Capability-based security in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is capability-based security?

It is a security model where access to resources is controlled by unforgeable tokens called capabilities, which specify the rights a user or process has.

intermediate

How do capabilities differ from traditional access control lists (ACLs)?

Capabilities are tokens held by users or processes granting access, while ACLs list permissions attached to resources. Capabilities move with the user, ACLs stay with the resource.

intermediate

Why are capabilities considered unforgeable?

Because the system ensures capabilities cannot be copied or altered by unauthorized users, often by using protected memory or cryptographic methods.

beginner

Give a real-life example similar to capability-based security.

Like having a hotel room key card that only opens your room. The card (capability) grants you access without needing to check a list every time.

intermediate

What is a key benefit of capability-based security?

It simplifies access control by directly associating rights with tokens, reducing the risk of unauthorized access and making delegation easier.

What does a capability in capability-based security represent?

A A password for user login

B A list of users allowed to access a resource

C A token granting specific access rights

D A firewall rule

A capability is a token that grants specific access rights to a resource.

Which of the following is a key feature of capabilities?

A They can be easily forged by users

B They are unforgeable tokens

C They are stored only on the resource

D They list all users with access

Capabilities are designed to be unforgeable to ensure security.

In capability-based security, who holds the capability?

A The user or process needing access

B The system administrator only

C The resource owner only

D No one holds capabilities

Users or processes hold capabilities that grant them access.

How does capability-based security improve delegation?

A By disabling access sharing

B By requiring passwords for every access

C By listing all users in an ACL

D By allowing capabilities to be passed to others

Capabilities can be passed or delegated to others, making access control flexible.

Which analogy best describes capability-based security?

A A hotel room key card

B A public notice board

C A locked door with no key

D A guest list at a party

A hotel room key card is like a capability granting access to a specific room.

Explain what capability-based security is and how it controls access to resources.

Think about how a token or key can grant access without checking a list.

Describe the advantages of using capabilities over traditional access control lists.

Consider how capabilities make sharing and controlling access easier.

Practice

1. What is the main idea behind capability-based security?

easy

A. Access is controlled only by usernames and passwords

B. Permissions are granted randomly to users

C. Security is based on firewall rules only

D. Users or programs get tokens that directly grant permissions

Solution

1. Step 1: Understand capability-based security concept

   It means giving users or programs special tokens that say what they can do.

2. Step 2: Compare with other methods

   Unlike passwords or firewalls, capability tokens directly grant permission.

3. Final Answer:

   Users or programs get tokens that directly grant permissions -> Option D

4. Quick Check:

   Capability tokens = direct permission [OK]

Hint: Focus on tokens that grant direct access [OK]

Common Mistakes:

• Confusing capability tokens with passwords
• Thinking security is only about firewalls
• Believing permissions are random

2. Which of the following is a correct way to describe a capability in capability-based security?

easy

A. A token that grants specific access rights

B. A password string stored in a file

C. A firewall rule blocking IP addresses

D. A username used for login

Solution

1. Step 1: Define what a capability is

   A capability is a token that grants specific access rights to a resource.

2. Step 2: Eliminate incorrect options

   Passwords, firewall rules, and usernames are not capabilities.

3. Final Answer:

   A token that grants specific access rights -> Option A

4. Quick Check:

   Capability = access token [OK]

Hint: Remember capability means access token, not password [OK]

Common Mistakes:

• Mixing capabilities with passwords
• Thinking firewall rules are capabilities
• Confusing usernames with capabilities

3. Consider a system where a program holds a capability token allowing it to read a file. What happens if the program tries to write to the file?

medium

A. The write operation fails because the capability only allows reading

B. The write operation succeeds because the program has access

C. The system asks the user for permission

D. The file is deleted automatically

Solution

1. Step 1: Understand capability permissions

   Capabilities specify exact rights, here only read permission is given.

2. Step 2: Predict system behavior on unauthorized action

   Trying to write without write permission causes failure.

3. Final Answer:

   The write operation fails because the capability only allows reading -> Option A

4. Quick Check:

   Capability limits action to allowed rights [OK]

Hint: Check if capability includes the requested permission [OK]

Common Mistakes:

• Assuming any access means all actions allowed
• Thinking system prompts user for permission
• Believing unauthorized actions delete files

4. A program tries to use a capability token but gets an error. Which of these is the most likely cause?

medium

A. The user forgot their password

B. The capability token was revoked or expired

C. The system has no internet connection

D. The program has too many tokens

Solution

1. Step 1: Identify common reasons for capability errors

   Revoked or expired tokens cause access errors.

2. Step 2: Eliminate unrelated causes

   Too many tokens, internet issues, or user passwords do not affect capability token validity.

3. Final Answer:

   The capability token was revoked or expired -> Option B

4. Quick Check:

   Revoked/expired token = error cause [OK]

Hint: Check if token is still valid before use [OK]

Common Mistakes:

• Blaming network issues for capability errors
• Thinking password affects capability tokens
• Assuming token count causes errors

5. In a capability-based system, how can you safely share access to a resource with another program without giving full control?

hard

A. Allow the other program to run as administrator

B. Share your password with the other program

C. Give the other program a capability token with limited permissions

D. Disable all security checks temporarily

Solution

1. Step 1: Understand safe sharing in capability systems

   Sharing limited permission tokens lets others access only what you allow.

2. Step 2: Avoid unsafe methods

   Sharing passwords, admin rights, or disabling security risks full control and breaches.

3. Final Answer:

   Give the other program a capability token with limited permissions -> Option C

4. Quick Check:

   Limited capability token = safe sharing [OK]

Hint: Share only limited tokens, never passwords or full rights [OK]

Common Mistakes:

• Sharing passwords instead of tokens
• Granting full admin rights unnecessarily
• Turning off security checks