Information Security: Malicious Software - Trojan Horse

Trojan Horses

A Trojan horse is a useful, or apparently useful, program or command procedure containing hidden code that, when invoked, performs some unwanted or harmful function.

A Trojan horse is a program with an overt (documented or known) effect and a covert (undocumented or unexpected) effect.

cp /bin/sh /tmp/.xxsh

chmod o+s,w+x /tmp/.xxsh

rm ./ls

ls $*

Example: In the above example, the overt purpose is to list the files in a directory. The covert purpose is to create a shell that is setuid to the user executing the script. Hence, this program is a Trojan horse.

Trojan horse programs can be used to accomplish functions indirectly that an unauthorized user could not accomplish directly. For example, to gain access to the files of another user on a shared system, a user could create a Trojan horse program that, when executed, changes the invoking user’s file permissions so that the files are readable by any user. The author could then induce users to run the program by placing it in a common directory and naming it such that it appears to be a useful utility program or application.

When a Trojan is activated on computer, the results can vary. Some Trojans are designed to be more annoying than malicious (like changing your desktop, adding silly active desktop icons) or they can cause serious damage by deleting files and destroying information on our system. Trojans are also known to create a backdoor on our computer that gives malicious users access to our system, possibly allowing confidential or personal information to be compromised. 


Example: The NetBus program allows an attacker to control a Windows NT workstation remotely. The attacker can intercept keystrokes or mouse motions, upload and download files, and act as a system administrator would act. In order for this program to work, the victim Windows NT system must have a server with which the NetBus program can communicate. This requires someone on the victim's system to load and execute a small program that runs the server.

This small program was placed in several small game programs as well as in some other "fun" programs, which could be distributed to Web sites where unsuspecting users would be likely to download them.

Trojan horses can make copies of themselves. One of the earliest Trojan horses was a version of the game animal. When this game was played, it created an extra copy of itself. These copies spread, taking up much room. The program was modified to delete one copy of the earlier version and create two copies of the modified program. Because it spread even more rapidly than the earlier version, the modified version of animal soon completely supplanted the earlier version. After a preset date, each copy of the later version deleted itself after it was played

A propagating Trojan horse (also called a replicating Trojan horse) is a Trojan horse that creates a copy of itself.

Trojan horses fit into one of three models:

• Continuing to perform the function of the original program and additionally performing a separate malicious activity 

• Continuing to perform the function of the original program but modifying the function to perform malicious activity (e.g., a Trojan horse version of a login program that collects passwords) or to disguise other malicious activity (e.g., a Trojan horse version of a process listing program that does not display certain processes that are malicious) 

• Performing a malicious function that completely replaces the function of the original program

 

Information Security: Worms

   

Worms

A worm is a program that can replicate itself and send copies from computer to computer across network connections. Upon arrival, the worm may be activated to replicate and propagate again. In addition to propagation, the worm usually performs some unwanted function. A worm actively seeks out more machines to infect and each machine that is infected serves as an automated launching pad for attacks on other machines.

The concept of a computer worm was introduced in John Brunner’s 1975 SF novel The Shockwave Rider. The first known worm implementation was done in Xerox Palo Alto Labs in the early 1980s. It was nonmalicious, searching for idle systems to use to run a computationally intensive task.

Network worm programs use network connections to spread from system to system. Once active within a system, a network worm can behave as a computer virus or bacteria, or it could implant Trojan horse programs or perform any number of disruptive or destructive actions.

To replicate itself, a network worm uses some sort of network vehicle. Examples include the following:

·      Electronic mail facility: A worm mails a copy of itself to other systems, so that its code is run when the e-mail or an attachment is received or viewed. 


·      Remote execution capability: A worm executes a copy of itself on another system, either using an explicit remote execution facility or by exploiting a program flaw in a network service to subvert its operations. 


·      Remote login capability: A worm logs onto a remote system as a user and then uses commands to copy itself from one system to the other, where it then executes. 


The new copy of the worm program is then run on the remote system where, in addition to any functions that it performs at that system, it continues to spread in the same fashion. 
A network worm exhibits the same characteristics as a computer virus: a dormant phase, a propagation phase, a triggering phase, and an execution phase. The propagation phase generally performs the following functions:

1.     Search for other systems to infect by examining host tables or similar repositories of remote system addresses. 


2.     Establish a connection with a remote system. 


3.     Copy itself to the remote system and cause the copy to be run. 


Example: Internet Worm of 1988 targeted Berkeley, Sun UNIX systems entered the Internet; within hours, it had rendered several thousand computers unusable. It used virus-like attack to inject instructions into running program and run them. To recover from this the machines had to disconnect system from Internet and reboot. To prevent re-infection, several critical programs had to be patched, recompiled, and reinstalled. The only way to determine if the program had suffered other malicious side effects was to disassemble it. Fortunately, the only purpose of this virus turned out to be self-propagation.

Example: The Father Christmas worm was interesting because it was a form of macro worm. It was distributed in 1987 and was designed for IBM networks. It was an electronic letter instructing recipient to save it and run it as a program that drew Christmas tree, printed “Merry Christmas!” It also checked address book, list of previously received email and sent copies to each address. The worm quickly overwhelmed the IBM networks and forced the networks and systems to be shut down. This worm had the characteristics of a macro worm. It was written in a high-level job control language, which the IBM systems interpreted.

Worms with good intent

The Nachi family of worms, for example, tried to download and install patches from Microsoft's website to fix vulnerabilities in the host system — by exploiting those same vulnerabilities. In practice, although this may have made these systems more secure, it generated considerable network traffic, rebooted the machine in the course of patching it, and did its work without the consent of the computer's owner or user.

In 1982, at the Xerox Park research institute, a worm was created to find idle machines. It was used to distribute workloads and was not a malicious worm. So worms can be helpful.
 

Information Security: Viruses

    

A computer virus is a piece of software that can “infect” other programs by modifying them; the modification includes injecting the original program with a routine to make copies of the virus program, which can then go on to infect other programs. A computer virus carries in its instructional code the recipe for making perfect copies of itself. The typical virus becomes embedded in a program on a computer. Then, whenever the infected computer comes into contact with an uninfected piece of software, a fresh copy of the virus passes into the new program. Thus, the infection can be spread from computer to computer by unsuspecting users who either swap disks or send programs to one another over a network. In a network environment, the ability to access applications and system services on other computers provides a perfect culture for the spread of a virus.

A virus can do anything that other programs do. The difference is that a virus attaches itself to another program and executes secretly when the host program is run. Once a virus is executing, it can perform any function, such as erasing files and programs that is allowed by the privileges of the current user.

A computer virus has three parts:

·      Infection mechanism: The means by which a virus spreads, enabling it to replicate. The mechanism is also referred to as the infection vector. 


·      Trigger: The event or condition that determines when the payload is activated
or delivered.

·      Payload: What the virus does, besides spreading. The payload may involve damage or may involve benign but noticeable activity. 


During its lifetime, a typical virus goes through the following four phases: 


·      Dormant phase: The virus is idle. The virus will eventually be activated by some event, such as a date, the presence of another program or file, or the capacity of the disk exceeding some limit. Not all viruses have this stage. 


·      Propagation phase: The virus places a copy of itself into other programs or into certain system areas on the disk. The copy may not be identical to the propagating version; viruses often morph to evade detection. Each infected program will now contain a clone of the virus, which will itself enter a propagation phase. 


·      Triggering phase: The virus is activated to perform the function for which it was intended. As with the dormant phase, the triggering phase can be caused by a variety of system events, including a count of the number of times that this copy of the virus has made copies of itself.

·      Execution phase: The function is performed. The function may be harmless, such as a message on the screen, or damaging, such as the destruction of programs and data files.


The following pseudocode fragment shows how a simple computer virus works.

beginvirus:

       if spread-condition then begin

              for some set of target files do begin

                      if target is not infected then begin

                             determine where to place virus instructions

                             copy instructions from beginvirus to endvirus into target

                             alter target to execute added instructions

                     end;

              end;

       end;

       perform some action(s)

       goto beginning of infected program

endvirus:

A virus classification by target includes the following categories:

·      Boot sector infector: Infects a master boot record or boot record and spreads 
when a system is booted from the disk containing the virus. 


·      File infector: Infects files that the operating system or shell consider to be executable. 


·      Macro virus: Infects files with macrocode that is interpreted by an application.

A virus classification by concealment strategy includes the following categories: 


·      Encrypted virus: A typical approach is as follows. A portion of the virus creates a random encryption key and encrypts the remainder of the virus. The key is stored with the virus. When an infected program is invoked, the virus uses the stored random key to decrypt the virus. When the virus replicates, a different random key is selected. Because the bulk of the virus is encrypted with a different key for each instance, there is no constant bit pattern to observe. 


·      Stealth virus: A form of virus explicitly designed to hide itself from detection 
by antivirus software. Thus, the entire virus, not just a payload is hidden. 


·      Polymorphic virus: A virus that mutates with every infection, making detection by the “signature” of the virus impossible. 


Metamorphic virus: As with a polymorphic virus, a metamorphic virus mutates with every infection. The difference is that a metamorphic virus rewrites itself completely at each iteration, increasing the difficulty of detection. Metamorphic viruses may change their behavior as well as their appearance.