From Wikipedia, the free encyclopedia
Malware or malicious software is software designed to infiltrate or damage a computer system without the owner's informed consent. It is a portmanteau of the words "malicious" and "software". The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code.
Many normal computer users are however still unfamiliar with the term, and most never use it. Instead, "(computer) virus" is used in common parlance and often in the general media to describe all kinds of malware. Another term that has been recently coined for malware is badware, perhaps due to the anti-malware initiative Stopbadware or corruption of the term "malware".
Software is considered malware based on the perceived intent of the creator rather than any particular features. It includes computer viruses, worms, trojan horses, spyware, adware, and other malicious and unwanted software. In law, malware is sometimes known as a computer contaminant, for instance in the legal codes of California, West Virginia, and several other U.S. states.
Malware should not be confused with defective software, that is, software which has a legitimate purpose but contains harmful bugs.
Many early infectious programs, including the Internet Worm and a number of MS-DOS viruses, were written as experiments or pranks generally intended to be harmless or merely annoying rather than to cause serious damage. Young programmers learning about viruses and the techniques used to write them might write one to prove that they can do it, or to see how far it could spread. As late as 1999, widespread viruses such as the Melissa virus appear to have been written chiefly as pranks.
A slightly more hostile intent can be found in programs designed to vandalize or cause data loss. Many DOS viruses, and the Windows ExploreZip worm, were designed to destroy files on a hard disk, or to corrupt the filesystem by writing junk data. Network-borne worms such as the 2001 Code Red worm or the Ramen worm fall into the same category. Designed to vandalize web pages, these worms may seem like the online equivalent to graffiti tagging, with the author's name or affinity group appearing everywhere the worm goes.
However, since the rise of widespread broadband Internet access, more malicious software has been designed for a profit motive. For instance, since 2003, the majority of widespread viruses and worms have been designed to take control of users' computers for black-market exploitation.  Infected "zombie computers" are used to send email spam, to host contraband data such as child pornography, or to engage in distributed denial-of-service attacks as a form of extortion.
Another strictly for-profit category of malware has emerged in spyware -- programs designed to monitor users' web browsing, display unsolicited advertisements, or redirect affiliate marketing revenues to the spyware creator. Spyware programs do not spread like viruses; they are generally installed by exploiting security holes or are packaged with user-installed software.
Infectious malware: viruses and worms
The best-known types of malware, viruses and worms, are known for the manner in which they spread, rather than any other particular behavior. Originally, the term computer virus was used for a program which infected other executable software, while a worm transmitted itself over a network to infect other computers. More recently, the words are often used interchangeably.
Today, some draw the distinction between viruses and worms by saying that a virus requires user intervention to spread, whereas a worm spreads automatically. Using this distinction, infections transmitted by email or Microsoft Word documents, which rely on the recipient opening a file to infect the system, would be classified as viruses, not worms.
Capsule history of viruses and worms
- Main articles: Computer virus, computer worm.
Before Internet access became widespread, viruses spread on personal computers by infecting programs or the executable boot sectors of floppy disks. By inserting a copy of itself into the machine code instructions in these executables, a virus causes itself to be run whenever the program is run or the disk is booted. Early computer viruses were written for the Apple II and Macintosh, but they became more widespread with the dominance of the IBM PC and MS-DOS system. Executable-infecting viruses are dependent on users exchanging software or boot floppies, so they spread heavily in computer hobbyist circles.
The first worms, network-borne infectious programs, originated not on personal computers, but on multitasking Unix systems. The first well-known worm was the Internet Worm of 1988, which infected SunOS and VAX BSD systems. Unlike a virus, this worm did not insert itself into other programs. Instead, it exploited security holes in network server programs and started itself running as a separate process. This same behavior is used by today's worms as well.
With the rise of the Microsoft Windows platform in the 1990s, and the flexible macro systems of its applications, it became possible to write infectious code in the macro language of Microsoft Word and similar programs. These macro viruses infect documents and templates rather than applications, but rely on the fact that macros in a Word document are a form of executable code.
