Validating an uploaded file's ContentType in ASP.NET

I'm storing some files in my database and since I'm storing them in binary format and not keeping any other information, I have to make sure that all of them are in the same format so that I'll be able to "serve" them later (If there's a simple way to infer the file type from a byte array, please tell, but that's not the focus here).

So, what I need to do is validate every file that is uploaded to make sure it's on the required format.

I've set up a FieldTemplate with a FileUpload control and a CustomValidator:

<asp:FileUpload ID="FileUpload" runat="server" />&nbsp;


<asp:CustomValidator
    ID="CustomValidator1"
    runat="server"
    ErrorMessage="PDF only."
    ControlToValidate="FileUpload"
    OnServerValidate="CustomValidator1_ServerValidate">
</asp:CustomValidator>

What I'm missing is the code to place in that CustomValidator1_ServerValidate method that checks the uploaded file to make sure it's in the right format (PDF in this case).

Thanks in advance.

Answers


Use the FileUpload.PostedFile.ContentType property to validate the MIME type ( should be application/pdf ). For security reasons, also validate that the file extension is appropriate ( .pdf ). You could have a static hashtable containing mappings from MIME type to file extension(s) and use as lookup to validate an extension.


Like ary said. This can all be spoofed. Take a .txt file, rename it to a pdf file and try getting the content type. It will be "application\pdf".

However there is one solution that I have used before. During my brief test with the PDF files, I figured out that the first 3 bytes were always the same. I tried only the first 3 bytes because it seemed enough. The value for the first three bytes is : 37, 80, 68.

So I read the bytes (InputFile1.FileContent.ReadByte()), compared them to the 3 bytes above and if they were the same, then I had a PDF file. Also I read somewhere that you should turn off the script execution for the upload directory in IIS. Hope it helps.


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