File Signature Checker

This .NET Standard 2.0 library contains code for checking file signatures in their byte marks.

The file signatures were taken fromt his helpful resource: https://filesignatures.net/ (Thanks to them!)

Supported Extensions

It currently contains file signatures for the following extensions:

• .png
• .jpg
• .jpeg
• .gif
• .doc
• .docx
• .ppt
• .pptx
• .xls
• .xlsx
• .pdf
• .mp3
• .wav
• .ogg
• .mpg
• .m4a
• .xml

I will add more signatures if they are requested, or if I desire them.

Base 64 Images

There is a method for checking base 64 strings to see whether they are images.

Use as FileSigCheck.IsValidBase64Image(base64string).

Currently, It will strip the metadata from the start of the string, ignoring the "image/*" MIME declarations.

Usage

A typical scenario in which I have implented this is with a file upload in .NET Core API:

static readonly string[] fileExtensions = { ".pdf", ".jpg", ".jpeg", ".png" };

public async Task<IActionResult> UploadImage([FromForm]IFormFile file)
{
    var ext = Path.GetExtension(file.FileName);

    /* Check the file extension in permissible file types */
    if(!fileExtensions.Contains(ext))
    {
        return StatusCode(StatusCodes.Status415UnsupportedMediaType);
    }
    /* Get the file data */
    using (var rs = file.OpenReadStream())
    {
        /* perform check  */
        if (!FileSecurity.IsValidFileSignature(file.FileName, rs, fileExtensions))
        {
            ModelState.AddModelError("File", $"Is not recognised as a {Path.GetExtension(file.FileName)} file.");
            return BadRequest(ModelState);
        }

        /* Then do what you would normally do with that file... */
    }

    return Ok();
}