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Linux Administrator's Security Guide
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Linux Physical and Console Security

By Kurt Seifried [email protected]

While the majority of random and remote attacks come in over the network physical and console security are important factors. In a perfect world every machine would be physically secure with access to the console (i.e. keyboard, reset switch and monitor) tightly controlled. Unfortunately this is not a perfect world it is rare to find a physically secure machine outside of a server room.

Contents

Physical security

Remember that an attacker may not want to break into your desktop machine or network, they may be looking for a quick way to make $200, and stealing a desktop computer is one way to do that. All systems should be securely fastened to something with a cable system, or locked in an equipment cage if possible. Case locks should be used when possible to slow attackers down (thefts of ram and CPU's are becoming increasingly popular). Some systems, like Apple G4's, when cable locked cannot be opened, if you need machines for public areas features like this are ideal. For secure rooms make sure that the walls go beyond the false ceiling, and below the raised floor, large vents should also be covered with bars if possible. Monitoring the room with CCTV and possibly requiring a remote person to "buzz"

you in (in addition to a lock, keypad or other access control device) is recommended.

With enough time and physical access an attacker will be able to gain access to the system, several methods of attack include:

  • Rebooting the system from other media such as floppy disk, CD-ROM, external SCSI drives and so on
  • Removing the case, and removing the BIOS battery to get around any BIOS restrictions
  • Using a default BIOS password to gain access to the BIOS
  • Rebooting the system and passing boot arguments to LILO
  • Installing physical monitoring devices such as KeyGhost
  • Stealing the system's disk(s)
  • Unplug the server, or turn the power bar off (a very effective DoS), if done several times this can lead to filesystem corruption.

These are just a few of many possible attacks. The primary goal is to stop them where possible, and failing that slow them down so that hopefully someone will notice the attacker tearing apart a system in someone's office. The installation of monitoring devices is becoming especially worrisome, as they are now available for purchase online for less then $100. An attacker can easily log all keystrokes for a long time period before the attacker comes back to retrieve them. Periodic physical inspections (in teams of two) of user machines for items like KeyGhost, modems are so on are a good idea.

Gaining access to office buildings is often trivial. While working for the government there was no access control to the building itself from 8 A.M. until 5 P.M. (after 5 P.M. a security guard forced you to sign in). Worse yet the building had a back entrance that was not monitored. If you were to enter at 4:30 P.M., hide in a bathroom for an hour or two (crouched on top of a toilet reading the latest Linux Journal) you could easily spend several hours fiddling with desktop systems and leave at your convenience without being seen by anyone. Cleaning staff also never questioned me when I stayed late (and conversely I never questioned them), you should train staff to approach people they do not recognize and ask them politely if they need assistance or are lost. While access to buildings cannot often be controlled effectively (to many entrances / different tenants / etc.) you can control access to your floor, a locked door with a CCTV monitoring it after hours is often a good deterrent.

"Practical Unix and Internet Security" from O'Reilly covers physical problems as well and is definitely worth buying.


Console security

With physical access to most machines you can simply reboot the system and ask it nicely to launch into single user mode, or tell it to use /bin/sh for init. You can enter the BIOS and tell the machine to boot from a floppy, doing a quick end run around most security. Alternatively you can simply enter the bios, disable the IDE controllers, put a password on the BIOS, rendering the machine largely unusable.


BIOS / Open Firmware security

Assuming the attacker does not steal the entire machine the first thing that they will usually try is to reboot the system and either boot from a floppy disk (or other removable media). If they can do this then any standard file protection is useless, the attacker declares themselves to be root, mounts the filesystem and gains complete access to it.

To secure a x86 BIOS you typically enter it by hitting "delete" or a function key during the boot process, the actual name and location of the BIOS password varies significantly, look for "security" or "maintenance". There are usually different levels of password protections, on some motherboards you can disable the keyboard until a password is typed in (the BIOS intercepts and blocks input until it sees the password entered on the keyboard), on others it only prevents accessing the BIOS. Generally speaking you want to block access to the BIOS, and lock the boot sequence to the first internal storage device (i.e. the first IDE disk or SCSI).

