A very few day ago i had to face the situation where hacker uploaded the malicious ASP script to the web server by changing its extension to .JPG through user interface which allow user to upload image file. Although developer team had put the validation on extension. But unfortunately extension of ASP script was .JPG and hence it is allowed extension. So hacker could upload that malicious script.
After this situation i just checking only extension for uploaded file is not the sufficient. We need to check content as well of the uploaded file.
Image File Header Information Table
|
Image File Header Information Table |
|||
|
File Format |
Offset |
Length |
Value |
|
JPG / JPEG |
0 |
4 |
0xFF, 0xD8, 0xFF, 0xE0 |
|
PNG |
0 |
4 |
0x89, 0x50, 0x4E, 0x47 |
|
TIF / TIFF |
0 |
4 |
0x49, 0x49, 0x2A, 0x00 |
|
GIF |
0 |
4 |
0x47, 0x49, 0x46, 0x38 |
|
BMP |
0 |
2 |
0x42, 0x4D |
|
ICO |
0 |
4 |
0x00, 0x00, 0x01, 0x00 |
Instead of checking only header, we could also check whole file content against its file format. But checking only header could serve our purpose and it is also speedy process than checking whole file content so i am not checking whole file content.
Following code snippet validate header of known image types (JPG, PNG, TIFF, GIF, BMP, ICO) Please do let me know if i have missed any image types.
NOTE: In code snippet fuImage refer to ASP.NET file upload control
protected void btnUpload_Click(object sender, EventArgs e)
{
// DICTIONARY OF ALL IMAGE FILE HEADER
Dictionary<string, byte[]> imageHeader = new Dictionary<string, byte[]>();
imageHeader.Add("JPG", new byte[] { 0xFF, 0xD8, 0xFF, 0xE0 });
imageHeader.Add("JPEG", new byte[] { 0xFF, 0xD8, 0xFF, 0xE0 });
imageHeader.Add("PNG", new byte[] { 0x89, 0x50, 0x4E, 0x47 });
imageHeader.Add("TIF", new byte[] { 0x49, 0x49, 0x2A, 0x00 });
imageHeader.Add("TIFF", new byte[] { 0x49, 0x49, 0x2A, 0x00 });
imageHeader.Add("GIF", new byte[] { 0x47, 0x49, 0x46, 0x38 });
imageHeader.Add("BMP", new byte[] { 0x42, 0x4D });
imageHeader.Add("ICO", new byte[] { 0x00, 0x00, 0x01, 0x00 });
byte[] header;
if (fuImage.HasFile)
{
// GET FILE EXTENSION
string fileExt;
fileExt = fuImage.FileName.Substring(fuImage.FileName.LastIndexOf('.') + 1).ToUpper();
// CUSTOM VALIDATION GOES HERE BASED ON FILE EXTENSION IF ANY
byte[] tmp = imageHeader[fileExt];
header = new byte[tmp.Length];
// GET HEADER INFORMATION OF UPLOADED FILE
fuImage.FileContent.Read(header, 0, header.Length);
if (CompareArray(tmp, header))
{
lblMessage.Text = "Valid ." + fileExt + " file.";
// VALID HEADER INFORMATION
// CODE TO PROCESS FILE
}
else
{
lblMessage.Text = "Invalid ." + fileExt + " file.";
// INVALID HEADER INFORMATION
}
}
else
{
lblMessage.Text = "Please select image file.";
}
}
private bool CompareArray(byte[] a1, byte[] a2)
{
if (a1.Length != a2.Length)
return false;
for (int i = 0; i < a1.Length; i++)
{
if (a1[i] != a2[i])
return false;
}
return true;
}
Any input on above is greatly appreciated...
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