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Linux.com :: Securing a fresh Linux install, part 2 http://www.linux.com/archive/feature/113

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Feature

Securing a fresh Linux install, part 2

By Mike Peters on April 21, 2004 (8:00:00 AM)

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In the first article in this series we began looking at ways to secure a newLinux server, starting with locking down services. Next, let's look at securingfiles and monitoring system logs.SUID and SGID files

SUID and SGID files are executables which, when run by a normal user, mayhave access to resources not normally available to the user running theprogram. For example, an SUID program could have the permissions:

-r-sr-xr-x 1 root root 11267 Jan 21 00:28 /usr/sbin/foo

The s in the owner's permission field in place of the usual x indicates that

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/usr/sbin/foo is SUID. If run by a normal user, the executable will run with theprivileges of the owner of the file, in this case root. In this case the programwill have access to the same system resources as root.

Below is an example of an SGID file:

-r-xr-sr-x 1 root foo 11267 Jan 21 00:28 /usr/sbin/foo

Here there is an s in the place of the group's executable bit, meaning the file isSGID and will be executed with the group permissions of the foo group.

SGID and SUID programs may be used by a cracker to gain elevatedpermissions on a system, so you should keep track of such files. You can findSUID and SGID files using find:

# find / -perm -4000 -o -perm -2000 -exec ls -ldb {} \; >> SUID_files.txt

This command finds all SUID or SGID files and lists them in a file calledSUID_files.txt. You can unset SUID or SGID privileges with the command chmod-s /usr/sbin/foo, but be warned, unsetting the SUID or SGID bit on someprograms may mean that they will no longer run. Periodically check for newfiles.

There should be no reason for users to have SUID files in their homedirectories so you should use the nosuid option in /etc/fstab for the partitioncontaining users $HOME directories. For example:

/dev/hda3 /home ext3 defaults,nosuid 1 1

World readable/writable files

Files should be world readable or writable only for very good reasons. Youshould check for such files the way we did above for SGID and SUID files:

# find / $ -perm -a+r -o -perm -a+w $ ! -type l >> world_readwrite.txt

Again, check through the list and remove permissions from files that do notneed to be world readable or writable, and run checks regularly for new worldreadable or writable files.

Files with no owner or group

Ownerless files can be an indication that someone has gained access to yoursystem. You should check regularly using the command # find / -nouser -o -nogroup.If you find any ownerless files, either delete them, or, if you know what theyare and wish to keep them, assign them to an appropriate user and group. For


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example, assign myfile to the user foo and the group bar you would issue thecommand # chown foo.bar myfile

Using umask

The umask command can be used to determine the permissions given to newlycreated files on your system. Addng the line umask 022 to the /etc/profile file tellsthe system that any files created by users should have the permissions 0644,or -rw-r--r--. This means users must explicitly make a file executable by usingchmod in their $HOME directory.

The immutable and append-only bits

With chattr, root can set files to be read-only or append-only. Setting theimmutable bit (making a file read-only) ensures that a file cannot be altered,even by root (of course root can remove the immutable bit and then alter thefile, so it's not watertight). Setting the append-only bit ensures that the

existing contents of a file cannot be changed, only added to. It is a good idea toset the append-only bit on log files: # chattr +a /var/log/messages.

You can set the immutable bit to make it more difficult to replace importantexecutables or change critical configuration files:

# chattr +i /usr/bin/ps

# chattr +i /etc/services

The attributes of files set by chattr can be displayed using the lsattr utility.

System logs

In order to trace any unwanted activity on your computer, you should keepcomplete and accurate logs. On Linux machines, logging is handled by thesyslog daemon, syslogd. Syslogd reads its configuration from the/etc/syslog.conf file. You can set the facilities to be logged, the log priority, andthe files in which to log information here. The default values in mostdistributions do not give you enough information.

A sensible log policy is to log almost everything in /var/log/messages and/var/log/syslog and then to have each individual facility log to its own separate

file, as shown in the example below:

--- Begin Example syslog.conf-----

# Log anything 'info' or higher, but lower than 'warn'.

# Exclude mail. This is logged elsewhere.

*.info;*.!warn;mail.none -/var/log/messages

# Log anything 'warn' or higher.


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# Exclude mail. This is logged elsewhere.

*.warn; -/var/log/syslog

# Debugging information is logged here.

*.=debug -/var/log/debug

# Kernel related logs:

kern.* -/var/log/kernel

# Private authentication message logging:

authpriv.* -/var/log/secure

# Cron related logs:

cron.* -/var/log/cron

# Mail related logs:

mail.* -/var/log/maillog

# Daemon related logs:

daemon.* -/var/log/daemonlog

# User related logs:

user.* -/var/log/userlog

# Mark logs:

mark.* -/var/log/marklog

# Emergency level messages go to all users consoles:

*.emerg *

--- End Example syslog.conf-----

Note the dash before the log files' names. This tells syslogd not to sync after

every log. The disadvantage of this is that log information may be lost in theevent of a system crash. Removing the dash, however, can cause a performanceloss with heavy logging.

If you want to be able to track logs in real time, you can open a log file usingthe command tail -f /var/log/messages. Alternatively you can have a permanent logconsole by adding the line

*.* /dev/tty8

to the end of your syslog.conf file. This displays logs in real time on /dev/tty8.(Be sure that tty8 exists, of course!)

In order to keep accurate logs, ensure that your system clock is accurate at alltimes. You should look to using Network Time Protocol (NTP) to maintain yoursystem clock's accuracy. The easiest way to do this is to regularly run ntpdatesome.time.server from a cron job.


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Finally, although it's not really related to your system logs, make sure youredirect root's mail to your normal user account so you don't miss anyimportant warning mail messages sent to root. You should do this either byplacing the line:

root: [email protected]

in /etc/aliases and running the command newaliases, or, alternatively, create afile named .forward in /root containing the address you want mail to beforwarded to.

In the third and final part of this series we'll look at security considerations forsome important networking tools.

Mike Peters is a freelance consultant and programmer and long-time Linux

user.

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Last 5 articles by this author:

Securing Apache Jul 15, 2004Encrypting partitions using dm-crypt and the 2.6 series kernel Jun 08,2004Chrooting Apache May 27, 2004Securing a fresh Linux install, part 3 Apr 22, 2004Securing a fresh Linux install, part 2 Apr 21, 2004

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#

Corrections?

Posted by: Administrator on April 22, 2004 09:31 AMI have two comments about this article:

(1) World readable files are very common on Linux. It's world writable filesthat should be rare. It is true if you want files private, like your home directory


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for example it should not be world readable -- and there are certain filesin /etc and elsewhere that should not be world readablebut ths is not a blanket statement.

(2) A umask of 022 seems just wrong to me. This is no security at all prettymuch. In fact it contradicts what you said a few paragraphs earlier about notwanting world readable files. If you want security use 007, 027, or 077depending on your needs, how you setup shares, and your group structure.

These are two basic mistakes. It makes me wonder how much the author reallyknows about security. I think your articiles are about an important topic, butplease present the topic with a little more care.

Rob

#

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