3 Easy Ways To Set Special Permission in Linux?

Written by

Ojash

Reviewed by

Akshat

Last updated: July 11, 2024

Expert verified

Special file permissions like SUID, SGID, and the Sticky Bit in Linux are really helpful, especially when performing tasks that usually require higher privileges. These settings allow you to execute crucial tasks more efficiently, all while keeping your system secure.

In this guide, I’ll walk you through each step to set up these permissions effectively. We’ll cover how to check, set, and, if necessary, remove these permissions, alongside some best practices to keep your system safe. So, let’s get started on enhancing your Linux system’s capabilities safely and efficiently.

What are Special Permissions in Linux?

Special permissions in Linux are additional settings that enhance security and functionality for files and directories. These special permissions help manage more precise security controls on files and directories, ensuring that only authorized users can perform specific actions. There are three main types:

1. Setuid (Set User ID): This permission lets a user run an executable file with the privileges of the file’s owner, not the user who is running it. For example, if a file owned by the root user has the setuid permission, anyone who runs that file will temporarily have root-level access during that time. This is useful for programs that need to perform tasks that require higher privileges than the user normally has.
2. Setgid (Set Group ID): Setgid is similar to setuid but applies to group permissions. When setgid is applied to a file, anyone who runs that file will inherit the group’s permissions of the file. If setgid is applied to a directory, any new files created within that directory will inherit the directory’s group rather than the group of the user who created the file. This ensures that all files in the directory are accessible to all users sharing the same group.
3. Sticky Bit: The sticky bit is used mainly on directories to control who can delete or rename files within that directory. When the sticky bit is set, only the file’s owner, the directory’s owner, or the root user can modify the files. This is important in public directories like /tmp, where many users can create files but shouldn’t be able to delete or modify others’ files. This helps prevent accidental or malicious deletions or alterations of files.

For giving root permissions to users in Linux, explore this step-by-step guide.

How to Set Special Permission in Linux?

Below, I’ll guide you through how to configure these special permissions step by step:

1. Setting SUID (Set User ID)

The Set User ID (SUID) permission is a special file permission in Linux that allows users to execute a file with the permissions of the file’s owner. This is particularly useful for performing tasks that typically require administrative privileges, such as changing their own passwords. Follow these steps to set these permissions:

1. Open your Terminal window.

2. To check the current permissions of a file, use the command:

ls -l filename

This will display the file’s permissions, allowing you to see if SUID is already set.

3. To enable SUID on a file, add a 4 to the beginning of the three-digit permission code using the chmod command:

chmod 4755 filename

In this command, 4 activates the SUID, 7 grants full permissions to the file owner, and 5 grants read and execute permissions to the group and others.

4. To confirm that SUID has been set, use the following command again:

ls -l

If SUID is properly set, an s will appear in the user’s permission field instead of the usual execute x.

5. If you need to remove SUID from a file, reset the permission without the leading 4.

chmod 0755 filename

This command removes the SUID bit and sets the standard permissions for owner, group, and others.

2. Setting SGID (Set Group ID)

Similar to SUID, Set Group ID (SGID) is a special type of permission used in Unix-like operating systems. It allows a user to execute a file with the permissions of the group that owns the file. This is useful for collaborative environments where users need to share the execution privileges. Here is how to set these permissions:

1. In the command window, to set the SGID bit, execute the following command:

chmod 2755 filename

Here, 2 activates the SGID bit, and the rest of the code (755) sets standard permissions (read, write, and execute for the owner, read and execute for the group and others).

2. After setting SGID, check that it’s applied correctly by running the following command:

ls -l

A correctly set SGID will display as an s in the group permissions field.

3. To remove SGID, modify the permissions by omitting the 2.

chmod 0755 filename

This command resets the permissions, removing the SGID bit.

3. Setting the Sticky Bit

The Sticky Bit is a permission setting used on directories to restrict file deletion within the directory. When set, it allows only the file owner, the directory owner, or the root user to delete files within the directory, regardless of other permissions. Here is the step-by-step guide to setting permissions:

1. Access your command prompt and use the chmod command with a 1 at the start of the permission digits to set the Sticky Bit:

chmod 1755 directoryname

This configuration sets the Sticky Bit (1), with the owner having full permissions and the group and others having read and execute permissions.

2. To ensure the Sticky Bit is set, use the following command on the directory:

ls -ld directoryname

If set correctly, a t will appear at the end of the permission field.

3. To remove the Sticky Bit, change the permissions and exclude the 1.

chmod 0755 directoryname

This removes the Sticky Bit and applies standard permissions to the directory.

Security Best Practices with Special Permissions

Managing special permissions in Linux, such as SUID, SGID, and the Sticky Bit, is crucial for maintaining system security. Here are six enhanced best practices to help you handle these permissions responsibly and effectively.

• 🔒 Limit Use of SUID and SGID: Apply SUID and SGID permissions only to programs that absolutely need them. Widely using these permissions can open up security holes, as they allow programs to run with elevated privileges. Be carefull and only assign these permissions to well-trusted and regularly maintained software.
• 🔎 Regular Audits: Frequently audit your system to identify and review any files with SUID or SGID permissions. Use commands like find / -perm /4000 to list all SUID files and find / -perm /2000 for SGID files. Regular checks help ensure that no unnecessary permissions are set, reducing potential security risks.
• 🚨 Monitor Changes: Implement monitoring tools like auditd, which can track and log changes made to file permissions. Setting up alerts for modifications to SUID, SGID, or Sticky Bit permissions can help you respond quickly to unauthorized changes, preventing potential exploits.
• 🛡️ Restrict Access: Limit access to files with special permissions to users who really need it. Use group policies and access control lists (ACLs) to fine-tune who can interact with sensitive files. Restricting their availability minimizes the chance of these powerful permissions being misused.
• 📚 Educate Users: Provide training for all users and administrators about the dangers and proper handling of special permissions. When people understand what SUID, SGID, and Sticky Bit entail and the security implications of misusing them, they are more likely to adhere to best practices.
• 🔄 Update Regularly: Always keep your Linux system and its applications up to date. Developers frequently release security updates that fix vulnerabilities, including those related to special permissions. Regular updates ensure that your system is protected against known threats and reduce the risks associated with outdated software.

Linux Special Permissions: In a Nutshell

In this article, I showed you how to set special file permissions in Linux, like Setuid, Setgid, and the Sticky Bit. You learned how to apply and remove these permissions using the chmod command, with best practices such as limiting their use, conducting regular audits, and keeping systems updated to maintain a secure environment.

To keep building your Linux skills, I recommend:

• Exploring how to compare two files in Linux to understand and manage file differences effectively.
• Learning about file encryption techniques in Linux to enhance the security of sensitive data.
• Checking out methods to view file timestamps in Linux to track and manage file modifications efficiently.

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