Today, worms are most commonly written for the Windows OS, although a small number are also written for Linux and Unix systems. Worms today work in the same basic way as 1988's Internet Worm: they scan the network for computers with vulnerable network services, break in to those computers, and copy themselves over. Worm outbreaks have become a cyclical plague for both home users and businesses, eclipsed recently in terms of damage by spyware.
Concealment: Trojan horses, rootkits, and backdoors
For a malicious program to accomplish its goals, it must be able to do so without being shut down by the user or administrator of the computer it's running on. Concealment can also help get the malware installed in the first place. By disguising a malicious program as something innocuous or desirable, users may be tempted to install it without knowing what it does. This is the technique of the Trojan horse or trojan.
Broadly speaking, a Trojan horse is any program that invites the user to run it, but conceals a harmful or malicious payload. The payload may take effect immediately and can lead to many undesirable effects, such as deleting all the user's files, or more commonly it may install further harmful software into the user's system to serve the creator's longer-term goals. Trojan horses known as droppers are used to start off a worm outbreak, by injecting the worm into users' local networks.
One of the most common ways that spyware is distributed is as a Trojan horse, bundled with a piece of desirable software that the user downloads off the Web or a peer-to-peer file-trading network. When the user installs the software, the spyware is installed alongside. Spyware authors who attempt to act legally may include an end-user license agreement which states the behavior of the spyware in loose terms, but with the knowledge that users are unlikely to read or understand it.
Once a malicious program is installed on a system, it is often useful to the creator if it stays concealed. The same is true when a human attacker breaks into a computer directly. Techniques known as rootkits allow this concealment, by modifying the host operating system so that the malware is hidden from the user. Rootkits can prevent a malicious process from being reported in the process table, or keep its files from being read. Originally, a rootkit was a set of tools installed by a human attacker on a Unix system where the attacker had gained administrator (root) access. Today, the term is used more generally for concealment routines in a malicious program.
A backdoor is a method of bypassing normal authentication procedures. Many computer manufacturers preinstall backdoors on their systems to provide technical support for customers.[verification needed] Hackers typically use backdoors to secure remote access to a computer, while attempting to remain hidden from casual inspection. To install backdoors hackers use either Trojan horse or computer worm.
Malware for profit: spyware, botnets, loggers, and dialers
During the 1980s and 1990s, it was usually taken for granted that malicious programs were created as a form of vandalism or prank. More recently, the greater share of malware programs have been written with a financial or profit motive in mind. This can be taken as the malware authors' choice to monetize their control over infected systems: to turn that control into a source of revenue.
Since 2003 or so, the most costly form of malware in terms of time and money spent in recovery has been the broad category known as spyware.  Spyware programs are commercially produced for the purpose of gathering information about computer users, showing them pop-up ads, or altering web-browser behavior for the financial benefit of the spyware creator. For instance, some spyware programs redirect search engine results to paid advertisements. Others often called "stealware" by the media overwrite affiliate marketing codes so that revenue goes to the spyware creator rather than the intended recipient.
Spyware programs are sometimes installed as Trojan horses of one sort or another.[verification needed] They differ in that their creators present themselves openly as businesses, for instance by selling advertising space on the pop-ups created by the malware. Most such programs present the user with an end-user license agreement which purportedly protects the creator from prosecution under computer contaminant laws. However, spyware EULAs have not yet been upheld in court.
Another way that financially-motivated malware creator can monetize their infections is to directly use the infected computers to do work for the creator. Spammer viruses, such as the Sobig and Mydoom virus families, are commissioned by e-mail spam gangs. The infected computers are used as proxies to send out spam messages. The advantage to spammers of using infected computers is that they are available in large supply (thanks to the virus) and they provide anonymity, protecting the spammer from prosecution. Spammers have also used infected PCs to target anti-spam organizations with distributed denial-of-service attacks.
In order to coordinate the activity of many infected computers, attackers have used coordinating systems known as botnets. In a botnet, the malware or malbot logs in to an Internet Relay Chat channel or other chat system. The attacker can then give instructions to all the infected systems simultaneously. Botnets can also be used to push upgraded malware to the infected systems, keeping them resistant to anti-virus software or other security measures.