Even if you do everything right there are still some ways for an attacker to subvert the boot process. Many older BIOS’s have universal passwords, generally speaking this practice has declined with modern systems, but you may wish to inquire with the vendor. Another potential problem to be aware of is that many add-on IDE and SCSI controller cards have their own BIOS, from which you can check the status of attached devices, choose a boot device, and in some cases format attached media. Many high-end network cards also allow you to control the boot sequence, letting you boot from the network instead of a local disk. This is why physical security is critical for servers. Other techniques include disabling the floppy drive so that attackers or insiders cannot copy information to floppy and take it outside. You may also wish to disable the serial ports in users machines so that they cannot attach modems, most modern computers use PS/2 keyboard and mice, so there is very little reason for a serial port in any case (plus they eat up IRQ's). Same goes for the parallel port, allowing users to print in a fashion that bypasses your network, or giving them the chance to attach an external CD-ROM burner or harddrive can decrease security greatly. As you can see this is an extension of the policy of least privilege and can decrease risks considerably, as well as making network maintenance easier (less IRQ conflicts, etc.). There are of course programs to get the BIOS password from a computer, one is available http://www.cgsecurity.org/, it is available for DOS and Linux.

If you decide to secure the BIOS on systems you should audit them once in a while if possible, simply reboot the machine and try to boot off of a floppy disk or get into the BIOS. If you can then you know someone has changed settings on the system, and while there may be a simple explanation (a careless technician for example) it may also be your first warning that an attack has occurred. There are several programs for Linux that allow an attacker with root access to gain the BIOS password, while this is a rather moot point it does bear mentioning (if an attacker has gained root access they can already do pretty much anything).


To secure a Sparc or UltraSparc boot prom send a break during boot-up, hit stop-a, and you should be presented with the ok> prompt. Setting your password is a simple matter of using the password command and typing the password in twice. You will then want to set the security-mode, using "setenv" from the default of none to command at the very least, and full if you are security conscious (warning, you will need the password to boot the machine).


ok
ok password

ok New password (only first 8 chars are used):*****
ok Retype new password: *****
ok
ok setenv security-mode full
ok


You can also set "security-mode" to

"command" which will require the password to access the open firmware but is less strict then "full". Do not lose this password, especially if you set the security-mode to full, as you may need to replace the PROM to recover the system. You can wipe the password by setting "security-mode" to "none".

Unfortunately if you are using Apple hardware you cannot secure the boot process in any meaningful manner. While booting if the user holds down the command-option-P-R keys it will wipe any settings that exist, there is no way to avoid this. About the only security related option you can set is whether the machine automatically reboots or not, this is useful for servers to prevent a remote attacker from changing the kernel for example (which require a system reboot). Hold down the command-option-O-F keys to access the OpenFirmware and from there you need to:

go> setenv auto-boot? False

However because a local attacker can easily flush the settings there is no inherent security. If you need to use Apple systems as servers then it is highly advisable to lock them in a cabinet of some sort. As workstations in a public area your best solution is to automate the reloading of the OS to speed recover time.


LILO security

LILO is the Linux boot loader, it handles all the nitty gritty tasks of getting the kernel into memory and bootstrapping them machine into something that resembles a useful computing device. LILO handles many things, from telling the kernel to look for multiple network cards to how much memory there is (assuming Linux won't detect how much your system has). There are several options you can pass to LILO to bypass most any account security or console security.

Booting Linux into single user mode with (assuming the label is "linux"):

linux single

This will dump you into a root level command prompt on many systems. From there you can simply add a new users with UID 0, format the disks or copy Trojan'ed binaries off a floppy disk. One way many vendors deal with this is by using sulogin, single user login, which prompts for root's password before letting you on. This will prevent people from accessing single user mode and getting directly to a root prompt however there is a another attack. You can specify which program will be used to init the system, instead of the default init you can use a command shell:

init=/bin/sh

Which will present you with a root prompt. The best way to defeat this and the single mode problem are to secure LILO and prevent the passing of boot arguments without a password. Many people argue that this somehow inhibits normal use of the system, however there should be no need normally for the user booting the system to pass LILO arguments (if an argument is needed at boot time it should be put into lilo.conf permanently). Generally speaking the only time you will need to pass LILO arguments is when something on the system breaks, at which point it's probably wise to send out a technician who knows the password to fix the system.

