Question

Unable to bypass sudo Password prompt on SSH

Posted on April 5, 2017
Security Ubuntu 16.04 Networking DigitalOcean
Asked by Nitin Prabhakar

I have deployed my public key on to authorized keys.

All was fine till a couple of hours ago, when Logging in via SSH with RSA PK Auth

started prompting for a sudo password.

I checked Auth.log , and it says

Public Key Accepted.

One weird thing I noticed was, There were Millions of entries that logged

Opened a session for root

Immediately after It said

Public key accepted

Probably because it prompted me for a sudo password?

There are also millions of entries logging

Maximum login attempts reached for root @ port 472 from an IP I dont recognise

which were all blocked thanks to the firewall.

Also weird is, Once I do login,

When I run

ps -aux | grep ssh

I get a long list of root logins on the SSH process

I kill them, and they create a new one.

Why?

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Nitin Prabhakar • April 5, 2017

Accepted Answer

I Found the culprit!

It was a process called

hugepages

and it was doing

dev-hugepages.mount

on startup.

systemctl list-unit-files | grep hu

Gave it out

dev-hugepages.mount | static
sigpwr-container-shutdown.service | static

This, was running a shell script , to disable thp (Transparent huge page) on startup.

Like so:

sudo hugeadm --thp-never

This was one of the tweaks to memalloc for the Redis server on the VPS

** Solution : **

remove the shell script it ran from under \etc\init.d

reboot droplet

@jtittle

thanks for pointing me to this :

That being said, it’s hard to tell what’s going on. It looks like something is being executed after login (authentication) and it’s attempting to run something that would require root or sudo, thus you’re being prompted to authenticate before whatever command is running tries to execute.

Jonathan Tittle • April 5, 2017

@SchrodingersCat

If you’re logged in as root and using sudo, i.e.

sudo [command]

… then you shouldn’t be prompted for a password as you’re already running as root, thus there’s no higher escalation.

If you’re logged in a as a non-root user and prefixing commands with sudo, then you’ll be asked to confirm that users password before the command executes – that’s by design. Without prompting for the users password, you’re effectively running as root and that defeats the purpose of a sudo user.

…

Public Key authentication has nothing to do with anything other than initial authentication between your computer and the server (or between two servers). If you’re being prompted at initial login, that’s because you’ve most likely put a passphrase on your key (which is ideal), but that’s as far as that goes.

…

As for the failed logins, seeing failures is normal as the IP of your Droplet is public and most likely belonged to a user before you had it. It’s common to see attempts to break in, and what this boils down to is security and properly securing your server.

Ideally, you should:

1). Create a sudo user; 2). Setup SSH keys for that user (with a passphrase on the key); 3). Set a password for the sudo user that differs from your passphrass; 4). Confirm that you’re able to login as the sudo user using the key. 5). Confirm that you’re able to run sudo [command] as the user, and it works; 6). Lock the root account so that it can’t be used to login.

Jonathan Tittle • April 5, 2017

@SchrodingersCat

When you run:

ssh user@hostname

… it’s not asking for a sudo password, it’s asking for the password associated with your private key. When you created your public/private key pair, you must have set a passphrase on it (which is good), so you’d need to enter in that passphrase to login.

The above applies if you’re using PuTTy on Windows or similar. On a Mac, you’d specify the path to your key using -i, i.e.

ssh user@hostname -i /path/to/privatekey

If it’s not asking for a key file, then it’s asking for the password of the user. If the user doesn’t have a password, then you won’t be able to login as that user and would need to login as root to change the users password using: