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- X Marks the Hole
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This excerpt from Real World Linux Security talks about specific techniques of crackers that do not depend on configuration errors or inherently insecure software. The techniques discussed include X Window vulnerabilities, physical security, physical actions, terminal device attacks and disk sniffing.
This chapter is from the book Real World Linux Security, 2nd Edition
This chapter is from the book
This chapter is from the book
Unlike Chapter 2, 'Quick Fixes for Common Problems,' which is concerned with configuration errors and old insecure versions of software that can be fixed quickly, this chapter talks about specific techniques of crackers that do not depend on configuration errors or inherently insecure software. The techniques discussed here are frequently starting points for a cracker breaking into your system, and your understanding of them and protection against them is critical to system security.
The topics covered in this chapter include:
- 'X Marks the Hole'
- Law of the Jungle—Physical Security'
- 'Physical Actions'
- 'Selected Short Subjects'
- 'Terminal Device Attacks'
- 'Disk Sniffing'
3.1 X Marks the Hole
DANGER LEVEL
X security is one of those things that many people ignore, and hope does not come back to infect them. On the systems that use it, which is most systems, the X subsystem has access to every keystroke of every user and screen output and X runs set-UID to root. A rogue X process can connect to a user's X display and capture keystrokes while that user is entering her password. Without adequate X security this is very hard to guard against.
For some of the highest security situations, it might be appropriate to not use X and to remove it from the system. However, most people cannot live without X, so let us examine how to make X more secure. As most SysAdmins know, the lowest level of X security is via host name validation. You enable a particular remote host to connect to your X display by requesting that xhost add it to the list of approved hosts. To add pentacorp.com the following command would be issued by any user that presently has access to the X server:
Security involving host names and IP addresses, such as this level of X security, can be broken easily by a variety of well-known methods; these are discussed in 'The rsh, rcp, rexec, and rlogin Services' on page 198 as these insecure services share this method of authentication.
To list the present access, issue the xhost command without arguments:
To turn off specified access change the '+' to a '-'.
Many users get lazy, so instead of issuing an xhost command for each of several systems, they enable all systems in the world access via
This lets any cracker on the Internet scan for systems having port 6000 accessible and take over that user's access. If this happens to root, the cracker 'owns' that system. This is a really good reason for you to have your firewall block all X access via the following. The example assumes that you have the Class A local network 10.0.0.0, that you want to allow unrestricted access for. If you want more security, you can limit access to the local machine.
An excellent and generally accepted solution is to run X sessions over SSH. It is important to read 'Wrapping SSH Around X' on page 417 in Part II carefully, as there are some gotchas2 if this is not done correctly that will result in unprotected X sessions.
The most important gotcha is if a user accidentally sets her $DISPLAY environment variable, usually in a shell startup file. This would bypass SSH's encryption and establish an unsecured X session over a 6000 series port. SSH will set it to the local machine but with a session greater than 0, and sshd will be serving that session and route it over its encrypted channel. The following is typical. Note that the host name is that of the server system, rather than that of the client system that the user is seated at.
Note that a $DISPLAY value of, say,
on a system means nothing more than 'connect to TCP port 10+6000, or 6010, on pentacorp.com.' This is illustrated in Figure 3.1, where Joe, the SysAdmin, is on his home system. Its host name is corbomite.homesys.com. He has used SSH to connect into the Pentacorp system pentacorp.com.
Figure 3.1 SSH-wrapped X session.
As you can see in Figure 3.1, all data traveling over the network is encrypted. The unencrypted data within a properly configured system may be sniffed only by root. Because SSH-wrapped X sessions will operate between the two systems over the SSH channel operating on TCP port 22, there is no need to allow ports starting at 6000 access from other systems. Thus, it is recommended that IP Chains block X's ports that start with TCP port 6000.
IP Chains has the added advantage of preventing those gotchas from happening. If a user makes a mistake that would allow unprotected X sessions, the IP Chains blocking of the 6000 series ports will prevent the unprotected X session from happening.
The ports program (discussed in 'Turn Off Unneeded Services' on page 86) or netstat may be used to see what ports are active. A good test is to start up a simple X application such as xclock and see whether port 6000 develops an active connection. If so, there is an error in configuration. Instead, you should see port 6010 in use by sshd (for the first session).
For those that do not want to use SSH-wrapped X (and that is going against advice) the following man pages cover X security. They are not particularly clear and that increases the chance of making a mistake resulting in less security.
One possibility might be to allow finer-grained control over which users on a remote system may access the X session of a particular local user.
