hardMultiple Choice
Bell-LaPadula MAC: Can a Secret Subject Read Confidential and Write to Top Secret?
In a Bell-LaPadula MAC model, which of the following operations is prohibited?
⚠ Common exam trap
ISC2 often tests the misconception that the *-property (no write down) applies to writing to higher-level objects, when in fact it only restricts writing to lower-level objects, and candidates may confuse it with the simple security property (no read up).
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
✓
A Top Secret subject writes to an Unclassified object
In the Bell-LaPadula model, the *-property (star property) prohibits a subject from writing to an object at a lower classification level. A Top Secret subject writing to an Unclassified object would cause a downgrade of sensitive information, violating this property. Therefore, option D is the prohibited operation.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A Top Secret subject reads a Confidential object
Why it's wrong here
Reading down from Top Secret to Confidential is expressly allowed by Bell-LaPadula's simple security property, which only forbids reading objects at a higher classification. It is tempting because the direction feels counterintuitive, but the model's concern is confidentiality leakage upward, not downward access.
- ✗
A Top Secret subject reads a Top Secret object
Why it's wrong here
A Top Secret subject reading a Top Secret object satisfies the simple security property, since the subject's clearance dominates the object's label. It is tempting to assume any Top Secret interaction is restricted, but Bell-LaPadula permits equal-level reads; only reads to higher classifications are prohibited.
- ✗
A Top Secret subject writes to a Top Secret object
Why it's wrong here
Writing from a Top Secret subject to a Top Secret object satisfies both the no-read-up and no-write-down properties, so Bell-LaPadula permits it. It is tempting because same-level access feels like it should be restricted, but the model's mandatory controls only prohibit flows that violate the dominance relation.
- ✓
A Top Secret subject writes to an Unclassified object
Why this is correct
A Top Secret subject writing to an Unclassified object violates the *-property (star property), which forbids writing down to lower sensitivity levels. Bell-LaPadula permits no write-down, preventing high-classification data leaking into less protected objects. The no-read-up simple property is unaffected here; only the write-down is prohibited.
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
Related to this question
Learn chapter
Security Operations Basics
Key term
Bell-LaPadula
A formal security model that prevents users from reading information at a higher classification level than their own and from writing information down to a lower classification level.
Key term
MAC
MAC (Media Access Control) is a unique hardware identifier assigned to network interfaces for communication on a local network segment.
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