Lastly, it is possible for a malware creator to profit by simply stealing from the person whose computer is infected. Some malware programs install a key logger, which copies down the user's keystrokes when entering a password, credit card number, or other information that may be useful to the creator. This is then transmitted to the malware creator automatically, enabling credit card fraud and other theft. Similarly, malware may copy the CD key or password for online games, allowing the creator to steal accounts or virtual items.
Another way of stealing money from the infected PC owner is to take control of the modem and dial an expensive toll call. Dialer (or porn dialer) software dials up a premium-rate telephone number such as a U.S. "900 number" and leave the line open, charging the toll to the infected user.
Vulnerability to malware
In this context, as throughout, it should be borne in mind that the “system” under attack may be of various types, e.g. a single computer and operating system, a network or an application.
Various factors make a system more vulnerable to malware:
- Homogeneity – e.g. when all computers in a network run the same OS, if you can break that OS, you can break into any computer running it.
- Bugginess – most systems containing errors which may be exploited by malware.
- Unconfirmed code – code from a floppy disk, CD or USB device may be executed without the user’s agreement.
- Over-privileged users – some systems allow all users to modify their internal structures.
- Over-privileged code – most popular systems allow code executed by a user all rights of that user.
An oft-cited cause of vulnerability of networks is homogeneity or software monoculture. In particular, Microsoft Windows has such a large share of the market that concentrating on it will enable a cracker to subvert a large number of systems. Introducing inhomogeneity purely for the sake of robustness would however bring high costs in terms of training and maintenance.
Most systems contain bugs which may be exploited by malware. Typical examples are buffer overruns, in which an interface designed to store data in a small area of memory allows the caller to supply too much, and then overwrites its internal structures. This may used by malware to force the system to execute its code.
Originally, PCs had to be booted from floppy disks, and until recently it was common for this to be the default boot device. This meant that a corrupt floppy disk could subvert the computer during booting, and the same applies to CDs. Although that is now less common, it is still possible to forget that one has changed the default, and rare that a BIOS makes one confirm a boot from removable media.
In some systems, non-administrator users are over-privileged by design, in the sense that they are allowed to modify internal structures of the system. In some environments, users are over-privileged because they have been inappropriately granted administrator or equivalent status. This is a primarily a configuration decision, but on Microsoft Windows systems the default configuration is to over-privilege the user. This situation exists due to decisions made by Microsoft to prioritize compatibility with older systems above security configuration in newer systems. As privilege escalation exploits have increased this priority is shifting for the release of Microsoft Windows Vista. As a result, many existing applications that require excess privilege (over-privileged code) are expected to have compatibility problems with Vista.
Malware, running as over-privileged code, can use this privilege to subvert the system. Almost all currently popular operating systems, and also many scripting applications allow code too many privileges, usually in the sense that when a user executes code, the system allows that code all rights of that user. This makes users vulnerable to malware in the form of e-mail attachments, which may or may not be disguised.
Given this state of affairs, users are warned only to open attachments they trust, and to be wary of code received from untrusted sources. It also common for operating systems to be designed so that device drivers need escalated privileges, while they are supplied by more and more hardware manufacturers, some of whom may be unreliable.
Eliminating over-privileged code
Over-privileged code dates from the time when most programmes were either delivered with a computer or written in-house, and repairing it would at a stroke render most anti-virus software almost redundant. It would, however, have appreciable consequences for the user interface and system management.
The system would have to maintain privilege profiles, and know which to apply for each user and program. In the case of newly installed software, an administrator would need to set up default profiles for the new code.
Eliminating vulnerability to rogue device drivers is probably harder than for arbitrary rogue executables. Two techniques, used in VMS, that can help are memory mapping only the registers of the device in question and a system interface associating the driver with interrupts from the device.
Other approaches are:
- Various forms of virtualization, allowing the code unlimited access only to virtual resources
- Various forms of sandbox or jail
- The security functions of Java, in
Such approaches, however, if not fully integrated with the operating system, would reduplicate effort and not be universally applied, both of which would be detrimental to security.