To secure LILO you must set a password, and use the restricted keyword. You can also set security on a per image basis, or on LILO as a whole. If you set security on a per image basis you will be able to tell LILO which image you wish to boot without needing a password, but you will not be able to pass the kernel arguments such as "single". If you set security on LILO as a whole then you will only be able to boot to the default image, specifying a different image will require the password.

boot=/dev/hda
map=/boot/map
install=/boot/boot.b
prompt
timeout=50
message=/boot/message
linear
default=linux

password=thisisapassword
restricted

image=/boot/vmlinuz-2.2.18
	label=linux
	read-only
	root=/dev/hda1

image=/boot/vmlinuz-2.2.17
	label=linux-old
	read-only
	root=/dev/hda1

This can be cumbersome, however it does prevent attackers from booting a different image.

boot=/dev/hda
map=/boot/map
install=/boot/boot.b
prompt
timeout=50
message=/boot/message
linear
default=linux

image=/boot/vmlinuz-2.2.18
	label=linux
	read-only
	root=/dev/hda1
	password=thisisapassword
	restricted

image=/boot/vmlinuz-2.2.17
	label=linux-old
	read-only
	root=/dev/hda1
	password=thisisapassword
	restricted

The above example will prevent attackers from messing with the "linux" and "linux-old" image, however if they need to boot the old kernel (because a driver is broken, or SMP support doesn't work quite right) then they will not need the password to do so.

Depending on the level of security you want you can either restrict and password protect all of LILO, or just the individual images. In any event if you are deploying workstations or servers you should use one to prevent users from breaking into systems in less than 10 seconds. While things like sulogin will prevent a user from getting a root prompt if they enter single user mode the attacker can always use "init=/bin/sh".

One minor security measure you can take to secure the lilo.conf file is to set it immutable, using the “chattr” command. To set the file immutable simply run the following command as root:

chattr +i /sbin/lilo.conf

and this will prevent any changes (accidental or otherwise) to the lilo.conf file. If you wish to modify the lilo.conf file you will need to unset the immutable flag run the following command as root:

chattr -i /sbin/lilo.conf

only the root user has access to the immutable flag.

Grub security

Grub is the default boot loader in Debian, Mandrake and possibly Red Hat 7.2. More on this to come.

Rebooting the system

If possible you should make rebooting the system more difficult, by default almost all Linux distributions have ctrl-alt-del enabled so that it reboots the machines. However, unlike Windows, this is not necessary. In Linux you can easily control the behavior of ctrl-alt-del, simply by editing the /etc/inittab file:

ca::ctrlaltdel:/sbin/shutdown -t3 -r now

You may wish to disable it entirely (simply comment it out with a #) or modify the command issued. Minimally you can create a bash script such as:

#!/bin/bash
#
# This script emails a message to root and then shuts 
# down the system after a 5 second delay
#
/bin/date | /bin/mail –s "ctrl-alt-del used" root
/bin/sleep 5s
/sbin/shutdown -t3 -r now


Now every time someone hits ctrl-alt-del you will have an email message in root's account logging it. You can also send that email to another system (i.e. to [email protected]). If you do this you may wish to introduce a small delay between the mail command and the shutdown command to ensure the email gets out before the mail system is turned off. Of course an attacker can always hit the reset switch, power button, unplug the system, or trip the breakers for an entire floor of computers. If the system is in a locked cabinet however this is quite a bit more conspicuous then simply using a three finger salute (ctrl-alt-del) to reboot the system.


Summary

An attacker with physical access to the console, or worse yet the hardware itself has many potential avenues of attack they can pursue. Worse yet if their goal is to simply deny use of service, or damage the software and hardware doing so is trivial. Flipping the power off and on during the boot process will often result in drive corruption, or they can simply pour a glass of water into the power supply. Locking servers up in secure rooms is critical, a competitor can easily afford to pay a janitor several thousand dollars to turn a server off, and pour pop into the power supply, resulting in a dead server when staff turns it back on. Workstations are typically more vulnerable as they are in accessible areas, but hopefully they are interchangeable with another workstation, with all data being stored on servers.

Linux Administrator's Security Guide
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  Copyright Kurt Seifried 2001 [email protected]. Published under the terms of the Open Content License Design by Interspire