EUID

EUID stands for Effective User ID. It is a concept primarily found in Unix-like operating systems. When a program is executed, it inherits the real user ID (RUID) of the user who launched it. However, the EUID represents the user ID that the process is currently operating under for privilege checking purposes. This distinction is crucial for security.

In standard operation, a process's EUID is the same as its RUID. This means the process has the same permissions as the user who started it. However, certain programs, known as setuid programs, can have their EUID set to a different value than their RUID. Typically, these are programs that need to perform actions that a regular user wouldn't normally have permission to do, such as changing a user's password or managing system services.

When a setuid program is executed, the operating system checks the file's permissions. If the setuid bit is set and the owner of the file is, for example, the root user, the EUID of the running process is temporarily set to root. This allows the program to perform its privileged operations. Once these operations are complete, the program should ideally reset its EUID back to the RUID to minimize the window of opportunity for security vulnerabilities. The EUID is a key mechanism for enforcing least privilege, ensuring that processes only have the permissions they absolutely need.