1_Set-UID_Mechanism
Linux password are stored in /etc/shadow file which is writable by root but not only by normal user.
How can we manage the password change?
We opt for a fine-grained access control: Unix provides a privileged program passwd that can only modify the /etc/shadow file.
There are 2 types of privileged program:
From the performance prospective, the Set-UID approach may be better, since it does not require any running background process. However, Set-UID approach has a much broader attack surface than the service approach. As we will see, this is due to environment variables.
The main issue is that in the Set-UID approach the env vars cannot be trusted since everyone can manipulate the env vars.
On the other hand, since service is started by a privileged parent process or OS, it can trust the env vars since they come from a trusted entity.
Although, still exists some attack surface on service approach to.
With the Set-UID bit, we can grant "superpower" to a given program. However, the behavior of such process are restricted so that it can perform only a well-defined and specific task.
