👉 Overview
👀 What ?
EUID (Effective User ID), RUID (Real User ID), and SUID (Set User ID) are integral parts of Unix and Linux security models, which manage user permissions and privileges.
🧐 Why ?
Understanding EUID, RUID, and SUID is important as they are fundamental to Unix and Linux system security. They govern what actions a user or process can perform on the system, such as file access and system command execution. Without a proper understanding of these concepts, system administrators may inadvertently grant excessive permissions to users or processes, leading to potential security risks.
⛏️ How ?
EUID, RUID, and SUID are used in the following way: when a user logs into a Unix or Linux system, they are assigned a RUID and EUID that match their user ID in the system's user account database. The SUID is a special permission bit that can be set on executable files, allowing users to run the file with the permissions of the file's owner. For instance, if a file has the SUID bit set and is owned by the root user, any user who executes this file will have the same privileges as the root user for that execution.
⏳ When ?
The concepts of EUID, RUID, and SUID were introduced with the Unix operating system in the 1970s and continue to be used in Unix-like systems, including Linux and MacOS, today.
⚙️ Technical Explanations
In Unix-like operating systems, each process has multiple user IDs associated with it that are crucial for controlling the process's access to system resources. These user IDs include the Real User ID (RUID), Effective User ID (EUID), and Set User ID (SUID).
The RUID is the ID of the user who initiated the process. It's the actual ID of the user who is interacting with the system, and it doesn't change during the process's life. It's important because it's used by the system to track who started the process and to apply any user-specific settings or restrictions.
The EUID, on the other hand, is the ID that the system uses when determining what resources the process can access. The EUID can be different from the RUID because, in some cases, users might need to run processes with more privileges than their standard user account allows. For instance, certain system commands or applications need to be executed with root privileges, even when initiated by non-root users. This is where the EUID comes into play.
The SUID is a special permission bit that can be set on executable files. If a file has the SUID bit set, when the file is executed, the EUID of the process is set to the owner of the file, rather than the user who initiated the process. This mechanism allows users to execute certain tasks that would otherwise require higher-level permissions. For example, changing a user's password requires root access to the system's password file. However, it wouldn't be secure to give every user root permissions, so the password change program has the SUID bit set and is owned by the root user. This way, when a user executes the password change program, it runs with the root's EUID, allowing the user to change their password without needing root access to the entire system.
It's important to note that while the SUID can provide necessary elevated permissions, it needs to be managed carefully. If misused, it can lead to significant security vulnerabilities, as it could potentially provide root access to malicious users or software.
Let's take a detailed look at EUID, RUID, and SUID with an example related to the passwd command in Unix:
- Real User ID (RUID): Suppose a user named 'Alice' logs into a Unix system. The system assigns Alice a unique numeric identifier, her RUID. Let's say this is '1001'. This ID signifies who initiated the process.
- Effective User ID (EUID): Alice decides to change her password. She uses the
passwdcommand, a program that requires root privileges as it needs to write to the system's password file. Thepasswdcommand has its SUID bit set and is owned by the root user. This means when Alice runspasswd, the EUID for this execution is '0' (the root's user ID), not '1001' (Alice's RUID).
Here's an illustration using command line:
alice$ id
uid=1001(alice) gid=1001(alice) groups=1001(alice)
alice$ ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 47032 May 16 2019 /usr/bin/passwd
The 's' in the permissions field (rwsr-xr-x) indicates the SUID bit is set for passwd. When Alice runs passwd, the process will run with the EUID of the file's owner (root).
- Set User ID (SUID): This is the mechanism that allows Alice to change her password without having root access to the entire system. It's a powerful feature, but if misused, it can lead to security vulnerabilities. So, it's crucial to ensure only trusted programs owned by the root user have the SUID bit set.
Remember, only a few system programs like passwd require the SUID bit because they need to perform specific tasks that should bypass usual permission checks. For most other tasks, the EUID and RUID remain the same, and processes run with the privileges of the initiating user.