/ VIRUSES in .NET Encode QR Code ISO/IEC18004 in .NET / VIRUSES

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10.2 / VIRUSES use .net vs 2010 qr code jis x 0510 drawer todevelop denso qr bar code for .net Microsoft Windows Official Website Execution phase: The qr-codes for .NET 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.

Most viruses carry out their work in a manner that is specific to a particular operating system and, in some cases, specific to a particular hardware platform. Thus, they are designed to take advantage of the details and weaknesses of particular systems. V IRUS S TRUCTURE A virus can be prepended or postpended to an executable program, or it can be embedded in some other fashion.

The key to its operation is that the infected program, when invoked, will first execute the virus code and then execute the original code of the program. A very general depiction of virus structure is shown in Figure 10.1 (based on [COHE94]).

In this case, the virus code, V, is prepended to infected programs, and it is assumed that the entry point to the program, when invoked, is the first line of the program. The infected program begins with the virus code and works as follows. The first line of code is a jump to the main virus program.

The second line is a special marker that is used by the virus to determine whether or not a potential victim program has already been infected with this virus. When the program is invoked, control is immediately transferred to the main virus program. The virus program may first seek out uninfected executable files and infect them.

Next, the virus may perform some action, usually detrimental to the system. This action could be performed every time the program is invoked, or it could be a logic bomb that triggers only under certain conditions. Finally, the virus transfers control to the original program.

If the infection. program V : {goto main ; 1234567; subroutine infect-executable : {loop: file : get-random-executable-file; if (first-line-of-file 1234567) then goto loop else prepend V to file; } subroutine do-damage : {whatever damage is to be done} subroutine trigger-pulled : {return true if some condition holds} main: main-program : {infect-executable; if trigger-pulled then do-damage; goto next;}. next: }. Figure 10.1 A Simple Virus CHAPTER 10 / MALICIOUS SOFTWARE program CV : {goto mai n; 01234567; subroutine infect-executable : {loop: file : get-random-executable-file; if (first-line-of-file 01234567) then goto loop; (1) compress file; (2) prepend CV to file; } main: main-program : {if ask-permission then infect-executable; (3) uncompress rest-of-file; (4) run uncompressed file;} }. Figure 10.2 Logic for a Compression Virus phase of the program i s reasonably rapid, a user is unlikely to notice any difference between the execution of an infected and an uninfected program. A virus such as the one just described is easily detected because an infected version of a program is longer than the corresponding uninfected one. A way to thwart such a simple means of detecting a virus is to compress the executable file so that both the infected and uninfected versions are of identical length.

Figure 10.2 [COHE94] shows in general terms the logic required. The key lines in this virus are numbered, and Figure 10.

3 [COHE94] illustrates the operation. We assume that program P1 is infected with the virus CV. When this program is invoked, control passes to its virus, which performs the following steps: 1.

For each uninfected file P2 that is found, the virus first compresses that file to produce P 2 , which is shorter than the original program by the size of the virus. 2. A copy of the virus is prepended to the compressed program.

3. The compressed version of the original infected program, P , is uncompressed. 1 4.

The uncompressed original program is executed.. CV 3 4 1 P1 P2 Figure 10.3 A Compression Virus 10.2 / VIRUSES In this example, the v QRCode for .NET irus does nothing other than propagate. As previously mentioned, the virus may include a logic bomb.

INITIAL INFECTION Once a virus has gained entry to a system by infecting a single program, it is in a position to potentially infect some or all other executable files on that system when the infected program executes. Thus, viral infection can be completely prevented by preventing the virus from gaining entry in the first place. Unfortunately, prevention is extraordinarily difficult because a virus can be part of any program outside a system.

Thus, unless one is content to take an absolutely bare piece of iron and write all one s own system and application programs, one is vulnerable. Many forms of infection can also be blocked by denying normal users the right to modify programs on the system. The lack of access controls on early PCs is a key reason why traditional machine code based viruses spread rapidly on these systems.

In contrast, while it is easy enough to write a machine code virus for UNIX systems, they were almost never seen in practice because the existence of access controls on these systems prevented effective propagation of the virus. Traditional machine code based viruses are now less prevalent, because modern PC OSs do have more effective access controls. However, virus creators have found other avenues, such as macro and e-mail viruses, as discussed subsequently.

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