File permissions and access control in Cybersecurity - Step-by-Step Execution

Concept Flow - File permissions and access control

User tries to access file

↓

Check file permissions

↓

Does user have permission?

No→Access Denied

Yes↓

Allow access to file

↓

Log access event

When a user tries to access a file, the system checks the file's permissions. If the user has the right permissions, access is granted; otherwise, it is denied.

Execution Sample

Cybersecurity

User: Alice
File: report.txt
Permissions: rw-r-----
Action: Read file

Alice tries to read a file with read and write permissions for owner, read for group, none for others.

Analysis Table

Step	User	File Permissions	Action	Permission Check	Result
1	Alice	rw-r-----	Read	Is Alice owner? Yes	Access Granted
2	Bob	rw-r-----	Read	Is Bob owner? No; Is Bob in group? No	Access Denied
3	Charlie	rw-r-----	Write	Is Charlie owner? No; Is Charlie in group? Yes; Write permission for group? No	Access Denied
4	Alice	rw-r-----	Write	Is Alice owner? Yes; Write permission for owner? Yes	Access Granted
5	Dave	rw-r-----	Read	Is Dave owner? No; Is Dave in group? No	Access Denied

💡 Access decisions are based on user identity and file permission bits.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
User	None	Alice	Bob	Charlie	Alice	Dave
Action	None	Read	Read	Write	Write	Read
Access Result	None	Granted	Denied	Denied	Granted	Denied

Key Insights - 2 Insights

Why does Bob get denied access even though the file has read permission for the group?

Bob is neither the owner nor a member of the file's group, so group permissions do not apply to him. This is shown in execution_table row 2.

Why can Alice write to the file but Charlie cannot, even though Charlie is in the group?

The group does not have write permission (only read), so Charlie cannot write. Alice is the owner and has write permission. See execution_table rows 3 and 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the access result for Charlie when trying to write?

A Access Granted

B Access Denied

C Access Pending

D Access Unknown

💡 Hint

Check execution_table row 3 under 'Result' column.

At which step does the user have both ownership and write permission?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at execution_table rows for 'User' and 'Permission Check' columns.

If Dave was added to the file's group, what would change in step 5?

A Access would be granted for read

B Access would be granted for write

C Access would still be denied

D No change in access

💡 Hint

Refer to execution_table row 5 and group permissions in 'File Permissions'.

Concept Snapshot

File permissions control who can read, write, or execute a file.
Permissions are set for owner, group, and others.
Access is granted only if the user matches owner or group and has the needed permission.
If not, access is denied.
This protects files from unauthorized use.

Full Transcript

File permissions and access control determine who can use a file and how. When a user tries to access a file, the system checks if the user is the owner, belongs to the file's group, or is others. Then it checks if the requested action (read, write, execute) is allowed for that category. If yes, access is granted; if no, access is denied. For example, a file with permissions rw-r----- means the owner can read and write, the group can read, and others have no access. If Alice is the owner, she can read and write. If Bob is not in the group, he cannot read. This system helps keep files safe and private.

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