File permissions and access control in Cybersecurity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are file permissions in a computer system?

File permissions are rules that control who can read, write, or execute a file. They help protect files from unauthorized access or changes.

beginner

Name the three basic types of file permissions.

The three basic types are: Read (view file contents), Write (modify file), and Execute (run the file as a program).

beginner

What is access control in cybersecurity?

Access control is a security method that decides who can use or see resources like files or systems. It ensures only authorized users can access sensitive data.

intermediate

How do user, group, and others relate to file permissions?

Permissions are often set separately for the file owner (user), a group of users, and everyone else (others). This helps manage access for different people.

intermediate

What does the 'chmod' command do in Unix-like systems?

The 'chmod' command changes file permissions, letting you set who can read, write, or execute a file.

Which permission allows a user to run a file as a program?

A Read

B Write

C Execute

D Delete

Execute permission lets a user run the file as a program or script.

In file permissions, who does 'group' refer to?

A The file owner only

B A set of users assigned to the file

C All users on the system

D Only system administrators

'Group' means a collection of users who share the same access rights to the file.

What does 'read' permission allow you to do?

A View the file contents

B Change the file contents

C Run the file as a program

D Delete the file

Read permission lets you open and see the contents of the file.

Which command is used to change file permissions in Unix/Linux?

A ls

B mv

C cp

D chmod

'chmod' is the command to modify file permissions.

Access control helps to:

A Restrict file access to authorized users

B Allow everyone to access all files

C Speed up file access

D Delete files automatically

Access control restricts file or system access to only authorized users.

Explain the three basic file permissions and why they are important.

Think about what each permission allows a user to do with a file.

Describe how access control works to protect files in a computer system.

Consider who can access files and how the system decides this.

Practice

1. What does the read permission allow a user to do with a file?

easy

A. Modify the contents of the file

B. View the contents of the file

C. Execute the file as a program

D. Delete the file

Solution

1. Step 1: Understand the meaning of read permission

   The read permission allows a user to open and see the contents of a file without changing it.

2. Step 2: Compare with other permissions

   Write permission allows changes, execute allows running the file, and delete is a separate action.

3. Final Answer:

   View the contents of the file -> Option B

4. Quick Check:

   Read permission = View file contents [OK]

Hint: Read means viewing file contents only [OK]

Common Mistakes:

• Confusing read with write permission
• Thinking read allows file execution
• Assuming read allows file deletion

2. Which of the following is the correct symbolic notation for giving the owner execute permission on a file in Unix/Linux?

easy

A. u+x

B. g+x

C. o+x

D. a+x

Solution

1. Step 1: Understand symbolic notation for permissions

   In Unix/Linux, 'u' stands for user (owner), 'g' for group, 'o' for others, and 'a' for all.

2. Step 2: Identify execute permission addition

   Adding execute permission is done with '+x'. So 'u+x' means add execute for owner.

3. Final Answer:

   u+x -> Option A

4. Quick Check:

   Owner execute permission = u+x [OK]

Hint: u means owner; +x adds execute permission [OK]

Common Mistakes:

• Using g+x or o+x for owner permissions
• Confusing a+x with owner only
• Misunderstanding symbolic letters

3. Given the file permission string -rwxr-x---, which users can execute the file?

medium

A. Owner and group members

B. Only the owner

C. Everyone

D. Only others

Solution

1. Step 1: Break down the permission string

   The string '-rwxr-x---' means: owner has read, write, execute; group has read and execute; others have no permissions.

2. Step 2: Identify who can execute

   Owner (rwx) can execute, group (r-x) can execute, others (---) cannot.

3. Final Answer:

   Owner and group members -> Option A

4. Quick Check:

   Execute permission for owner and group [OK]

Hint: Look for 'x' in owner and group parts [OK]

Common Mistakes:

• Ignoring group execute permission
• Assuming others have execute
• Misreading permission string

4. A user tries to write to a file but gets a 'Permission denied' error. The file permissions are -rw-r--r--. What is the most likely cause?

medium

A. User has execute permission but not write

B. File is read-only for everyone

C. User is not the owner and has no write permission

D. File is locked by another process

Solution

1. Step 1: Analyze the permission string

   '-rw-r--r--' means owner can read and write; group and others can only read.

2. Step 2: Understand user's permission

   If the user is not the owner, they only have read permission, so writing is denied.

3. Final Answer:

   User is not the owner and has no write permission -> Option C

4. Quick Check:

   Write denied if not owner with write permission [OK]

Hint: Write permission only for owner here [OK]

Common Mistakes:

• Assuming read means write allowed
• Confusing execute with write permission
• Ignoring ownership in permissions

5. You want to allow a group of users to read and execute a script but prevent them from modifying it. Which permission setting should you apply?

hard

A. Set group permissions to ---

B. Set group permissions to rw-

C. Set group permissions to rwx

D. Set group permissions to r-x

Solution

1. Step 1: Understand required permissions

   Users need to read and execute but not write (modify) the script.

2. Step 2: Match permissions to requirements

   'r-x' means read and execute allowed, write denied. 'rw-' allows write, 'rwx' allows all, '---' denies all.

3. Final Answer:

   Set group permissions to r-x -> Option D

4. Quick Check:

   Read + execute, no write = r-x [OK]

Hint: r-x means read and execute, no write [OK]

Common Mistakes:

• Choosing rw- which allows writing
• Choosing rwx which allows all permissions
• Choosing --- which denies access