Home > Articles > Security > Network Security

This chapter is from the book

3.11 The X11 Window System

X11 [Scheifler and Gettys, 1992] is the dominant windowing system used on UNIX systems. It uses the network for communication between applications and the I/O devices (the screen, the mouse, and so on), which allows the applications to reside on different machines. This is the source of much of the power of X11. It is also the source of great danger.

The fundamental concept of X11 is the somewhat disconcerting notion that the user's terminal is a server. This is quite the reverse of the usual pattern, in which the per-user, small, dumb machines are the clients, requesting services via the network from assorted servers. The server controls all of the interaction devices. Applications make calls to this server when they wish to talk to the user. It does not matter how these applications are invoked; the window system need not have any hand in their creation. If they know the magic tokens—the network address of the server—they can connect.

In short, we give away control of our mouse, keyboard, and screen.

Applications that have connected to an X11 server can do all sorts of things. They can detect keypresses, dump the screen contents, generate synthetic keypresses for applications that will permit them, and so on. In other words, if an enemy has connected to your keyboard you can kiss your computer assets good-bye. It is possible for an application to grab sole control of the keyboard when it wants to do things like read a password. Few users use that feature. Even if they did, another mechanism that can't be locked out will let you poll the keyboard up/down status map.

The problem is now clear. An attacker anywhere on the Internet can probe for X11 servers. If they are unprotected, as is often the case, this connection will succeed, generally without notification to the user. Nor is the port number difficult to guess; it is almost always port 6000 plus a very small integer, usually zero.

One application, the window manager, has special properties. It uses certain unusual primitives so that it can open and close other windows, resize them, and so on. Nevertheless, it is an ordinary application in one very important sense: It, too, issues network requests to talk to the server.

A number of protection mechanisms are present in X11. Not all are particularly secure. The first level is host address-based authentication. The server retrieves the network source address of the application and compares it against a list of allowable sources; connection requests from unauthorized hosts are rejected, often without any notification to the user. Furthermore, the granularity of this scheme is tied to the level of the requesting machine, not an individual. There is no protection against unauthorized users connecting from that machine to an X11 server. IP spoofing and hijacking tools are available on the Internet.

A second mechanism uses a so-called magic cookie. Both the application and the server share a secret byte string; processes without this string cannot connect to the server. But getting the string to the server in a secure fashion is difficult. One cannot simply copy it over a possibly monitored network cable, or use NFS to retrieve it. Furthermore, a network eavesdropper could snarf the magic cookie whenever it was used.

A third X11 security mechanism uses a cryptographic challenge/response scheme. This could be quite secure; however, it suffers from the same key distribution problem as does magic cookie authentication. A Kerberos variant exists, but of course it's only useful if you run Kerberos. And there's still the issue of connection-hijacking.

The best way to use X11 these days is to confine it to local access on a workstation, or to tunnel it using ssh or IPsec. When you use ssh, it does set up a TCP socket that it forwards to X11, but the socket is bound to 127.0.0.1, with magic cookie authentication using a local, randomly generated key on top of that. That should be safe enough.

3.11.1 xdm

How does the X server (the local terminal, remember) tell remote clients to use it? In particular, how do X terminals log you in to a host? An X terminal generates an X Display Manager Control Protocol (XDMCP) message and either broadcasts it or directs it to a specific host. These queries are handled by the xdm program, which can initiate an xlogin screen or offer a menu of other hosts that may serve the X host.

Generally, Xdm itself runs as root, and has had some security problems in the past (e.g., CERT Vendor-Initiated Bulletin VB-95:08). Current versions are better, but access to the xdm service should be limited to hosts that need it. There are configuration files that tell xdm whom to serve, but they only work if you use them. Both xauth and xhost should be used to restrict access to the X server.

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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