Cisco Cisco Email Security Appliance C170 Guia Do Utilizador

Complex viruses avoid detection with signature scanning by using two popular techniques:

Encryption. The data inside the virus is encrypted so that anti-virus scanners cannot see the 
messages or computer code of the virus. When the virus is activated, it converts itself into a working 
version, then executes.

Polymorphism. This process is similar to encryption, except that when the virus replicates itself, it 
changes its appearance.

To counteract such viruses, the engine uses a technique called emulation. If the engine suspects that a 
file contains such a virus, the engine creates an artificial environment in which the virus can run 
harmlessly until it has decoded itself and its true form becomes visible. The engine can then identify the 
virus by scanning for a virus signature, as usual.

Using only virus signatures, the engine cannot detect a new virus because its signature is not yet known. 
Therefore the engine can use an additional technique — heuristic analysis.

Programs, documents or email messages that carry a virus often have distinctive features. They might 
attempt unprompted modification of files, invoke mail clients, or use other means to replicate 
themselves. The engine analyzes the program code to detect these kinds of computer instructions. The 
engine also searches for legitimate non-virus-like behavior, such as prompting the user before taking 
action, and thereby avoids raising false alarms.

By using these techniques, the engine can detect many new viruses.

When a virus has been detected, McAfee can repair (disinfect) the file. McAfee can usually repair any 
file in which a virus has been found, after which the file can be used without risk. The precise action 
taken depends on the virus. 

Occasionally, there can be limitations when it comes to disinfecting files because it is not always 
possible to return a file to its original state. Some viruses overwrite part of the executable program which 
cannot be reinstated. In this instance, you define how to handle messages with attachments that could 
not be repaired. You configure these settings on a per-recipient basis using the Email Security Feature: 
the Mail Policies > Incoming or Outgoing Mail Policies pages (GUI) or the 

policyconfig -> antivirus

 

command (CLI). For more information on configuring these settings, see 

.