I am working on an ASP web page that handles file uploads. Only certain types of files are allowed to be uploaded, like .XLS, .XML, .CSV, .TXT, .PDF, .PPT, etc.
I have to decide if a file really has the same type as the extension shows. In other words if a trojan.exe was renamed to harmless.pdf and uploaded, the application must be able to find out that the uploaded file is NOT a .PDF file.
What techniques would you use to analyze these uploaded files? Where can I get the best information about the format of these files?
One way would be to check for certain signatures or magic numbers in the files. This page has a handy list of known file signatures and seems quite up to date:
http://www.garykessler.net/library/file_sigs.html