Academic research on malware: a brief overview
The notion of a self-reproducing computer program can be traced back to 1949 when John von Neumann presented lectures that encompassed the theory and organization of complicated automata [Ne49]. Neumann showed that in theory a program could reproduce itself. This constituted a plausibility result in computability theory. Fred Cohen experimented with computer viruses and confirmed Neumann's postulate. He also investigated other properties of malware (detectability, self-obfuscating programs that used rudimentary encryption that he called "evolutionary", and so on). His doctoral dissertation was on the subject of computer viruses [Co86]. Cohen's faculty advisor, Leonard Adleman (the A in RSA) presented a rigorous proof that, in the general case, algorithmically determining whether a virus is or is not present is Turing undecidable [Ad88]. This problem must not be mistaken for that of determining, within a broad class of programs, that a virus is not present; this problem differs in that it does not require the ability to recognize all viruses. Adleman's proof is perhaps the deepest result in malware computability theory to date and it relies on Cantor's diagonal argument as well as the halting problem. Ironically, it was later shown by Young and Yung that Adleman's work in cryptography is ideal in constructing a virus that is highly resistant to reverse-engineering by presenting the notion of a cryptovirus [YY96]. A cryptovirus is a virus that contains and uses a public key. In the cryptoviral extortion attack, the virus hybrid encrypts plaintext data on the victim's machine using the virus writer's public key. In theory the victim must negotiate with the virus writer to get the plaintext back (assuming there are no backups). Analysis of the virus reveals the public key, not the needed private decryption key. This result was the first to show that computational complexity theory can be used to devise malware that is robust against reverse-engineering.
Another growing area of computer virus research is to mathematically model the infection behavior of worms using models such as Lotka-Volterra equations, which has been applied in the study of biological virus. Various virus propagation scenarios have been studied by researchers such as propagation of computer virus, fighting virus with virus like predator codes [ToKa02], virus war- fighting between worms [TaKh06], effectiveness of patching etc.
Emerging vectors and pathways
Wikis and Blogs
Innocuous wikis and blogs are not immune to hijacking. It has been reported that the German edition of Wikipedia has recently been used as an attempt to vector infection. Through a form of social engineering, users with ill intent have added links to web pages that contain malicious software with the claim that the web page would provide detections and remedies, when in fact it was a lure to infect.
Targeted SMTP Threats
Targeted SMTP threats also represent an emerging attack vector through which malware is propagated. As users adapt to widespread spam attacks, cybercriminals distribute crimeware to target one specific organization or industry, often for financial gain.
- Computer insecurity
- Digital Rights Management (DRM)
- Firewall (networking)
- Social engineering (security)
- Spy software
- Targeted threat
- Vulnerability (computing)
- ^ Wikipedia Hijacked to Spread Malware
- ^ "Protecting Corporate Assets from E-mail Crimeware," Avinti, Inc., p.1
- [Ne49] John von Neumann, "Theory of Self-Reproducing Automata", Part 1: Transcripts of lectures given at the University of Illinois, Dec. 1949, Editor: A. W. Burks, University of Illinois, USA, 1966.
- [Co86] Fred Cohen, "Computer Viruses", PhD Thesis, University of Southern California, ASP Press, 1988.
- [Ad88] L. M. Adleman, "An Abstract Theory of Computer Viruses", Advances in Cryptology---Crypto '88, LNCS 403, pages 354-374, 1988.
- [YY96] A. Young, M. Yung, "Cryptovirology: Extortion-Based Security Threats and Countermeasures," IEEE Symposium on Security & Privacy, pages 129-141, 1996.
- [Ze03] Zeltser, L, Skoudis, E, Stratton, W. O., Teall, H, "Malware: Fighting Malicious Code", Prentice Hall, 2003.
- [ToKa02] H. Toyoizumi, A. Kara. Predators: Good Will Mobile Codes Combat against Computer Viruses. Proc. of the 2002 New Security Paradigms Workshop, 2002
- [ TaKh06] Zakiya M. Tamimi, Javed I. Khan, Model-Based Analysis of Two Fighting Worms, IEEE/IIU Proc. of ICCCE '06, Kuala Lumpur, Malaysia, May 2006, Vol-I, Page 157-163.
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