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linux vulnerabilities

what are the ways that a hacker can gain access to a linux system? no need to give step-by-step details on how to hack but give applications that may have loopholes and/or how to make a linux box secure. for example, the bind vulnerability.

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Well ANY service that allow a user from outside to access the system legitemately is open for atack. It's like the old saign, if the lock has a key, it can be picked...
So, the real trick is to not have any unnecesary services open, and to configure those that need to be open as secure as possible. Those services that are open, should also be kept up to date via updates from the redhat site for instance ( good example is the bind exploit which quickly had a "cure" via redhat.com and others ), I think the password is to do maintenance and checks regularly and to update as soon as a known exploit becomes available.
There are also certain services that although "legitemate" are less secure than others, telnet is one of the biggest culprits, cause when loggin in to a server the password is contained within the actual network packet, and anyone sniffing the connection will see your password. That is the main reason why telnet is rarely used on machines connected to the internet anymore, and the industry standard is now SSH.
Same goes for poorly configured ftp servers. Allowing anonymous access to your ftp server can cause you alot of problems with users being able to wander around the system. Although they cannot edit or delete files, they usually can download files that might contain valuable data for a cracker.
There are also malicious crackers looking to inflict damage to your system by attacking the services on your box in the hope of bringing your box down. And again, with all of these "expliots" the best weapon in your arsenal is regular updates and a well configured system with as little as possible services running.
common sense tips that I apply :)

1) run only the necessary applications /services
2) patch, patch and patch..
3) subscribe to security mailling list like CERT, SANS, etc
4) run good tools : Snort (IDS), Tripwire, Nessus
5) implement a free software based firewall if you need more security (IP Filter, IPfw, IP Chains)
6) use encryption wherever possible (SSH instead of telnet)
7) pick a hard password to crack


i forgot tcpwrappers tool....  :)
to control access by IP/domain and logging.
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Yep, good info from everyone.  Here's a few things that you can do to try to make a more secure box :

- start from a barebones installation (nothing but the basic packages).
- remove all clear text authentication apps that were installed by default (ftp, telnet, etc).
- remove all apps that create any listening ports (including linuxconf).
- remove all unneccessary tools that hackers might like (like ip-utils).
- install ssh if remote access is necessary
- run any necessary network apps in their own chroot environments.

- rename the "id" command, and create a script that updates your ipchains rules to block the ip address of anyone that runs it (cute trick I learned from one of the security gurus, works quite well...  What's the first thing a hacker wants to know when they get into your box, hehe :->)

+ everything that psimation and haho suggested...
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