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Need a quick security fix? Whether its sudo or lilo, tcp or icmp, you'll find short and straight-to-the point guidance in our listing of Security Tips.
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Posted by Eric Lubow
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Some attackers, prior to attacking a host, (or users nmaping a host) will check to see if the host is alive. They do this by 'ping'ing the host. In order to check if the host is up, they will use an ICMP echo request packet.
To disallow these types of packets, use iptables:
iptables -A INPUT -p icmp --icmp-type echo-request -j DROP
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Posted by Eric Lubow
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World writable files can be left around by users wanting to make things easier for themselves. It is necessary to be careful about who can write to which files. To find all world writable files:
find /dir -xdev -perm +o=w ! \( -type d -perm +o=t \) ! -type l -print
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Posted by Eric Lubow
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Setuid and Setgid files have the potential to be very hazardous if they are accessible by the wrong users on the system. Therefore it is handy to be able to check with files fall into this category.
find /dir -xdev -type f -perm +ug=s -print
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Posted by Eric Lubow
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Just in case someone else who has access to the superuser account decided to alter the password file and potentially make themselves a superuser. This is a method to check:
awk -F: '$3 == 0 { print $1, "is a superuser!" }' /etc/passwd
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Posted by Eric Lubow
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Some users like to have a passwordless account. To check this you need to look at the /etc/shadow account with the following command line:
awk -F: '$2 == "" { print $1, "has no password!" }' /etc/shadow
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Posted by Eric Lubow
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The default umask (usermask) on most systems should be 022 to ensure that
files are created with the permissions 0644 (-rw-r--r--). To change the
default umask setting for a system, edit /etc/profile to ensure that you umask
is appropriate for your setup. |
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Posted by Eric Lubow
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Two SSH configuration options that can be set to improve security should be checked on your production server. UsePrivilegeSeparation is an option, when enabled will allow the OpenSSH server to run a small (necessary) amount of code as root and the of the code in a chroot jail environment. StrictModes checks to ensure that your ssh files and directories have the proper permissions and ownerships before allowing an SSH session to open up. The directives should be set in the /etc/ssh/sshd_config as follows:
UsePrivilegeSeparation yes
StrcitModes yes
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Posted by Eric Lubow
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On a production server that is in a common area (although this should not be the case, some situations are inevidable). To avoid an accidental CTRL-ALT-DEL reboot of the machine, do the following to remove the necessary lines from the /etc/inittab file:
# sed -i 's/ca::ctrlaltdel:/#ca::ctrlaltdel:/g' /etc/inittab
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Posted by Eric Lubow
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Make use of security tools out there to test your server's weaknesses. Nmap
is an excellent port scanning tool to test to see what ports you have open. On
a remote machine, type the command:
# nmap -sTU
Starting nmap 3.70 ( http://www.insecure.org/nmap/ ) at 2006-08-10 13:51 EST
Interesting ports on eric (172.16.0.1):
(The 3131 ports scanned but not shown below are in state: closed)
PORT STATE SERVICE
22/tcp open ssh
113/tcp open auth
Nmap run completed -- 1 IP address (1 host up) scanned in 221.669 seconds
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Posted by Eric Lubow
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Users who may be acting up or aren't listening can still be controlled.
Using a program called 'skill' (signal kill) which is part of the 'procps'
package.
Halt/Stop User eric: skill -STOP -u eric
Continue User eric: skill -CONT -u eric
Kill and Logout User eric: skill -KILL -u eric
Kill and Logout All Users: skill -KILL -v /dev/pts/*
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