feat: a3

sheet05/a3/a.txt

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+Passwords are stored in the /etc/shadow file, which is restricted to the root user. 
+A standard user cannot write to it directly. However, the passwd executable is owned by root and has the SUID permission set. 
+When a standard user runs passwd, the SUID bit tells the system to execute the program with the privileges of root, 
+giving the program the temporary permissions to update /etc/shadow

sheet05/a3/b.txt

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+The script runs with root privileges because the setuid bit is set. 
+Since it just asks for a username and saves the new hash to /etc/shadow, 
+and there is no validation checking if the user running the program is actually changing their own password, 
+someone could simply run the program, type root as the username, and set a new password for the root user. 
+The script would then overwrite the actual root password in /etc/shadow.