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Unix System Security

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Unix System Security

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Unix system security l.jpg

Unix System Security

UNIX system security can be divided into three main areas of concern. Two of these areas, account security and network security, are primarily concerned with keeping unauthorized users from gaining access to the system. The third area, file system security, is concerned with preventing unauthorized access, either by legitimate users or crackers, to the data stored in the system.

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Physical Security

Often the subject of internal security is overlooked. However, often it is fairly easy for someone to get access to systems they are not supposed to have access by simply walking up to a valid users desk. This can be the cleaning staff or a disgruntled (ex)employee making a visit. This is the easiest type of security to implement and should definitely be included in any security plan.

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  • Console security

  • Machines and consoles need to be secure. A person can simply turn off a computer if one has access to it. If they have access to the console, they can often interrupt the boot process to get access to the root prompt. If this doesn't work, they can keep guessing the root password in hopes of compromising the system. For these reasons (and more), the computers and associated consoles should be kept in a secure room. A limited number of people should have access to this room, of course with a limited number of keys. Some places actually have security guards let people into the computer rooms for guaranteed secure access.

  • If your data is sensitive, be certain to verify that there are no alternative methods for getting into the room. This includes hidden spare keys in an unsecured place, gaps in the raised floors that go past the locked access point, and space above the ceilings.

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Locking Program for Console

trap “” 1 2 3

banner terminal

banner locked

read key

while true

do

echo “Enter your password: \c”

stty -echo

read pw

stty sane

if [ “$pw” = icisa ]

then

break

else

echo “Wrong password. You are an Unauthorised user.”

fi

done

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  • Data Security

  • Companies that value their data need a detailed backup recovery scheme. This includes on site backups for least amount of down time, a copy of this data off site in case of computer room disasters, as well as contingency plans in place. Unfortunately, an easy way to get access to a companies data is to gain access to backup tapes and sensitive printouts. Hence, all sensitive information should be stored in locked cabinets. Backup tapes sent off site should be in locked containers. Old sensitive printouts and tapes should be destroyed.

  • To protect against computer damage from power outages, be certain to have your computers on a UPS. This provides consistent power, protects against outages, as well as protects the computer from power spikes. Ideally, there should be a backup generator for production systems. For non-production systems, there should be a automatic way to shutdown the computer if the power has switched to the UPS for more than 1/2 the time the UPS is rated to supply.

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Unix Network Security

  • Once you put a computer on a network, you allow many more people potential access to the machine. Without networks, often a machine is not useful. The key to network security is to allow only those functions that the users actually need. Make those services as secure as possible. By disabling non-used functions, you have much less monitoring/securing.

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  • Telnet Security

  • Convince your users to use SSH (secure shell). SSH provides encrypted traffic to prevent snooping. If you MUST use telnet, at least close down which IPs you accept telnet from and turn off root login.

  • FTP Security

  • As with other services, if you do not need this functionality, turn it off. You can turn off incoming FTP or simply certain users. If you need full FTP functionality, be certain to enable logging and monitor syslog. If possible, use secure ftp (comes with ssh). Standard FTP is known to be a security hazard as it sends passwords in clear text.

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  • Since you only want valid users using FTP, make sure you have /etc/ftpusers include all system accounts (uucp, bin, daemon, sys, adm, lp, root, ...). If there are other users who do not need FTP, also place them in this file. Only allow the users that actually need them. Give them the least amount of access possible. Do not allow writable directories unless absolutely necessary. If writable directories are needed, sometimes write only directories can be utilized.

  • Modem Security

  • Having modems hooked up to one central point makes security easier. All modems should have additional dial-up password for additional security. To do this, setup /etc/d_passwd (see d_passwd man page). Once you are finished, verify that passwords are not guessable by using CRACK. As usual, use one password per user. Be certain to disable the account when the user no longer needs access. All dial-up modems should log users out upon disconnect (hupcl in /etc/gettydefs)

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Unix Account Security

  • If your accounts are not secure, then your other steps won't help much. There is general password security as well as special steps to take for each type of account.

    • Password Security

    • You want to make sure all accounts have a non-guessable password. To ensure that the passwords are not guessable, use crack on a regular basis. In addition, be certain that passwords are changed from time to time. Ideally, use one time passwords such as skey. Accounts should be disabled when there are several bad logins in a row.

