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Chapter: Cryptography and Network Security

Network Virus Countermeasures

Network Virus Countermeasures
The ideal solution to the threat of viruses is prevention: The next best approach is to be able to do the following:



Antivirus Approaches


The ideal solution to the threat of viruses is prevention: The next best approach is to be able to do the following:


·        Detection: Once the infection has occurred, determine that it has occurred and locate the virus.  


·        Identification: Once detection has been achieved, identify the specific virus that has infected a program.  


·        Removal: Once the specific virus has been identified, remove all traces of the virus from the infected program and restore it to its original state. Remove the virus from all infected systems so that the disease cannot spread further.  


If detection succeeds but either identification or removal is not possible, then the alternative is to discard the infected program and reload a clean backup version.


There are four generations of antivirus software:


·        First generation: simple scanners  


·        Second generation: heuristic scanners


·        Third generation: activity traps  


·        Fourth generation: full-featured protection  



A first-generation scanner requires a virus signature to identify a virus.. Such signature-specific scanners are limited to the detection of known viruses. Another type of first-generation scanner maintains a record of the length of programs and looks for changes in length.


A second-generation scanner does not rely on a specific signature. Rather, the scanner uses heuristic rules to search for probable virus infection. One class of such scanners looks for fragments of code that are often associated with viruses.


Another second-generation approach is integrity checking. A checksum can be appended to each program. If a virus infects the program without changing the checksum, then an integrity check will catch the change. To counter a virus that is sophisticated enough to change the checksum when it infects a program, an encrypted hash function can be used. The encryption key is stored separately from the program so that the virus cannot generate a new hash code and encrypt that. By using a hash function rather than a simpler checksum, the virus is prevented from adjusting the program to produce the same hash code as before.


Third-generation programs are memory-resident programs that identify a virus by its actions rather than its structure in an infected program. Such programs have the advantage that it is not necessary to develop signatures and heuristics for a wide array of viruses. Rather, it is necessary only to identify the small set of actions that indicate an infection is being attempted and then to intervene.


Fourth-generation products are packages consisting of a variety of antivirus techniques used in conjunction. These include scanning and activity trap components. In addition, such a package includes access control capability, which limits the ability of viruses to penetrate a system and then limits the ability of a virus to update files in order to pass on the infection.


The arms race continues. With fourth-generation packages, a more comprehensive defense strategy is employed, broadening the scope of defense to more general-purpose computer security measures.


Advanced Antivirus Techniques


More sophisticated antivirus approaches and products continue to appear. In this subsection, we highlight two of the most important.


Generic Decryption


Generic decryption (GD) technology enables the antivirus program to easily detect even the most complex polymorphic viruses, while maintaining fast scanning speeds . In order to detect such a structure, executable files are run through a GD scanner, which contains the following elements:


·        CPU emulator: A software-based virtual computer. Instructions in an executable file are interpreted by the emulator rather than executed on the underlying processor. The emulator includes software versions of all registers and other processor hardware, so that the underlying processor is unaffected by programs interpreted on the emulator.   


·        Virus signature scanner: A module that scans the target code looking for known virus signatures.  


·        Emulation control module: Controls the execution of the target code.  

Digital Immune System


The digital immune system is a comprehensive approach to virus protection developed by IBM]. The motivation for this development has been the rising threat of Internet-based virus propagation.Two major trends in Internet technology have had an increasing impact on the rate of virus propagation in recent years:


Integrated mail systems: Systems such as Lotus Notes and Microsoft Outlook make it very simple to send anything to anyone and to work with objects that are received.  


Mobile-program systems: Capabilities such as Java and ActiveX allow programs to move on their own from one system to another.

·        A monitoring program on each PC uses a variety of heuristics based on system behavior, suspicious changes to programs, or family signature to infer that a virus may be present. The monitoring program forwards a copy of any program thought to be infected to an administrative machine within the organization.


·        The administrative machine encrypts the sample and sends it to a central virus analysis machine.


·        This machine creates an environment in which the infected program can be safely run for analysis. Techniques used for this purpose include emulation, or the creation of a protected environment within which the suspect program can be executed and monitored. The virus analysis machine then produces a prescription for identifying and removing the virus.


·        The resulting prescription is sent back to the administrative machine.


·        The administrative machine forwards the prescription to the infected client.


·        The prescription is also forwarded to other clients in the organization.


·        Subscribers around the world receive regular antivirus updates that protect them from the new virus.


The success of the digital immune system depends on the ability of the virus analysis machine to detect new and innovative virus strains. By constantly analyzing and monitoring the viruses found in the wild, it should be possible to continually update the digital immune software to keep up with the threat.


Behavior-Blocking Software


Unlike heuristics or fingerprint-based scanners, behavior-blocking software integrates with the operating system of a host computer and monitors program behavior in real-time for malicious actions. Monitored behaviors can include the following:


·        Attempts to open, view, delete, and/or modify files;  


·        Attempts to format disk drives and other unrecoverable disk operations;  


·        Modifications to the logic of executable files or macros;  


·        Modification of critical system settings, such as start-up settings;  


·        Scripting of e-mail and instant messaging clients to send executable content; and  


·        Initiation of network communications.  


If the behavior blocker detects that a program is initiating would-be malicious behaviors as it runs, it can block these behaviors in real-time and/or terminate the offending software. This gives it a fundamental advantage over such established antivirus detection techniques as fingerprinting or heuristics.


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