Home > Articles > Security > Network Security

This chapter is from the book

9.8 Intentional and Accidental Interactions

Computer virus researchers have observed a set of interesting behaviors that are the result of intentional and accidental interactions between various kinds of malicious code. This section describes common interactions.

9.8.1 Cooperation

Some computer viruses accidentally cooperate with other malicious code. For example, computer worms might get infected with a standard file infector virus as they pass through already infected nodes. It is common to encounter multiple infections on top of in-the-wild computer worms. It is not uncommon to find three or even more different viruses on the top of a worm carrier. This can help both the network worm and the standard file infector virus in various ways.

A worm can take advantage of the infection of an unknown file infector virus. If the file infector virus is unknown to antivirus products, the computer worm body might not be detectable. For example, in some cases the worm body will be embedded deep inside the virus code, leaving little chance for the antivirus program to find it. See Figure 9.19 for an illustration.

In Step A, the computer becomes infected with a worm. In Step B, the worm successfully penetrates a new remote system. That computer, however, is already infected with a virus that infects exactly the same type of files in which the worm propagates itself. Thus the file infector virus attaches itself to the worm. In Step C, the multiple infection arrives at a new computer. When the worm is executed, the file infector virus runs (in most cases, it will run before the worm's code) and infect other objects.

In Step D, the combination arrives at a system protected by antivirus software that knows the file infector virus on top of the computer worm carrier—but does not know the computer worm underneath. If the antivirus can disinfect the file infector virus, it might create a file object that is not exactly the same as the original worm. For example, the binary file of the worm might get larger or smaller, and important fields in its header might also change. (Of course, an antivirus is just one possible agent that might interact with other malicious programs.)

Thus a "mutant" worm will have the chance to propagate itself further and infect a new system in Step E. In practice, no antivirus software would consider the Step E case a variant of the original worm. However, antivirus programs need to address this issue. For example, the MD5 checksums of the "mutant" worm body are clearly different, and if the antivirus or content-filtering software uses such checksums to detect the original worm, it will fail to detect the "mutant" worm.

Figure 9.19Figure 9.19 The accidental interaction of a worm infected with a file infector virus.

An intentional variation of such cooperation appeared in the "symbiosis project" of GriYo. He released the mass-mailing worm, W32/Cholera, in infected form. The worm was infected with the polymorphic W32/CTX virus and became a carrier. The result was a quick succession of both W32/Cholera and W32/CTX infections worldwide. As a result, W32/CTX was reported on the Wildlist.

File infector viruses such as W32/Funlove often infect other worms and often multiple times. Such file infector viruses occasionally disappear from the top 50 charts but suddenly attack again during major computer worm outbreaks. This kind of activity was seen increasingly when the W32/Beagle worm appeared in the wild. As discussed earlier, some variants of Beagle send a password-protected attachment to recipients. Because the worm creates the archive (ZIP) file on the local system, the executable file of the worm can easily get infected by viruses such as Funlove before the executable gets packed. Thus a virus like Funlove also enjoys being password-protected36. Because several antivirus softwares had problems detecting the password-protected attachments reliably, "traveler viruses" could take advantage of this accidental cooperation.

A form of cooperation also exists with the previously mentioned W32/Borm creation, which infects Back–Orifice-compromised systems. W32/Borm does not attempt to kill Back Orifice; it simply takes advantage of compromised systems to propagate. Similarly, the aforementioned Mydoom had a backdoor that was utilized by the Doomjuice worm to spread itself.

With macro and script viruses, "body snatching" attacks often occur. Two or more script or macro viruses might form a new creation as they accidentally propagate each other's code.

9.8.2 Competition

Competition between malicious codes was also experienced among computer viruses. Several viruses attack other viruses and disinfect them from the systems that they have compromised. An example of this is the Den_Zuko boot virus37, which disinfects the Brain virus. These viruses are often called "benefical viruses" or "antivirus" viruses.

Antiworm computer worms started to become more popular in 2001 with the appearance of the CodeRed worm and the counterattacking, CodeGreen. (However, antiworm worms had been experienced previously on other platforms, such as Linux.)

