Email, while useful, can be a dangerous thing. Reading an email is usually safe if you have the latest security patches, however some attachments can be harmful. Any type of file can be attached to an email, including .exe program files. Look for the common warning signs so you can avoid viruses, worms, and Trojans.
Do not open or download files with file extensions that do not look familiar to you. For example, if you don’t usually receive .dot files, then you should be cautious when you see one in your email. It is especially important to remember when you don’t recognize or trust the sender, or when you’re not expecting to receive an attachment.
Be exceptionally cautious of files with double extensions, such as reports.pdf.exe. The only extension that matters is the last one. In this example, EXE represents an executable file that will run when you download. Files with double extensions are almost always dangerous.
Be vigilant in what you open!
Potentially dangerous file types
.exe - EXEcutable. This is the most dangerous and common attachment
type. In a recent article, it was mentioned "think of it as possibly
executing your PC!"
.com - Another executable. Indicates COMmand files
.bat - BATch file. This runs a process when opened.
.vbs - VBScript file. These attachments automatically execute.
.scr - Screen Saver file. Can contain malicious code.
.pif - Performs text based command line functions in MS Dos
.doc - Microsoft Word, Wordpad. Only harmful if they contain macros
.xls - Excel spreadsheet data. Only harmful if they contain macros
.ppt - PowerPoint Presentation. Only harmful if they contain macros
.zip - compressed file storage
Below are more commonly safe e-mail attachments. A PDF can still be dangerous, if you are not expecting it, always double check.
.jpg or .jpeg - Picture file (Joint Pictures Expert Group)
.gif - Picture File (Graphics Interchange Format). Pictures can be
.mpg or .mpeg - Movie clip (Motion Picture Expert Group)
.bmp - Picture file (BitMaP)
.txt - ASCII Text file
.pdf - Portable Document Format. Can be read by Adobe Acrobat Reader