Home > Articles > Security > Network Security

Even Nastier: Traditional RootKits

📄 Contents

  1. So, What Do Traditional RootKits Do?
  2. Defending Against Traditional RootKits
When attackers get root access on a machine, a RootKit allows them to maintain that access by implementing a back door and hiding evidence of system compromise. Ed Skoudis shows how to avoid being RootKitted in this article.
This article is an excerpt from the book Counter Hack: A Step-by-Step Guide to Computer Attacks and Effective Defenses (Prentice Hall PTR, 2001), by Ed Skoudis.
Like this article? We recommend

Traditional RootKits are a very insidious form of Trojan horse back door. Traditional RootKits raise the ante by altering or replacing existing system components, as shown in Figure 1. Rather than running as a foreign application (such as Netcat or Back Orifice 2000), traditional RootKits replace critical operating system executables to let an attacker have back-door access and hide on the system. By replacing system components, RootKits can be far more powerful than application-level Trojan horse back doors.

Figure 1 Comparing application-level Trojan horse back doors with traditional RootKits.

Traditional RootKits have been around for over a decade, with the first very powerful RootKits detected in the early 1990s. Many of the early RootKits were kept within the underground hacker community and distributed via Internet Relay Chat for a few years. Throughout the 1990s and into the new millennium, traditional RootKits have become more and more powerful and radically easier to use. Now, traditional RootKit variants are available that practically install themselves, allowing an attacker to "RootKit" a machine in less than 10 seconds.

Traditional RootKits are available for a variety of platforms but have traditionally focused on UNIX systems, such as Solaris, SunOS, Linux, AIX, HP-UX, and so on. While some Windows NT/2000 RootKits are available that replace Dynamic Link Libraries (DLLs) or otherwise alter the system, the vast majority of RootKits are focused on UNIX systems.

So, What Do Traditional RootKits do?

Contrary to what their name implies, RootKits do not allow an attacker to gain root access to a system. RootKits depend on the attacker already having root access, which was likely obtained using buffer overflows, taking advantage of programming errors, or performing other attacks. Once an attacker gets root access on a machine, a RootKit is a suite of tools that allows the attacker to maintain root-level access by implementing a back door and hiding evidence of a system compromise.

The Centerpiece of Traditional RootKits on UNIX: /bin/login Replacement

So, how do RootKits implement back doors? To understand RootKit backdoors, it's important to know what happens when you access a UNIX machine. When you log into a UNIX system, whether by typing at the local keyboard or accessing the system across a network using telnet or another application, the /bin/login program runs. The system uses the /bin/login program to gather and check the user's ID and password. The /bin/login program is one of the most fundamental security tools in UNIX, requiring users to provide their user ID and password to the system for authentication during login. /bin/login gathers the user ID and password typed by the user and consults the password file to determine whether the password was accurate. If the password was accurate, the /bin/login routine allows the user into the system.

A RootKit replaces /bin/login with a modified version that includes a back-door password for root access. If the attacker uses the back-door root password, the modified /bin/login program will give access to the system. Even if the sysadmin alters the legitimate root password for the system (or wipes the password file clean), the attacker can still log in as root using the back-door password. So, a RootKit's /bin/login routine is a back door because it can be used to bypass normal system security controls. Furthermore, it is a Trojan horse because, although it looks like a normal, happy login routine, it is really an evil back door.

Figure 2 shows a user logging onto a system before and after a traditional RootKit is installed. In this example, the /bin/login routine is replaced with a back-door version from the widely used Linux RootKit, lrk5. Note the subtle differences in behavior of the original login routine and the new back-door version.

Figure 2 Behavior of /bin/login before and after installation of a generic RootKit.

In Figure 2, the first difference we notice in the before and after picture is the inclusion of the system name before the login prompt on the RootKitted system, which says bob login: instead of simply login:. Additionally, when we tried to log in as root, the original /bin/login requested our password. The system is configured to disallow incoming telnets as root, so it gathered the password and wouldn't allow the login. The original /bin/login just displayed the login: prompt again. The RootKitted /bin/login however, displayed a message saying, "root login refused on this terminal."

Of course, a more sophisticated attacker would first observe the behavior of the login routine and very carefully select (or even construct) a Trojan horse /bin/login routine to make sure that it properly mimics the behavior of the original /bin/login routine. However, if the behavior of your login routine ever changes, as shown in Figure 2, this could be a tip-off that something is awry with your system. You should investigate immediately if the behavior of your login routine changes. The difference could be due to a patch or system configuration change, or it could be a sign of something sinister.

To detect back doors like this, system administrators often run the /bin/login routine through strings, a UNIX program that shows all sequences of consecutive characters in a binary file. If an unfamiliar sequence of characters is found, it may be a back-door password. After all, the binary of /bin/login could have the back-door password in it, which it uses to compare to see if the attacker is trying to get in. A mysterious appearance of a string could indicate a back-door password.

The majority of RootKit developers knew of this strings technique and developed a clever means for foiling it. The back-door password is split up and distributed throughout the /bin/login binary and is not a sequence of consecutive characters. The password is assembled in real time only when the /bin/login routine is executed to check if the back-door password has been entered. Therefore, the strings routine will not find the password in the executable because it is not a sequence of characters.

Furthermore, when a user logs into a UNIX system, the /bin/login program records the login in the wtmp and utmp files. These accounting files are used by various programs, such as the who command, to show who is currently logged into the system. The RootKit version of /bin/login skips this critical step if the back-door root password is used. Therefore, a system administrator that runs the who command will not be able to see the attacker's root-level log in.

Traditional RootKits: Hide Everything Else!

RootKits go far beyond just replacing the /bin/login programs. The same techniques applied to /bin/login to hide critical evidence about an attacker's presence are also employed against numerous other programs used by a system administrator. Table 1 shows some of the programs that are commonly replaced by RootKits to mask the attacker's activities on the system.

Table 1 Programs Typically Replaced by RootKits

Program RootKit Replaces

Program's Original Function

Behavior of RootKit Version


Displays disk utilization, showing how much disk space is available

Lies about available disk space, hiding the sectors taken up by attacker's tools and sniffer logs


Allows users to find files and directories, such as programs and recently modified files, in the file system

Lies about presence of attacker's files, such as sniffer programs and other tools, hiding their presence


Shows status of interfaces, including an indication of which interfaces are in promiscuous mode

Masks promiscuous mode so administrator cannot detect sniffer on the local system


Allows users to login to the system

Allows users to log into the system, but also provides a backdoor root-level password for the attacker


Shows the contents of a directory

Lies about presence of RootKit files, hiding their presence


Can be used to show processes listening on various TCP and UDP ports

Lies about specific ports used by the attacker, masking the fact that a process is listening there


Displays a list of running processes on the system

Lies about any processes the attacker wants to hide

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