Because IIS could be exploited more than once, CodeGreen could easily attack CodeRed-infected systems. The worm sent a similarly malformed GET request to the remote target nodes to CodeRed, which had in front the message shown in Listing 9.9.

Listing 9.9 The Front of a CodeGreen GET Request

GET /default.ida?Code_Green_<I_like_the_colour-_-><AntiCodeRed-

The worm also carried the following messages shown in Listing 9.10.

Listing 9.10 Other Messages of the CodeGreen Worm

HexXer's CodeGreen V1.0 beta CodeGreen has entered your system
it tried to patch your system and
to remove CodeRedII's backdoors

You may uninstall the patch via
SystemPanel/Sofware: Windows 2000 Hotfix [Q300972]

get details at "http://www.microsoft.com".
visit "http://www.buha-security.de

CodeGreen removed the CodeRed infections from systems and also removed the backdoor components of other CodeRed variants. Furthermore, it downloaded and installed patches to close the vulnerability.

Similar attacks against W32/Blaster worm were experienced when the W32/Welchia worm began its antiworm hunt against Blaster, which started the "worm wars" (as I decided to call it after Core Wars).

Another enthralling example is the W32/Sasser worm. Sasser targeted an LSASS vulnerability that was previously exploited by variants of Gaobot worm. Thus Gaobot's author was not impressed because Gaobot needed to compete with Sasser for the same targets. Consequently, the W32/Gaobot.AJS38 worm was developed with a vampire attack. I decided to call this kind of attack a "vampire" based on the Core War vampire attack. Vampire warriors can steal their enemies' souls (see Chapter 1, "Introduction to the Games of Nature," for details).

Gaobot.AJS is a vampire because it attacks Sasser when the two worms are on the same machine. Instead of simply killing Sasser, Gaobot.AJS modifies Sasser's code in a very tricky way. As a result of the modification, Sasser can still scan for new targets and even exploit them successfully. However, when Sasser connects to its shellcode on the compromised system to instruct it to download and execute a copy of Sasser via FTP, the code modifications of Gaobot.AJS will get control. In turn, Gaobot.AJS sends commands to Sasser's shellcode on the remote machine and instructs it to download a copy of Gaobot.AJS's code instead of Sasser's. Furthermore, Gaobot closes the connection to the remote machine so Sasser cannot propagate but is used as a Gaobot propagation agent in a parasitic manner.

Another gripping example is the W32/Dabber worm, which appeared right after Sasser. As mentioned, Sasser's shellcode is instructed to download a copy of Sasser via FTP. On the attacker system, Sasser implements a crude FTP server. However, this routine of Sasser had a simple buffer overflow vulnerability that could be exploited. (Indeed, worms can have their own vulnerabilities!) Dabber was released to exploit Sasser's vulnerability to propagate itself. It scans for targets that were compromised by Sasser and attempts to connect to Sasser's vulnerable "FTP server" to exploit it successfully.

It is expected that competitions between malicious programs will become more and more common in the future.

9.8.3 The Future: A Simple Worm Communication Protocol?

Although increased competition among malicious programs is likely, it also makes sense for attackers to invest in cooperating techniques. For example, computer worms could use a special protocol such as simple worm communication protocol (SWCP) to exchange information, as well as plug-ins ("genes") among different families of computer worms that support SWCP. Computer worms could swap payloads, exchange information about systems to attack, or even collect e-mail addresses and share them with the other worms that occasionally communicate using SWCP. I highly anticipate that such techniques will appear in the very near future.

Of course, communication can have other forms. For instance, viruses could "reproduce sexually"39 to cross their genomes to produce offspring, which can evolve or devolve. The closest currently known example of accidentally "sexually reproducing" computer viruses can be found in macro viruses which occasionally swap, or snatch their macros ("genes") as discussed in Chapter 3, "Malicious Code Environments,". However, specifically written binary viruses could possibly demonstrate similar behavior that would lead to further evolution in computer viruses on their own.

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