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  • Be certain that passwords are not written down. Often people will use their license plate numbers or children's names. Unfortunately, these are easy to guess passwords. Also, they will use passwords from their favorite hobby. Have your password dictionary include checking these passwords.

  • Having no .netrc files strengthens security.

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Passwords

  • The password is the most vital part of UNIX account security. If a cracker can discover a user's password, he can then log in to the system and operate with all the capabilities of that user. If the password obtained is that of the super-user, the problem is more serious: the cracker will have read and write access to every file on the system. For this reason, choosing secure passwords is extremely important. Unix passwords should be longer (i.e. more than 6 characters at least) However, if the user ``insists'' that a shorter password be used (by entering it three times), the program will allow it. No checks for obviously insecure passwords (see below) are performed. Thus, it is incumbent upon the system administrator to ensure that the passwords in use on the system are secure.

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Selecting Passwords

  • Be certain that passwords are not written down. Often people will use their license plate numbers or children's names. Unfortunately, these are easy to guess passwords. Also, they will use passwords from their favorite hobby. Have your password dictionary include checking these passwords.

  • Having no .netrc files strengthens security. The object when choosing a password is to make it as difficult as possible for a cracker to make educated guesses about what you've chosen. This leaves him no alternative but a brute-force search, trying every possible combination of letters, numbers, and punctuation. A search of this sort, even conducted on a machine that could try one million passwords per second (most machines can try less than one hundred per second), would require, on the average, over one hundred years to complete. With this as our goal, and by using the information in the preceding text, a set of guidelines for password selection can be constructed:

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  • Don't use your login name in any form (as-is, reversed, capitalized, doubled, etc.).

  • Don't use your first or last name in any form.

  • Don't use your spouse's or child's name.

  • Don't use other information easily obtained about you. This includes license plate numbers, telephone numbers, social security numbers, the brand of your automobile, the name of the street you live on, etc.

  • Don't use a password of all digits, or all the same letter. This significantly decreases the search time for a cracker.

  • Don't use a word contained in (English or foreign language) dictionaries, spelling lists, or other lists of words.

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  • Don't use a password shorter than six characters.

  • Use a password that is easy to remember, so you don't have to write it down.

  • Use a password that you can type quickly, without having to look at the keyboard. This makes it harder for someone to steal your password by watching over your shoulder.

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Password Policies

  • Although asking users to select secure passwords will help improve security, by itself it is not enough. It is also important to form a set of password policies that all users must obey, in order to keep the passwords secure. First and foremost, it is important to impress on users the need to keep their passwords in their minds only. Pass- words should never be written down on desk blotters, calendars, and the like. Further, storing passwords in files on the computer must be prohibited. In either case, by writing the pass- word down on a piece of paper or storing it in a file, the security of the user's account is totally dependent on the security of the paper or file, which is usually less than the security offered by the password encryption software.

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  • A second important policy is that users must never give out their passwords to others. Many times, a user feels that it is easier to give someone else his password in order to copy a file, rather than to set up the permissions on the file so that it can be copied. Unfortunately, by giving out the password to another person, the user is placing his trust in this other person not to distribute the password further, write it down, and so on. Finally, it is important to establish a policy that users must change their passwords from time to time, say twice a year. This is difficult to enforce on UNIX, since in most implementations, a password-expiration scheme is not available. However, there are ways to implement this policy, either by using third-party software or by sending a memo to the users requesting that they change their passwords. This set of policies should be printed and distributed to all current users of the system. It should also be given to all new users when they receive their accounts. The policy usually carries more weight if you can get it signed by the most ``impressive'' person in your organization (e.g., the president of the company).

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Checking Password Security

  • The procedures and policies described in the previous sections, when properly implemented, will greatly reduce the chances of a cracker breaking into your system via a stolen account. However, as with all security measures, you as the system administrator must periodically check to be sure that the policies and procedures are being adhered to. One of the unfortunate truisms of password security is that, ``left to their own ways, some people will still use cute doggie names as passwords'' . The best way to check the security of the passwords on your system is to use a password-cracking program much like a real cracker would use. If you succeed in cracking any passwords, those passwords should be changed immediately.

