even with schema validation, the parser processes DTD declarations before validation occurs, meaning the entity expansion and data exfiltration happen at parse time, not during validation. In real-world scenarios, this is why many security frameworks (e.g., OWASP) recommend disabling DTD or using JSON as a simpler alternative, as JSON lacks entity expansion capabilities entirely.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the CAS-004 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Application Environment, Configuration and Security — This question tests Application Environment, Configuration and Security — Read the scenario before looking for a memorised answer..
The correct answer is: Disable Document Type Definition (DTD) processing in the XML parser — Option A is correct because XXE attacks exploit the XML parser's ability to process external entities defined in a DTD. By disabling DTD processing entirely, the parser cannot resolve or fetch external resources, which neutralizes the primary vector for XXE injection. This is a standard security hardening step for XML parsers like libxml2, Xerces, or .NET's XmlReader, often achieved by setting properties such as `LIBXML_NOENT` to false or `XmlReaderSettings.DtdProcessing` to `Prohibit`.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
This CAS-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 CAS-004 exam.
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