Home > Articles > Operating Systems, Server

Quick and Easy Linux Hacking and How to Avoid It

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

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


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:

xhost +pentacorp.com

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

xhost +

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, that you want to allow unrestricted access for. If you want more security, you can limit access to the local machine.

ipchains -A input -p tcp -j ACCEPT -s    -d 6000:60631
ipchains -A input -p tcp -j DENY -s    -d 6000:6063

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.

$ echo $DISPLAY

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 1Figure 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:

X -nolisten tcp :0

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.

// dataP is user supplied data from the network
char    *policy = *dataP;
int     nPolicies;
// Oh dear, we can set nPolicies to -1
nPolicies = *policy++;
while (nPolicies) {
// Do some stuff in a loop

To fix this code, change

while (nPolicies) {


while (nPolicies > 0) {

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.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020