X does offer the Secure Keyboard option while typing confidential data. This option prevents keystrokes from being intercepted by malevolent X programs that ordinarily can intercept keystrokes from any X server that grants them access.
Another solution for most 'desktop' Linux systems is simply to disable X from listing on TCP port 6000 at all. To do this, supply the argument -nolisten tcp to X. An easy way to do this is to add the following line to $HOME/.xserverrc:
You will want to use ports or netstat to verify that this causes TCP port 6000 not to be opened any more. This will prevent any other systems from displaying X-generated images on your system or reading your keyboard but will not prevent local processes from using X via the X unix socket.
But wait! There's more. The X server has a nasty DoS that can be generated remotely.3 A malformed packet can cause the X server to run a busy loop for two billion iterations due to this packet being able to specify a count of –1 (when a legitimate packet specifies a small positive number), that then is decremented repeatedly until 0 is reached.
The following buggy code in the AuthCheckSitePolicy() function of Xserver/os/secauth.c is the culprit.
To fix this code, change
This illustrates an important programming technique that should be common but is not. Specifically, this technique applies when testing for boundary conditions, such as when to end a loop or follow an if statement. Instead of testing for an exact match, such as the buggy code's test for nPolicies exactly matching 0, test for meeting or exceeding the boundary condition. Thus, if a bug causes the value to skip past the boundary, the condition will be detected. Over 25 years of C programming, following this technique has prevented a number of small bugs in the author's code from being big bugs.
Although the code could be fixed as discussed above, recompiling X is painful. An alternative is to use any of the techniques discussed earlier to prevent the wrong kind of machines from getting close to X. This problem is known to affect XFree86 versions 3.3.5, 3.3.6, and 4.0. It causes X to lock up for roughly a few minutes, depending on processor speed. On 4.0 you can wiggle the mouse but X still is frozen until the loop completes or X is killed.
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Posted by3 years ago
Archived
SSH and Starting Off Your Pine64 Ubuntu Server - v .4 pt. 1
Part 3 is UP. Part 2 has not been posted.
This is the First part of a 3 part guide to setting up your pine64 with longsleep's Ubuntu imageas a homeserver. Most of the instructions will be Debian/Ubuntu agnostic - and depending on reception I get, I may write explicit debian instructions as well.
By the end of this guide:
- Your Pine64 will be properly setup, up-to-date and secure
- You will be able to log on to your Pine64 image remotely and securely with no password.
- You will be minutes away from setting up any of the hundreds of services in the native Ubuntu repos
- Torrenting - w/ Transmission or other clients
- Use OpenVPN
- Serving files to computers on your network - SMB, CFS or NFS
- Serving media to smartTV's and other media devices using DLNA - MiniDLNA
- Serving files across the internet with services like Syncthing
- In a later tutorial I will show you how to setup containers - more on this then.
- these are just the tip of the iceberg - if you can find someone doing it online with arm - it can be a click away
1. First thing first, follow longsleep's instructions on how to create a bootable sd-card.
2. Now obviously there is the required power source and network connectivity, so put the newly bootable SD Card in your pine64, connect ethernet cable, connect power. Boot times may very, my pines have never taken more than 3 min to boot - but there are stories of this happening - so maybe take a small break before continuing.
3. It is now time to find the pine on your network. If you have access to your network's router (such as admin access to your asus, linksys etc) you will be able to just wait until it boots - and it should pop-up under the nice gui in 'connected devices' or 'network map'.
Alternatively, if you connect your pine to a monitor and after log-in type
For eth0 you will find an 'inet' address- this is your pine's network-wide IP address.
On my network, and for the purposes of this discussion we will assume the Pine64 starts out at 192.168.1.11
4. This is a bit of pick your own adventure - if you are access your pine from a Mac or Linux the rest of this guide will be almost line for line the same. For windows, you need to download an ssh client....further directions coming.
For Mac and Linux users, bring up a terminal - and type
it should prompt for a password- at first boot it is ubuntu.
A successful login will show a little blurb about linux.
Now you're in!
5. My first session is usually just houskeeping. set root password - this is an admin account - meaning it has full control - a strong, unique password is advised
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6. Now either setup a new user or alter the ubuntu user- if you create a new user and don't change the ubuntu passwd - be advised that it is strongly encouraged that you delete the ubuntu user account
7. User accounts are almost there - now let's set the time.
Find your timezone among all available timezones with:
9. to check it:
10. Now let's use longlseep's tools to do the hardwork.
11. Now I typically run through some application installs and upgrades before my first reboot - but you can reboot and then do this after - it is personal preference
this will update the information on your system about available packages and versions
This is going to install some packages that I find super helpful - and basically won't be able to live without on an ssh box:
- man is a package that will download and setup a database of manuals - 'man pages' - on your pine.
