Topic in Operating Systems
Protection is the set of OS mechanisms that decide which processes and users may access which resources, while security defends against attacks. Protection questions use the principle of least privilege, protection domains, the access matrix, and its implementation as access control lists or capability lists. Access-control models such as DAC, MAC, RBAC and the Bell-LaPadula confidentiality model are regularly compared. On the security side, know threats like trojan horses, trap doors, buffer overflows, viruses and worms, along with authentication, password hashing with salts, symmetric versus asymmetric encryption, and hardware flaws such as Spectre and Meltdown.
Below are 30 practice questions from a pool of 210 Protection & Security MCQs, one of 12 topics in Operating Systems. Each shows the correct answer with an explanation; when you are ready, take a timed quiz to test recall under exam conditions.
Each question below shows the correct answer with a full explanation. Use these to build conceptual understanding before attempting a timed quiz.
Q1. Authentication is the process of:
Authentication verifies the identity of a user or process, typically through something the user knows (password), has (smart card/token), or is (biometrics like fingerprint).
Q2. A worm differs from a virus in that:
Unlike viruses that need a host program, worms are standalone programs that self-replicate across networks without human intervention. They exploit network vulnerabilities to spread automatically.
Q3. A firewall is:
A firewall is a network security system that monitors and filters incoming and outgoing network traffic based on an organization's security policies and predefined rules.
Q4. Protection in operating systems refers to:
Protection is an internal mechanism that controls access of processes and users to system resources (CPU, memory, files) based on defined policies, ensuring authorized access only.
Q5. The principle of least privilege states that:
The principle of least privilege requires that every process, user, and program operates using the least set of privileges necessary to complete the task, minimizing the damage from errors or malicious actions.
Q6. A Trojan horse in computer security is:
A Trojan horse is a program that appears legitimate and useful but contains hidden malicious functionality. Unlike viruses and worms, Trojans do not replicate themselves.
Q7. Security in operating systems deals with:
Security involves defending the system against internal and external threats, including unauthorized access, malicious attacks (viruses, worms, trojans), denial-of-service attacks, and data breaches.
Q8. An access control matrix is:
An access control matrix has rows for subjects (users/processes) and columns for objects (files/resources). Each cell contains the access rights (read, write, execute) that the subject has on the object.
Q9. Authorization determines:
Authorization determines the access rights and permissions of an authenticated user - what resources they can access and what operations (read, write, execute) they can perform on them.
Q10. Encryption is the process of:
Encryption transforms readable data (plaintext) into an unreadable format (ciphertext) using a cryptographic algorithm and key. Only authorized parties with the correct key can decrypt and read the data.
Q11. A virus in computer security is:
A computer virus is a piece of malicious code that attaches itself to a legitimate program. When the host program runs, the virus executes and can replicate itself to other programs or files.
Q12. Capability lists are a row-based implementation of the access control matrix where:
A capability list is associated with each subject (user/process) and contains pairs of (object, access rights). It is a row-wise view of the access control matrix. A capability is an unforgeable token of authority.
Q13. In UNIX file permissions, the permission mode 755 means:
In octal notation, 755 means: owner has rwx (7=4+2+1), group has r-x (5=4+0+1), and others have r-x (5=4+0+1). This is common for executable files and directories.
Q14. Access Control Lists (ACLs) are a column-based implementation of the access control matrix where:
An ACL is associated with each object (e.g., file) and lists all subjects (users/groups) and their access rights. It is a column-wise view of the access control matrix.
Q15. What is a protection domain?
A protection domain specifies the resources (objects) a process can access and the operations it can perform. Processes execute within a domain that limits their access rights, following least privilege.
Q16. A buffer overflow attack exploits:
Buffer overflow occurs when a program writes data beyond the boundary of allocated buffer memory. Attackers exploit this to overwrite return addresses or other control data, potentially executing malicious code.
Q17. A denial-of-service (DoS) attack:
A DoS attack floods a system, server, or network with excessive requests, consuming resources and making the service unavailable to legitimate users. A DDoS (Distributed DoS) uses multiple attacking systems.
Q18. What is the difference between symmetric and asymmetric encryption?
Symmetric encryption (AES, DES) uses one shared key. Asymmetric encryption (RSA, ECC) uses a key pair: public key for encryption, private key for decryption. Asymmetric is slower but solves key distribution.
Q19. Multifactor authentication combines:
Multifactor authentication requires two or more different types of credentials: something you know (password), something you have (token/phone), and/or something you are (fingerprint/face), providing stronger security.
Q20. What is a rootkit?
A rootkit is malware designed to gain administrator (root) access while actively hiding its presence from the OS and security tools. It can modify OS components, intercept system calls, and hide files/processes.
Q21. What are Spectre and Meltdown vulnerabilities?
Spectre and Meltdown are hardware vulnerabilities in modern CPUs. They exploit speculative execution and out-of-order execution to read memory that should be inaccessible, including kernel memory and other processes' data.
Q22. What is the role of a digital certificate in security?
A digital certificate binds a public key to an entity's identity (person, organization, server). It is issued and signed by a trusted Certificate Authority (CA), enabling verification of the public key's authenticity.
Q23. What is mandatory access control (MAC) and how does it differ from discretionary access control (DAC)?
In MAC (e.g., SELinux), the system enforces centrally-defined security policies that users cannot override. In DAC (e.g., UNIX permissions), the resource owner decides who can access their resources.
Q24. What is the concept of security rings (protection rings) in OS architecture?
Protection rings are a hierarchical mechanism where Ring 0 (innermost, kernel mode) has highest privilege, and Ring 3 (outermost, user mode) has least privilege. Hardware enforces that outer rings cannot access inner ring resources directly.
Q25. What is a side-channel attack?
Side-channel attacks extract secrets by observing physical characteristics of computation: timing (how long operations take), power consumption, electromagnetic emissions, or cache access patterns (Spectre, Meltdown).
Q26. What is Address Space Layout Randomization (ASLR)?
ASLR randomizes the memory locations of stack, heap, libraries, and code at each execution. This makes it difficult for attackers to predict target addresses for buffer overflow or return-to-libc attacks.
Q27. What is the Bell-LaPadula security model?
Bell-LaPadula is a formal security model enforcing confidentiality. Its rules: 'no read up' (a subject cannot read data at a higher security level) and 'no write down' (cannot write data to a lower level), preventing information leakage downward.
Q28. What is Role-Based Access Control (RBAC)?
RBAC assigns access rights to roles (e.g., admin, editor, viewer) rather than individual users. Users are assigned roles, and inherit the permissions associated with those roles, simplifying access management.
Q29. What is the Biba integrity model and how does it differ from Bell-LaPadula?
Biba is the dual of Bell-LaPadula, focusing on integrity instead of confidentiality. Its rules: 'no read down' (cannot read lower-integrity data) and 'no write up' (cannot write to higher-integrity objects), preventing corruption of trusted data.
Q30. What is a sandbox in operating system security?
A sandbox is a restricted execution environment where untrusted code runs with limited permissions. If the code is malicious, the damage is contained within the sandbox and cannot affect the wider system.
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