Customers may not know the difference between a virus and a worm, but understanding the different types of malware will help you protect their systems -- and pass the SSCP certification exam. This excerpt, from the Official (ISC)²® Guide to the SSCP® CBK® Domain 7: Malicious Code, by Diana-Lynn Contesti, takes a look at double file extensions. Download the full Domain as a PDF to learn about the different types of malware and their payloads.
Viruses, worms and Trojan horses all make use
of the double file extension. The Windows operating systems allows the creation
of files names with a number of spaces in it. This trick is intended to
fool users into believing that the file they are viewing cannot be executed.
as in this example:
PLAIN.TXT.EXE
The .EXE at the end of the spaces, makes the program executable. Unfortunately
in e-mail, users will only see the .TXT and potentially believe that
the file is simply a Text file. This is why much has been done to educate users
on not running e-mail attachments.
As a number of file extensions can be used to deliver or contain malicious
code, it is recommended that the administrators block specific File Extensions
at the Firewall. Table 7.3 is a partial list of suggested file extensions
that should be blocked.
It is difficult for end users to understand all the file extensions that can
be used and those that may be considered dangerous or Executable. Therefore,
it is a good idea to develop a list of extensions that will be blocked at
the Firewall by default. Every organization is unique and the list that is correct
for one organization may not be correct for another. It is a good idea to
educate users on some of the basic file extensions that you may not be able
to block (i.e., .EXE, .PIF, .SCR, .COM).
A complete list of file extensions and their meanings is available at
The File Extension Source; also view Every File Extension in the World from WhatIs.com.
Table 7.3 A Partial List of File Extensions That Should Be Blocked
| File Extension |
Descriptions |
| .API |
Acrobat Plug-in
Used to view Adobe Acrobat files |
| .BAT |
Batch processing file
Used to execute a series of commands in a sequential order |
| .BPL |
Borland package libraries
Used in programs developed with the Delphi software language |
| .CHM |
Compiled HTML Help file
Could include a link that would download and execute malicious code |
| .COM |
Command File
Contains scripts and executables for DOS or Windows |
| .DLL |
Dynamic Link Library
Executable code that is shared by other programs on the system |
| .DRV |
Device Driver
Used to extend the hardware support of a Windows machine |
| .EXE |
Windows binary executable program |
| .OCX |
Object linking and embedding (OLE) control
Used to orchestrate the interaction of several programs on a Windows
machine |
| .PIF |
Program Information File
Used to tell windows how to run non-Windows applications |
| .SCR |
Screen saver programs
Includes binary executable code |
| .SYS |
System configuration file
Used to establish system settings |
| .VB |
Visual Basic® files (VBE and VBS)
Used to script in visual basic which is built into many Windows-based
machines |
| .WSH |
Windows Script Host Settings File
Used to configure the script interpreter program on Windows machines |
Official (ISC)²® Guide to the SSCP® CBK®
By Diana-Lynn Contesti, Douglas Andre, Eric Waxvik, Paul A. Henry, Bonnie A. Goins
Published by (ISC)2 Press
ISBN # 9780849327742; Copyright 2007; Pages: 573; Edition: 1st
Chapter: Domain 7: Malicious Code
By Diana-Lynn Contesti, CISSP-ISSAP, ISSMP, SSCP
