I know the password of gqqtest as I can login via ssh.
> psftp gqqtest@172.25.9.11 Using username "gqqtest". gqqtest@172.25.9.11's password: Remote working directory is /home/gqqtest Now I use another account to login, and once in Ubuntu, I use su to switch to gqqtest.
gqq@172.25.9.11:~$ su - gqqtest Password: su: Authentication failure gqq@172.25.9.11:~$ sudo su - gqqtest [sudo] password for gqq: gqqtest@172.25.9.11:~$ exit logout Just to verify, I tried sudo su - gqqtest and succeeded.
Why doesn't su allow me to switch to non-root even with correct password?
I searched here and there, eg. Why does su fail with "authentication error"?, they are all dealing with root or empty password.
$ type -a su; stat $(command -v su) su is /usr/bin/su su is /bin/su File: /usr/bin/su Size: 67816 Blocks: 136 IO Block: 4096 regular file Device: fd00h/64768d Inode: 4200703 Links: 1 Access: (0754/-rwxr-xr--) Uid: ( 0/ root) Gid: ( 1123/ wheel) Access: 2021-02-02 15:53:54.794314648 +0800 Modify: 2020-07-21 15:49:28.000000000 +0800 Change: 2021-02-02 15:53:02.314466458 +0800 Birth: - 1 Answer
Your su is missing the setuid bit for some reason. Without it, su doesn't have the access need to authenticate and switch to another user. I assume your system is broken in more ways if that's the case, but this particular problem can be solved using:
sudo chmod u+s /usr/bin/su 
