The correct answer is a weak JWT signing algorithm vulnerability. This is because the server’s behavior of returning a 200 OK for both valid and invalid tokens indicates it is not verifying the JWT signature at all, a classic sign that the application accepts the “none” algorithm or uses an easily guessable symmetric key. In this scenario, an attacker can forge arbitrary tokens by simply setting the algorithm header to “none” or by brute-forcing a weak secret, thereby bypassing authentication entirely. On the CISSP exam, this tests your understanding of cryptographic failures in the Software Development Security domain, often appearing as a scenario where a server fails to reject malformed tokens. A common trap is confusing this with a token expiration or injection issue, but the key clue is the uniform 200 response. Memory tip: “None means none—no signature, no security.”
CISSP Security Assessment and Testing Practice Question
This CISSP practice question tests your understanding of security assessment and testing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
During a penetration test, a tester discovers that the target web application responds to HTTP requests with a "200 OK" status for both valid and invalid session tokens on a particular API endpoint. The application uses JSON Web Tokens (JWT) for authentication. Which of the following vulnerabilities is MOST likely present?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Weak JWT signing algorithm
The application returns a 200 OK for both valid and invalid session tokens, which indicates that the server is not validating the JWT signature. This behavior is characteristic of a weak JWT signing algorithm vulnerability, such as when the server accepts 'none' as the algorithm or uses a symmetric key that can be easily guessed. An attacker can forge arbitrary tokens by setting the algorithm to 'none' or by using a known weak secret, gaining unauthorized access.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Weak JWT signing algorithm
Why this is correct
A weak or missing signature verification allows any JWT to be accepted.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Session fixation
Why it's wrong here
Session fixation involves tricking a user into using a session ID chosen by the attacker; not related to token acceptance.
✗
Missing authentication
Why it's wrong here
The application does authenticate (returns 200 for valid tokens), but it also accepts invalid tokens.
✗
Insecure direct object reference
Why it's wrong here
IDOR allows access to unauthorized resources via direct reference; not about token validation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the distinction between 'missing authentication' and 'weak authentication'—the trap here is that candidates see '200 OK for invalid tokens' and incorrectly assume no authentication is required, when in fact authentication is present but the signing mechanism is flawed.
Detailed technical explanation
How to think about this question
JWT libraries in many languages (e.g., jsonwebtoken for Node.js, PyJWT for Python) have a known vulnerability where if the algorithm is set to 'none', the signature is ignored entirely. In a real-world scenario, an attacker can modify the JWT header to `{"alg":"none"}` and remove the signature, and the server will accept it if it does not enforce algorithm validation. This is explicitly warned against in RFC 7518, which states that the 'none' algorithm should only be used when the JWT is secured by other means.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Security Assessment and Testing — This question tests Security Assessment and Testing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Weak JWT signing algorithm — The application returns a 200 OK for both valid and invalid session tokens, which indicates that the server is not validating the JWT signature. This behavior is characteristic of a weak JWT signing algorithm vulnerability, such as when the server accepts 'none' as the algorithm or uses a symmetric key that can be easily guessed. An attacker can forge arbitrary tokens by setting the algorithm to 'none' or by using a known weak secret, gaining unauthorized access.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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