CAS-004 Security Engineering Practice Question
An IoT device manufacturer wants to ensure the security of over-the-air (OTA) firmware updates. Which TWO measures are essential to protect the update process?
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
✓
Sign the firmware with a trusted code signing certificate
Option A is correct because signing the firmware with a trusted code signing certificate lets the device verify authenticity and integrity via the vendor's public key, ensuring the OTA image genuinely originates from the manufacturer and has not been tampered with. Option C is correct because a secure boot chain validates that signature against a hardware-rooted trust anchor before the update is applied, preventing malicious or corrupted firmware from ever executing. Together, signing plus signature verification in a secure boot chain form the essential cryptographic trust path for OTA updates. Option B is not sufficient because a simple checksum detects only accidental corruption, not deliberate tampering, since it is not cryptographically bound to a secret or key. Option D is wrong because a hardcoded key can be extracted from the device and reused by attackers, and encryption alone does not prove authenticity. Option E is wrong because permitting downgrades enables rollback attacks that reintroduce known vulnerabilities.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Sign the firmware with a trusted code signing certificate
Why this is correct
Code signing binds the firmware to a trusted publisher's private key, so devices can verify authenticity and integrity before flashing. This directly satisfies the OTA constraint: preventing attackers from pushing tampered or malicious firmware images to fielded devices.
- ✗
Use a simple checksum for integrity verification
Why it's wrong here
A checksum detects accidental corruption but is not cryptographic, so an attacker who alters firmware can recompute it; a digital signature verified against a trusted key is needed. It tempts because checksums are the standard, cheap integrity check for ordinary file transfers.
- ✓
Implement a secure boot chain that verifies the signature before applying the update
Why this is correct
A secure boot chain validates each stage's signature against a hardware-rooted trust anchor before execution, so a correctly signed but unauthorised image is rejected. This enforces the OTA requirement that only verified firmware is applied, blocking rollback and tampering attacks.
- ✗
Encrypt the firmware using a hardcoded key
Why it's wrong here
A hardcoded key is extractable from device firmware, so anyone recovering it can forge or decrypt updates; keys must be unique per device and stored in secure hardware. It tempts because encrypting firmware at all is correct, and hardcoding is the quickest way to ship it.
- ✗
Allow firmware downgrades to previous versions
Why it's wrong here
Permitting downgrades lets an attacker replay an older, signed-but-vulnerable image, defeating rollback protection; the device must refuse versions below the installed one. It tempts because downgrades are legitimately needed to recover from faulty releases, but that must be a controlled, authorised exception.
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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.