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Email is almost a necessity nowadays. Very few companies and business can survive without this form of communication whether it is emailing a simple message, travel itinerary, purchase confirmation, forgotten passwords or a whole gamut of other things. If email consisted of only text messages and no file attachments it would be virtually harmless. The thing that makes email dangerous are those file attachments and the recipients not understanding the consequences of double-clicking an executable file.

The problem with email and why viruses, trojan horses and worms are so dangerous is that it offers a very easy medium for hackers to spread their malicious code. It preys on people who don’t understand what an executable file is or what it can do. In short, an executable file is code which can be written a whole multitude of programming languages, that runs locally on your PC. Once run locally, it can do whatever it was designed to do, which might be displaying a picture, installing malware, deleting files, installing a program to capture and record key presses to remember passwords and account numbers and then email the results to the hacker. The code can do anything the creator wants; benign or malicious.

Below is a list of file extensions which are considered executable by the Windows XP operating system. You should never double-click, run or save any file with the extensions listed below unless you are expecting the file attachment and it is coming from someone you know. Just because someone you know sends you a file doesn’t mean it’s safe. Their computer could have easily been hijacked and the hacker is sending his malicious code to all people in your friends address book; that being you.

Below is a list of file extensions that are associated with executible code. You do not want to save these files, double-click them, or execute them because they could easily contain malicious code:

ADE - Microsoft Access Project Extension
ADP - Microsoft Access Project
BAS - Visual Basic Class Module
BAT - Batch File
CHM - Compiled HTML Help File
CMD - Windows NT Command Script
COM - MS-DOS Application
CPL - Control Panel Extension
CRT - Security Certificate
DLL - Dynamic Link Library
DO* - Word Documents and Templates
EXE - Application
HLP - Windows Help File
HTA - HTML Applications
INF - Setup Information File
INS - Internet Communication Settings
ISP - Internet Communication Settings
JS - JScript File
JSE - JScript Encoded Script File
LNK - Shortcut
MDB - Microsoft Access Application
MDE - Microsoft Access MDE Database
MSC - Microsoft Common Console Document
MSI - Windows Installer Package
MSP - Windows Installer Patch
MST - Visual Test Source File
OCX - ActiveX Objects
PCD - Photo CD Image
PIF - Shortcut to MS-DOS Program
POT - PowerPoint Templates
PPT - PowerPoint Files
REG - Registration Entries
SCR - Screen Saver
SCT - Windows Script Component
SHB - Document Shortcut File
SHS - Shell Scrap Object
SYS - System Config/Driver
URL - Internet Shortcut (Uniform Resource Locator)
VB - VBScript File
VBE - VBScript Encoded Script File
VBS - VBScript Script File
WSC - Windows Script Component
WSF - Windows Script File
WSH - Windows Scripting Host Settings File
XL* - Excel Files and Templates