Just like vehicle protection measures, you can stop a lot of
potential intruders but never all. Most hackers are of the "script
kiddie" variety, non-programmers using commonly available hacker
tools to penetrate systems if only for their own amusement. Since
their skills are limited, they hunt for the easy kill rather than a
challenge, so our mission here is to make your computer as
unattractive as possible. The rules outlined below are a checklist
of things you must do to properly secure your system. At the end of
this article you'll find useful links that will help you actually
implement these measures.

Rule 1: Mind your passwords. It is simply incredible that in this day
and age most computer users persist in choosing the weakest passwords,
typically because they're easy to remember. Forget convenience: Under
no circumstance should you ever use real words or combinations of
real words, nor should you use numbers that have any significance to
you such as your birth date. Script kiddies will use password guessing
tools and it might take only seconds to guess yours. Additionally,
using the same or similar password everywhere is foolish, sort of
like leaving your credit cards and house keys in your car! The best
policy is to use random sequences of characters for passwords.

Rule 2: Use encrypted connections. The Internet tools most commonly
used on workstations and servers are inherently insecure. For
instance, never use the popular Telnet, POP, or FTP programs unless
passwords are passed in a strongly encrypted format over the Internet.
Sadly, authentication is typically in plain text, which makes life
easy for hackers using packet sniffers to automatically grab scores
of plain text passwords from Internet noise while they sleep. Remote
shell sessions such as with Telnet should be completely encrypted if
you use such sessions to switch to other userids or root id, lest you
want to give script kiddies a big break! SSH is the best substitute
for Telnet and can also be used instead of FTP. By the same token,
sensitive information should NEVER be sent via email. Note that any
encryption between client and server requires that both ends support
the encryption method.

Rule 3: Hide! Cloaking your computer's Internet presence via a
firewall is the first line of defense. All hackers use scanning tools
to quickly find weaknesses in a computer's Internet interfaces.
Firewalls work at the packet level and not only can detect scan
attempts but also block them, thus making your system look
unattractive or even hide its existence altogether.

Rule 4: Don't trust anybody! Be very careful about what programs you
download from the Internet. Do not install software from little known
sites especially. Programs can hide "trojans" that once launched will
modify your system and/or connect to remote servers without your
knowledge, perhaps allowing hackers to slip into your system.
Nowadays, most download sites will include a checksum, typically PGP
or MD5 encoded, which can be used to verify that the program is
authentic before you install it. It's not too paranoid to make it a
rule to never trust anything from the Internet, as even the biggest
website could be subverted by hackers and you wouldn't notice.

Rule 5: If you're managing a server, limit what other users can do on
the system. From limiting them to certain areas of the filesystem and
which applications they can run to limiting their memory and process
usage, there are many things you can do to reduce the serious risk
posed by reckless users.

Rule 6: Keep your system up to date. Vulnerabilities are continuously
being found in popular programs and operating system kernels, some of
which might allow an intruder to gain root access to your system.
Unfortunately, word of such "holes" travels quickly in hackerdom, so
it's important to be on top of them yourself. Software creators post
"patches" and/or new versions of their programs to fix those defects.
It's hard to keep track of them all, but you can save yourself a lot
of grief by subscribing to mailing lists that warn you of such
incidents. Again, software producers often maintain such lists
themselves, as do sites specializing in computer security, and it's
usually free to subscribe. You can also reduce the risks by avoiding
using new applications or those that a long history of
vulnerabilities.

Rule 7: Disconnect from the Internet when you're not actually using
it. This is the ultimate protection to stop intrusions, but of course
it doesn't stop trojans unless you literally unplug your physical
connection. In the case of servers, where your connection is always
on, disable any daemons that aren't needed or seldom used. Note that
the daemons running by default on a newly installed operating system
are quite often the most vulnerable, and most hosting services won't
bother to disable them for you.

Rule 8: Don't continue using a computer that you know has been
compromised by hackers. To do so allows the hacker to potentially
gain access to more systems and exposes you to the possible theft of
some of your sensitive data. The only solution is to reformat the
hard disk(s) and re-install the operating system, or smash your
computer to bits if you prefer!

Rule 9: Use every security mechanism you can, not just one or a few.
They serve different purposes and thus complement each other. Always
rolling-up the car windows and locking the doors is a good start, but
you wouldn't really call that a security system, would you?!

Stick to the rules or let the script kiddies rule the day!

Some useful links you can follow to get further knowledge on the
subjects discussed today:

SSH:
http://www.ssh.com http://www.openssh.org http://www.fsecure.com

Vulnerability reports:
http://www.securityfocus.org http://cve.mitre.org

Firewalls:
http://www.zonelabs.com http://www.firewallguide.com

Other Security Sites:

http://www.net-security.org/