Internet Security and Privacy - The File-Extensions List

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In this list we use the file-extensions to lookup if a file could be dangerous or not. The file-extension is the code, including the period, following the file-name. File-extensions are used by the system to determine the different file types, such as image-files, spreadsheets, text-documents, system-files and so on. Thanks to the extensions, the computer "knows" with which program he must open a file (e.g. to view a photo with an image viewer and not with a word-prcessing program). But you don't need to know much about these extensions to detect dangerous files. Just remember that some types of files can be dangerous, while others can not. Safe file-types, such as jpeg-images and many more, do not contain any system code, while dangerous file-types do contain code which may manipulate the system.

In the list, the [*] symbol before each extension replaces the filename. The filename can be anything and has no importance, it's only the extension that matters here. After each extension, you can see what kind of file it belongs to (the file-type), but no need to remember that, just checking the extension will do.
All files with these extensions are potentially dangerous, but of course, this does not necessarily mean they contain a virus, it's only possible. Most of the legitime system files on a computer have one of the potentially dangerous extensions, because these files can and may manipulate the system.

As there are hundreds of file-extensions in use, and given the continue changes in technology, no list can ever be complete. But here you find the most common used file-types which are used to spread viruses and other malware.



Remarks on the file-extension list

A lot of file-formats can be considered as safe, other are safe enough when regarding some precautions.

Graphic Formats

most graphic ("picture or image") formats are safe:

the most common (pixel-orientated) graphic formats such as *.jpg; *.jpeg; *.bmp; *.gif; *.tif; *.tiff; and others are safe

but -although also used to display graphics- some vector-based graphic formats can be dangerous (e.g. the *.wmf (Windows MetaFile) format)

if the *.pcd format is used in a Kodak Photo CD Image it's normally safe if you received your photo-cd from a trusted vendor; only if there could be tampered with the code there could be a security issue

(note: for more (not-security-related) info on graphic files, see the topics on file- and graphic formats)

Text Format


the raw text format (*.txt) is safe

Microsoft Office Documents


Word-, Excel-, Access- and other MsOffice-documents with the extensions *.doc(x); *.xls(x); and so on, are virtually always safe, although they can contain malicious code embedded in macro-scripts; but when using the default settings, Ms Office will detect the code and prompt the user what to do; in case of doubt, click "no" to open the file without executing the macro-code (it's also a good idea to check the appropriate security level settings in the different components of the MsOffice program suite)




documents in the Adobe Portable Document Format (*.pdf) may contain embedded malicious Acrobat JavaScript code which may be executed by Adobe Acrobat Reader


Multimedia, Audio and Movie Files


media-files, such as *.avi; *.mp3; *.wmv; *.mov; *.mpg; *.mpeg; *.ram; *.rm; and many more... are also safe most of the time, although some of them (e.g. *.asx; *.wmv and other) may contain malicious code (e.g. code to redirect you to a website which installs unwanted items onto your system) you can avoid this by playing the files with a media player which does not read the malicious code, instead of the standard

windows media player (look for media player in the software top-list selection)



widely used compressed archive-formats are (among others) *.zip and *.rar (files with extension '.zip' or '.rar')

archive-files are single files containing other files in a compressed format; this is done 'cause of two major reasons: by compressing the files into an archive, a lot of space can be saved, resulting in an archive-file smaller than all the separate non-compressed files together, and secondly, if you download a program from the internet, you only need to download one single archive-file, instead of all the program files separately, this making the download faster and more secure

concerning virii and malicious code, it's obvious that it's impossible to determine if an archive-file is dangerous or not, without first knowing what's in it; so the archive must be opened to check the files within

this can be done without actually unpacking and/or opening the separate files in the archive, with the help of a file-compression program (viewing the contents of an archive is harmless, just unpacking a malicious file is harmless too, as long as you do not open (and "execute") the file)

then, if -as an example- you received an archive-file, and it contains only pictures (with "safe" extensions such as *.jpg; *.bmp and so on), it's safe to unpack the archive and open the files; so you only have to check the file-extensions in the list (more info about archive-files, unpacking and installing programs in the software installation info-topics)



The "potentially dangerous" file-extension list :



*.exe Windows Executable (Program)
*.hlp Windows Help-File
*.hta Hypertext Program/Script
*.inf Setup-Information
*.ins Windows Internet Naming Service
*.isp Internet Communication Setup-File
*.its Internet Document Set
*.js JavaScript Source Code
*.jse JavaScript Coded Scriptfile
*.key Registry Entries
*.mht Web Archive File
*.msc Microsoft Common Console Document
*.msh Microsoft-Shell
*.msi Microsoft Windows Installer-Package
*.msp Microsoft Windows Installer-Update
*.mst Ms Win Installer/Ms Visual Test Source File
*.nch Outlook Express Folder File
*.ops Ms Office Profile Settings File
*.pdf Adobe Portable Document Format
*.pif Shortcut to MS-DOS-Program
*.plg Developer Studio Build LogFile
*.prf Microsoft Outlook Profile Settings
*.prg Program File
*.pst Ms Exchange / Ms Outlook Address File
*.rar Compressed Archive File
*.reg Registry Entries
*.scf Windows Explorer Command
*.scr Screensaver Program
*.shb Windows Document Shortcut
*.shs Shell Scrap-Object
*.url Internet-Shortcut
*.vb VBScript File
*.vbe VBScript Coded Script File
*.vbs VB Script File / VBS Script File
*.wmf Windows MetaFile (Graphic Vector Format)
*.ws Windows Script File
*.wsc Windows Script Component
*.wsf Windows Script File
*.wsh Windows Script Host Setup-File
*.zip Compressed Archive File



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Related topics : File formats and extensions - Basic security and privacy - E-mail security and privacy - Web security and privacy


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