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Decoding T-SQL `CAST` in C#/VB.NET: A Comprehensive Guide

In today’s digital landscape, security vulnerabilities like SQL injection attacks are unfortunately prevalent. One such attack, related to the notorious Asprox botnet, attempts to execute SQL commands through ASCII encoded BINARY strings. This can pose a serious security threat to your application. In this blog post, we’ll tackle the challenge of decoding these SQL commands specifically focusing on the CAST function.

Understanding the Problem

Let’s take a closer look at the situation at hand. An example of an encoded command looks like this:

DECLARE%20@S%20NVARCHAR(4000);SET%20@S=CAST(0x44004500...06F007200%20AS%20NVARCHAR(4000));EXEC(@S);--

In this, CAST(0x44004500...06F007200 AS NVARCHAR(4000)) is the part we need to decode, typically done within a SQL Server environment. However, for security reasons or convenience, you may want to do this outside of SQL Server, using C# or VB.NET.

Steps to Decode the T-SQL Command

1. Setting Up the Decode Tool

The first step is to create a simple tool which translates the encoded string without relying on SQL Server. The primary task is to decode the hexadecimal string.

2. Identify the Encoding Method

When dealing with hexadecimal values, it’s essential to understand the encoding method being used. In this example, the case involves converting a hexadecimal string to a character format.

3. Parsing the Hexadecimal String

You need to parse the hexadecimal string pair by pair. Since a byte is represented by two hexadecimal characters, we can construct a byte array and then convert this array into the corresponding string.

Here’s the crucial piece of code:

while (!boolIsDone)
{
    bytURLChar = byte.Parse(txtURLText.Text.Substring(intParseIndex, 2), System.Globalization.NumberStyles.HexNumber);
    bytURL[intURLIndex] = bytURLChar;
    intParseIndex += 2;
    intURLIndex++;

    if (txtURLText.Text.Length - intParseIndex < 2)
    {
        boolIsDone = true;
    }
}
txtDecodedText.Text = Encoding.UTF8.GetString(bytURL);

4. Using Double Conversion

An important concept here is the double conversion of the encoded string. The final decoding can be efficiently achieved by utilizing this method:

Convert.ToString(Convert.ToChar(Int32.Parse(EncodedString.Substring(intParseIndex, 2), System.Globalization.NumberStyles.HexNumber)));

This method takes the substring of two characters, parses it into an integer, and then converts it into its respective character.

5. Completing the Decoder

Finally, you can loop through the input, convert each byte pair appropriately, and concatenate these characters to form the final decoded string.

6. Testing Your Tool

After your tool is set up, run a few tests with known inputs to ensure that it accurately decodes them. If you encounter unexpected outputs, double-check your parsing logic and make sure you’re converting correctly.

Conclusion

Decoding T-SQL commands from encoded strings may seem daunting, but with the right approach using C#/VB.NET, it can be simplified significantly. The key is to understand the hexadecimal representation and to execute a proper parsing routine.

For those interested, I have also posted my little application on CodePlex, where the decoding logic is shared for anyone to modify or leverage in their own projects.

By following the steps laid out in this guide, you’ll be equipped to decode SQL commands effectively while enhancing the security of your applications.

Decoding T-SQL CAST in C#/VB.NET: A Comprehensive Guide