hardMultiple ChoiceObjective-mapped
CAS-004 JWT algorithm confusion vulnerability Practice Question
A security architect is evaluating a web application that uses JSON Web Tokens (JWTs) for authentication. The application uses an RSA256 asymmetric signing algorithm. The architect discovers that the JWT library accepts tokens with the algorithm set to 'none' if the public key is not provided during verification. Which of the following attacks is most likely to succeed if the application does not enforce algorithm validation?
⚠ Common exam trap
The CAS-004 exam often tests the distinction between algorithm confusion attacks (which involve key reuse) and signature exclusion attacks (which exploit the 'none' algorithm), and the trap here is that candidates confuse the 'none' algorithm vulnerability with the more complex key confusion attack described in option A.
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
✓
Signature exclusion attack using the 'none' algorithm
The JWT library accepts tokens with the algorithm set to 'none' when the public key is not provided during verification. This allows an attacker to forge a JWT with the 'none' algorithm, bypassing signature verification entirely. The attack succeeds because the application fails to enforce a whitelist of allowed algorithms, as recommended by RFC 7518.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Algorithm confusion (key confusion) attack where the attacker uses the public key as an HMAC secret
Why it's wrong here
This attack targets libraries that use the same key for both HMAC and RSA, but the scenario describes a library that accepts 'none' algorithm, not HMAC.
- ✗
Timing attack to brute-force the private key
Why it's wrong here
Timing attacks target side-channel leakage, not algorithm validation bypass.
- ✗
Header injection attack to modify the JWT header
Why it's wrong here
Header injection is about modifying headers in requests, not exploiting JWT algorithm handling.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The CAS-005 exam frequently reuses these exact scenarios with slightly different constraints.
✓Signature exclusion attack using the 'none' algorithmCorrect answer▾
✗Algorithm confusion (key confusion) attack where the attacker uses the public key as an HMAC secretWrong answer — click to see why▾
Why this is wrong here
This attack targets libraries that use the same key for both HMAC and RSA, but the scenario describes a library that accepts 'none' algorithm, not HMAC.
✗Timing attack to brute-force the private keyWrong answer — click to see why▾
Why this is wrong here
Timing attacks target side-channel leakage, not algorithm validation bypass.
✗Header injection attack to modify the JWT headerWrong answer — click to see why▾
Why this is wrong here
Header injection is about modifying headers in requests, not exploiting JWT algorithm handling.
Analysis generated from the official CAS-005blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
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About these practice questions
Courseiva writes every CAS-005 question from scratch — 968 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CAS-005 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-005 exam.