Home > Articles > Security > Network Security

Daily Security Tips from Ed Skoudis - Week of February 3, 2003

Keep the hackers at bay and your assets locked up with some advice from one of InformIT's most popular writers, Ed Skoudis.
Like this article? We recommend

Security Tip for Friday, February 7th, 2003

Remove Unnecessary Stuff That Attackers Use

When installing and configuring the computers that support your critical infrastructure, such as mail servers, DNS servers, and web servers, make sure you remove software that is not needed but is often used by attackers. For example, does your DNS server really need a TFTP client? This program is used to move files around without any authentication, something you surely don't require on your DNS server. Yet attackers' strategies often include manipulating TFTP to move files to a target. So, remove TFTP, as well as a variety of other tools used by the bad guys that are not needed on production systems. Consider removing C language compilers, Perl interpreters, and X Window system tools if they are not required on systems.

Security Tip for Thursday, February 6th, 2003

Educate Your Software Development Team About Avoiding Buffer Overflows

Buffer overflow vulnerabilities continue to plague the security of our computer systems. A buffer overflow can allow an attacker to execute arbitrary commands on a victim machine, letting the attacker take over the system. Buffer overflows are caused by software developers who forget to verify the size of user-entered data before moving it around in memory. You need to make sure your developers always properly check the size of user input in their programs. For a good description of secure coding practices in Linux and UNIX, have your software developers read the document at http://www.dwheeler.com/secure-programs/. For secure software development in Windows, buy your developers the book Writing Secure Code by Howard and Leblanc.

Additionally, there are several notable free automated source code security analysis tools that look for flaws. Feed your source code to these tools, and they'll provide a high-level search for common security mistakes made by your developers. These free tools include:

ITS4: http://www.cigital.com/its4/download.html

RATS: http://www.securesoftware.com/rats.php

Flawfinder: http://www.dwheeler.com/flawfinder/

Security Tip for Wednesday, February 5th, 2003

Protect Your Linux Kernel

Computer attackers who break into systems are increasingly modifying them at the most fundamental level, the kernel itself. The kernel controls which programs run, the use of memory, access to the hard drive, and use of the network interface. By controlling the kernel, the attacker really owns the machine! One of the easiest mechanisms for controlling a Linux kernel involves using loadable kernel modules (LKMs). Normal LKMs are used to dynamically extend the kernel to support new hardware and other services. Malicious LKMs can be used by an attacker to gain backdoor access and to hide on a system.

To prevent such attacks, for sensitive systems, such as firewalls, web servers, mail servers, and DNS servers, limit attackers' (and your own) ability to dynamically alter your kernel. Build a kernel that does not support LKMs, known as a monolithic kernel. For a recipe about how to build a monolithic kernel, check out http://www.redhat.com/docs/manuals/linux/RHL-7.3-Manual/custom-guide/s1-custom-kernel-monolithic.html. Alternatively, you can use the free Linux Loadable Security Module (LSM) for limiting what LKMs can do, available at http://www.packetstormsecurity.com/linux/security/lsm.tar.gz. Finally, a free tool called St. Michael can prevent and detect some malicious module behavior, available at http://sourceforge.net/projects/stjude.

Security Tip for Tuesday, February 4th, 2003

Make the Internet a Safer Place – Deploy Egress Anti-Spoof Filters

Suppose an attacker takes over one of your Internet accessible servers, such as your web server, DNS server, or mail server. The attacker could use your machine to launch a packet flood against another victim system in a Distributed Denial of Service (DDoS) attack. These attacks send a flood of packets, each with a spoofed source IP address so that the flood victim cannot trace the attack back. To minimize the chance of an attacker launching a flood from one of your own systems, you should implement egress anti-spoof filters at your border router or firewall connecting your network to the Internet. Most people have deployed incoming anti-spoof filters, which block incoming traffic whose source address is assigned to inside systems. But less than 50 % of networks have deployed egress anti-spoof filters. These filters block all outgoing packets whose source address is not on your network. If your web server suddenly starts spewing packets with a source address not on your network, an egress anti-spoof filter would stop it. If everyone implemented egress anti-spoof filters in addition to their current incoming anti-spoof filters, the Internet would be much less susceptible to DDoS floods.

Security Tip for Monday, February 3rd, 2003

Fight Back Against RootKits

RootKits are nasty suites of tools used by attackers to maintain control of victim machines. If bad guys take over your system, they could use a RootKit to set up a backdoor and hide out on your box. RootKits replace critical components of your operating system with the attacker's own malicious code. Some RootKits alter critical system binary applications (such as the UNIX login, ps, and ifconfig programs), DLLs, or even the underlying kernel itself. In addition to using a file integrity checking tool like Tripwire (www.tripwire.com), you should also regularly run a RootKit detection tool. One of the best RootKit checkers for Linux, BSD, and Solaris is called chkrootkit, available at http://www.chkrootkit.org/. This nifty free tool can detect over forty different RootKits, as well as look for symptoms of RootKit infestation.

Check back each business day for a new security tip!

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