Home > Articles > Operating Systems, Server

Linux Security: The Seven Most Deadly Sins

Bob Toxen tells you the seven worst things you could possibly do to compromise the security of your Linux system. Toxen guarantees that if these precautions are not followed, your system will be broken into within the first twelve months of operation.
This chapter is from the book

If you are new to security, I recommend that you start with Chapter 2, "Quick Fixes for Common Problems." At first glance, this chapter sounds simple and sweet. However, it covers the security problems that are so severe they are likely to result in a rather unpleasant and expensive break-in. These problems affect all unhardened Linux systems regardless of their use. This chapter is not full of fluff, theoretical ideas, or impractical suggestions. Instead, it is a hundred pages of no-nonsense, thoroughly explained, practical advice that has proven successful for many systems over the course of time.

Chapter 2's section entitled "The Seven Most Deadly Sins" explains exactly these. Indulge any of these seven sins and almost certainly your systems will be broken into in one to twelve months. I find that the difference between Linux and Windows is that with knowledge and a little effort, Linux can be made very secure. I provide that knowledge in Real World Linux Security, Second Edition, along with some humor and real world examples of successes and failures. Following the easy advice in the pages of this section will boost the security of an "out of the box" Linux system by an order of magnitude. Shall we begin?

Bob Toxen

These are the seven common problems most likely to allow major damage to occur to your system or bank account. If any of these are currently a problem on your systems, you should take care of them immediately.1

2.2.1 Weak and Default Passwords (#1)

As a system administrator, you are aware of the system breaches possible on your Linux machine. You have taken the time and effort to devise a difficult-to-guess password that uses at least eight characters, uses both letters and numbers, upper- and lowercase letters, and possibly some punctuation. Your root password is awesome—no one could guess it in a hundred years. (OK, some obsessive with a decrypt package could destroy it in a few days except that you use shadow passwords, but that is another story.)

How are your users doing? Choke, cough, gag, hack. Every account is a possible entry point. Have your users followed your advice, company policy, or threats to devise a good password? Are they being as careful as you are? Probably not. Now it is your turn to don the black hat and think like your enemy.

It is critically important to read "Avoiding Weak and Default Passwords" on page 42.

Using programs like crack (which cracks passwords), can you break into your users' accounts? You definitely will need to get written management approval to conduct this level of security audit. There are notable cases of unauthorized audits landing people in jail or at least on the unemployment rolls. Randal Schwartz is one.

You might even install a module in the passwd program that automatically tries to break a user's proposed new password. Though the standard passwd program makes very simple tests, there are more sophisticated routines that include much of crack's capability. One way to do this is to make use of the cracklib capability in the Pluggable Authentication Modules (PAM) enhancements to the passwd program. The cracklib library analyzes passwords to determine whether they are easily crackable. PAM offers additional security for Linux systems and other operating systems too. Edit the /etc/pam.d/passwd file to include:

passwd password requisite /usr/lib/security/pam_cracklib.so retry=3
passwd password required /usr/lib/security/pam_pwdb.so use_authtok

This will cause the PAM-enabled passwd program to load these dynamically loadable program libraries. PAM now is standard with Red Hat. On some systems these are in /lib instead of /usr/lib.

There is more documentation on configuring PAM-enabled utilities at:

http://www.kernel.org/pub/linux/libs/pam/

Another good source for PAM information is:

http://www.sun.com/software/solaris/pam/

On Slackware, this capability will be enabled if the following line is present in /etc/login.defs (and the dictionary is installed):

CRACKLIB_DICTPATH    /var/cache/cracklib/cracklib_dict

See also "Restricting Login Location and Times" on page 315.

Avoid default passwords as if your job depended on it.

I know what Cisco uses as the default password for many of its routers because a client's unhardened Linux system was cracked the night it was put on the Web. The password was used to determine which randomly chosen IPs on the Internet accepted Cisco's default password. A system at the same location, where I had been allowed to do "light" hardening months earlier, remained uncompromised. The next day, the list had grown to hundreds of IPs before the system was taken offline. A better design would have been for Cisco to force the administrator to pick a decent password before the device started accepting traffic. For years now, most Linux distributions have been forcing an initial nondefault password before routing network traffic.

So much for Cisco's advertising claim that their routers are more secure than Linux or UNIX systems because they "have security designed in and they use proprietary equipment" rather than using open source. It took me only a minute with Google to find a list of the default passwords for hundreds of popular switches.

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