- These can be accessed with the command 'man' - eg 'man man' will show you the manual for using the commands in man - 'man bash' etc...
- nano is the only text editor I can stand on the command line.
- wget and curl are useful - there is significant overlap - but I just prefer having them both
- iputils-ping is a basic network utility that I often use without thinking about - and so I just install it to get around having to download it later.
- libpam-systemd is a library that handles how your linux box will authenticate and handle some systemctl commands. Most simply, if you type 'sudo systemctl reboot' - pam- using libpam-systemd - will exit you out of your ssh session - this is nice to have
- LPT: apt-get install will prompt you for confirmation if it needs to install a dependency unspecified by you - you can get around this by appending -y to the end of your command.
sudo apt-get install man nano wget curl iputils-ping libpam-systemd
12.Now let's download & install available upgrades for our existing packages (that we found out about with sudo apt-get update) using:
after it prompts you and completes- this will take a minute -as some of the packages need quite a lot of configuration - we are going to go through our first reboot
13.if this guide has been successful it will pop out a few lines like 'Connection to 192.168.1.11 closed.'
This means, instead of getting stuck in limbo - as services were being closed down on your pine - your ssh session was closed - libpam-systemd worked!
Now you can re-ssh into the box - after you give it the minute to reload. If you are going to delete the ubuntu account make sure you ssh in using another account!
retype your password - and you are now re-logged in.
14.before we delete the user ubuntu - or move on to further stuff - we need to make sure your account has the ability to utilize administrator control:
now add yourself using the same syntax as the root account: e.g. # User privilege specification root ALL=(ALL:ALL) ALL username ALL=(ALL:ALL) ALL
use Ctrl+c to attempt to exit, save it.
now exit the root account by typing:
You should be back to the original ssh login account
to verify that the changes have stuck:
it will ask you to type your password and boom you are now a full admin on this newly secured Pine64.
15. You know your new user has all this power and you have all these new command line utilities - but let's add a degree of nerdiness and ease to this:
let's create/send your pine a new public key for your computer - that will enable you to login from your current computer with no password - but arguably more security.
exit your ssh connection:
now on your computer type
press enter or walk through the instructions - adding a passphrase or creating a specific location for your key is optional
again on your computer type:
it will prompt you for your pine64 username password... type that in
it should be done- to verify:
if it does prompt for a password it failed - on my system I had to manually add the location of my key's private location with 'IdentityFile' before it into /etc/ssh/ssh_config
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If you are unscathed by now, congrats, you are done - you can download applications from the repos and be up and running in minutes.
The two things that I thought you might consider adding to your ssh security are using a different port besides the standard 22 for ssh (security through obscurity). Not the greatest defense, but every layer helps. Though that can be a problem for applications that expect ssh to be on 22. The other is populate the hosts.allow file with the IP addresses of the internal LAN, which is of far greater utility.
comment - thanks /u/Groaker2
16.But one last thing I like to do, that might break your network config if done incorrectly - is to set a static route for the pine64.
This is something I do for two reasons - one it allows me to add new pine64's to my network - and it allows me to verify that I know the network settings are good and how everything is being setup. For instance, if your dhcp server decides to change your address someday, and you are using the Pine64 as a print server - this will just be one added layer of complexity.
first step is to verify your own network settings:
the necessary components for setting this up are the broadcast address - here it is 'Bcast:' followed by, in my case, 192.168.1.255 the netmask - here it is 'Mask:' followed by, in my case 255.255.255.0
and an available ip address that conforms to your network numbering structure - MAKE SURE IT IS FREE BEFORE YOU TRY TO REBOOT
and last but not least find your gateway by typing:
The first line of the table should have a gateway - my example gateway is 192.168.1.1
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and by the end of this you are going to want it to look like:
ctrl+c, save and exit -
moment of truth wait a minute for the system to reboot and try to ssh to the NEW ip address
ssh-keys should still work and you should be back in your newly configured pine
Thanks to everyone in the subreddit and on the Pine64 forums -- especially longsleep for the images and hosting all the stuff
1.7.10 Mods
If anyone is super lazy - this script does everything before the first boot in the guide above: pineStart.bash - it is complete and working and takes 13+ minutes to run through everything.
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