AHMED ZBYR

AHMED ZBYR

Biker, EV Enthusiast

Encountered a frustrating issue on a Linux server while trying to use sudo? Specifically, the error message: sudo: effective uid is not 0, is sudo installed setuid root? This post details how to diagnose and resolve this problem, which usually stems from incorrect file permissions on the sudo binary.

The Problem:

The error indicates that sudo isn’t running with the necessary privileges. sudo relies on the Set User ID (SUID) bit to temporarily elevate a user’s privileges to that of the file owner (in this case, root) when executed. If this bit is missing or the permissions are incorrect, sudo won’t function correctly.

Here’s the scenario where I initially encountered the issue, while attempting to install mysql-server:

[zahmed@ahmed-server ~]$ sudo yum install mysql-server
sudo: effective uid is not 0, is sudo installed setuid root?
[zahmed@ahmed-server ~]$ sudo su
sudo: effective uid is not 0, is sudo installed setuid root?

Diagnosis:

The first step is to examine the permissions of the /usr/bin/sudo executable. Log in as the root user (or use su -) and run the following command:

[root@ahmed-server home]# ls -l /usr/bin/sudo
---x--x--x. 1 root root 123832 Nov 22  2013 /usr/bin/sudo

Notice that the output is missing the ‘s’ in the owner permissions. This means the SUID bit is not set. The correct permissions should look like this: -rwsr-xr-x.

Solution:

The fix is to set the SUID bit and correct the execute permissions. As root, execute the following command:

[root@ahmed-server home]# chmod 4755 /usr/bin/sudo

Let’s break down chmod 4755 /usr/bin/sudo:

  • chmod: This is the command for changing file permissions.
  • 4755: This represents the desired permissions in octal format.
    • 4000: Sets the SUID bit.
    • 700: Grants read, write, and execute permissions to the owner (root).
    • 555: Grants read and execute permissions to the group and others.

Verification:

After applying the chmod command, verify the permissions again:

[root@ahmed-server home]# ls -l /usr/bin/sudo
-rwsr-xr-x. 1 root root 123832 Nov 22  2013 /usr/bin/sudo

The output should now show -rwsr-xr-x, confirming that the SUID bit is correctly set.

Now, switch back to the regular user and test sudo:

[root@ahmed-server home]# su zahmed
[zahmed@ahmed-server home]$ sudo su
[root@ahmed-server home]#

If everything is configured correctly, you should now be able to use sudo without the “effective uid is not 0” error.

Understanding Setuid and Setgid Bits

Here’s a quick overview of the different bit settings and their impacts:

  • Setuid (SUID) - chmod 4755 /path/to/file: When a program with the SUID bit set is executed, it runs with the effective user ID of the owner of the file, rather than the user executing the program. This is crucial for programs like sudo that need to perform privileged operations.
  • Setgid (SGID) - chmod 2755 /path/to/file: Similar to SUID, but for groups. When a program with the SGID bit set is executed, it runs with the effective group ID of the group owner of the file. This is often used for shared directories where all members of a group need to have the same access rights.
  • Setuid and Setgid Combined - chmod 6755 /path/to/file: Sets both the SUID and SGID bits.

Important Security Considerations:

  • Carefully consider the implications of setting the SUID or SGID bits. Improperly configured SUID/SGID binaries can create significant security vulnerabilities. Only apply these bits to programs that absolutely require them and have been thoroughly vetted.
  • Regularly audit file permissions to ensure that SUID/SGID bits haven’t been inadvertently set on sensitive files.
  • Keep your system and software up to date to patch any potential vulnerabilities that could be exploited through SUID/SGID binaries.

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