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Accounts Without Passwords

  • Some sites have installed accounts with names such as ``who,'' ``date,'' ``lpq,'' and so on that execute simple commands. These accounts are intended to allow users to execute these commands without having to log in to the machine. Typically these accounts have no password associated with them, and can thus be used by anyone. Many of the accounts are given a user id of zero, so that they execute with super-user permissions. The problem with these accounts is that they open potential security holes. By not having passwords on them, and by having super-user permissions, these accounts practically invite crackers to try to penetrate them. Usually, if the cracker can gain access to the system, penetrating these accounts is simple, because each account executes a different command. If the cracker can replace any one of these commands with one of his own, he can then use the unprotected account to execute his program with super-user permissions. Simply put, accounts without passwords should not be allowed on any UNIX system.

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Root Accounts

  • It is easiest to track changes and security violations when very few people who have root access, The root password needs to be a strong non-guessable password. In addition, change the root password every 3 months & whenever someone leaves company. Always logout of root shells; never leave root shells unattended.

  • The only place where root should be able to log onto directly should be the console (as specified in /etc/security). Only root should have UID 0.

  • Check root dot files for security holes. Aliases should have full pathnames. Root should NEVER have "." in path. The root dot files should ONLY have 700 permissions. The minimal umask for root is 022 (rwxr-xr-x). It is better to have a umask of 077 (rwx------) but often this isn't practical.

  • To avoid trojan horse programs, always use full pathnames. Also, never allow non-root write access to ANY directories in root's path. If possible, do not create root's tmp files in publicly writable directories.

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Guest Accounts

  • As with any account, only create guest accounts for the time it s needed. Remove the account when its purpose is completed. Use non-standard account names for guest accounts. Do not use "guest". Instead use account names such as: "fixomni" or "oratmp".

  • Guest accounts should have a strong password and a restricted shell. If reasonable, give guest accounts a strong umask such as 077.

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User Accounts

  • User accounts should not be shared. Remove user accounts upon termination. Disable login for well known accounts that do not need direct login access (bin,daemon,sys,uucp,lp,adm).

  • User accounts should have a strong password and in some cases, a restricted shell. If reasonable, give guest accounts a strong umask such as 077.

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File System Security

  • File system security is about making sure your users can only do what you want them to be able to do. This means that you want system programs to be secure and users to only be able to write where you want them to be able to do so.

    • NFS Security

    • Only run NFS as needed, apply latest patches. When creating your /etc/exports file, be certain to use limited access flags when possible such as read only or no suid. By using fully qualified hostnames, you are guaranteed that only the host you want to be able to access the file system can access it.

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  • Device Security

  • Device files /dev/null, /dev/tty & /dev/console should be world writeable but NEVER executable. Most other device files should be unreadable and unwriteable by regular users.

  • Script Security

  • Never write setuid/setgid shell scripts (can break out). Instead, write a compiled program in a language like "C". Scripts should ALWAYS have full pathnames.

  • Program Security

  • Always get your programs from a known source. Verify that it hasn't been hampered with via checksum. If you are compiling your own program, make sure you know that the compiler hasn't been tampered with as well.

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  • General Security Measures

  • Create minimal writable filesystems (esp. system files/directories!). Generally, users should only be able to write in their own directories, and /tmp. In addition, there will be directories for a specific group to write in. This way you control how each user can access specific areas of the system.

  • Make sure that important files are only accessible by authorized personnel. Use setuid/setgid only where necessary.

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Unix Security Testing

  • Security is an ongoing process. However, once the software is installed and initial checks completed, it can be a simple maintenance task of installing latest patches, verifying user accounts, running security software (and their extensions) and monitoring logs.

    • Always have latest security OS patches installed

    • Subscribe to security mailing lists. Read security newsgroups. Keep up to date on the latest security procedures.

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  • Test your system

  • Test w/ SATAN (network security), COPS (Various system checks), TIGER (ways for root to be compromised), and CRACK (password checker). Install Tripwire to detects changes to files).

  • Monitor log files

  • Check btmp, wtmp, syslog, sulog etc. regularly. Set up automatic email or paging to warn system administrators of any suspicious behavior. Also check your snort logs.

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Thankyou

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Presentation prepared byMr. Dharampal,Data Processor &Mr.Srinivas sastry,Section Officero/o the Accountant General(A&E)I,M.P.

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