Databases, Systems & Networks

As someone says, security is a not a product, but a process. While SSH protocol itself is cryptographically secure by design, someone can wreak havoc on your SSH service if it is not administered properly, be it weak passwords, compromised keys or outdated SSH client.

As far as SSH authentication is concerned, public key authentication is in general considered more secure than password authentication. However, key authentication is actually not desirable or even less secure if you are logging in from a public or shared computer, where things like stealth keylogger or memory scraper can always a possibility. If you cannot trust the local computer, it is better to use something else. This is when « one-time passwords » come in handy. As the name implies, each one-time password is for single-use only. Such disposable passwords can be safely used in untrusted environments as they cannot be re-used even when they are stolen.

One way to generate disposable passwords is via Google Authenticator. In this tutorial, I am going to demonstrate another way to create one-time passwords for SSH login: OTPW, a one-time password login package. Unlike Google Authenticator, you do not rely on any third party for one-time password generation and verification.

What is OTPW?

OTPW consists of one-time password generator and PAM-integrated verification routines. In OTPW, one-time passwords are generated apriori with the generator, and carried by a user securely (e.g., printed in a paper sheet). Cryptographic hash of the generated passwords are then stored in the SSH server host. When a user logs in with a one-time password, OTPW’s PAM module verifies the password, and invalidates it to prevent re-use.

Step One: Install and Configure OTPW on Linux

Debian, Ubuntu or Linux Mint:

Install OTPW packages with apt–get.

$ sudo apt-get install libpam-otpw otpw-bin

Open a PAM configuration file for SSH (/etc/pam.d/sshd) with a text editor, and comment out the following line (to disable password authentication).

#@include common-auth

and add the following two lines (to enable one-time password authentication):

auth       required     pam_otpw.so
session    optional     pam_otpw.so

Fedora or CentOS/RHEL:

OTPW is not available as a prebuilt package on Red Hat based systems. So let’s install OTPW by building it from the source.

First, install prerequites:

$ sudo yum git gcc pam-devel
$ git clone https://www.cl.cam.ac.uk/~mgk25/git/otpw
$ cd otpw

Open Makefile with a text editor, and edit a line that starts with « PAMLIB= » as follows.

On 64-bit system:

PAMLIB=/usr/lib64/security

On 32-bit system:

PAMLIB=/usr/lib/security

Compile and install it. Note that installation will automatically restart an SSH server. So be ready to be disconnected if you are on an SSH connection.

$ make
$ sudo make install

Now you need to update SELinux policy since /usr/sbin/sshd tries to write to user’s home directory, which is not allowed by default SELinux policy. The following commands will do. If you are not using SELinux, skip this step.

$ sudo grep sshd /var/log/audit/audit.log | audit2allow -M mypol
$ sudo semodule -i mypol.pp

Next, open a PAM configuration file for SSH (/etc/pam.d/sshd) with a text editor, and comment out the following line (to disable password authentication).

#auth       substack     password-auth

and add the following two lines (to enable one-time password authentication):

auth       required     pam_otpw.so
session    optional     pam_otpw.so

Lire la suite…

Using the same password on different servers allows attackers to access your accounts if cracker manage to steal your password from a less secure server. This is true for online website accounts too. So solution is to create unique passwords for server accounts like your email, sftp and ssh accounts. General guideline to create a strong and unique password is as follows:

Creating a strong and unique password for Linux or Unix-like systems

1. Create a password with mix of numbers, special symbols, and alphabets.
2. Make sure your password is hard to guess. You can use tool such as makepasswd to create hard to guess password.
3. Do not use simple words like « password« , « 123456« , « 123abc » or « qwerty« .
4. Use a unique password for all your server accounts.
5. A minimum password length of 12 to 14 characters should be used. See how to configure CentOS / RHEL / Fedora Linux based server password quality requirements.
6. Generating passwords randomly where feasible. You can do this with a simple shell scriptfunction.
7. If possible use two-factor authentication.
8. Use pam_crack to ensure strong passwords and to check passwords against a dictionary attack.

But, how do you test the effectiveness of a password in resisting guessing and brute-force attacks under Linux? The answer is simple use cracklib-check command.

Install cracklib on a Linux based system

Type the following yum command to install on RHEL and friends:
# yum install cracklib

Type the following apt-get command to install on Debian/Ubuntu and friends:
# apt-get install libcrack2

Lire la suite…