As an example, consider a hypothetical gourmet food e-commerce web site. This site displays a map of the world to the user, and as the user navigates the mouse pointer over each country, the page uses Ajax programming to connect back to the web server and retrieve a list of goods originating in that country. The following C# code snippet shows the web method in which the database is queried:

[System.Web.Services.WebMethod]
public System.Collections.IEnumerable GetProducts(string country)
{
// update the select command to use the country parameter
this.SqlDataSource1.SelectCommand = "SELECT * FROM [Product] WHERE Country = '" + country + "'";
// query the database and return the results
return this.SqlDataSource1.Select(DataSourceSelectArguments.Empty);
}

Some readers may notice a glaring security hole in this code. The database query is being constructed on the fly with un-validated user input being sent directly to the database. This insecure programming technique creates a vulnerability to SQL injection attacks, which are potentially devastating to the web application and its users. SQL injection vulnerabilities allow attackers to execute their own SQL queries and commands against the database, rather than those that the developers of the web site intended. The entire database, including customer names, addresses, and credit card numbers, could be downloaded by such a command. The prices of the products could be modified. The entire database itself could be permanently deleted. Clearly, this is a very serious issue. If the developer fails to notice the problem, the next line of defense is the QA team.

The average developer will probably do a quick, cursory test of the application before passing it to the QA department, without checking thoroughly for SQL injection vulnerabilities or other important problems. Instead, he will mouse over a few countries on the map, check that the displayed results match those in the database, and then pass the code off. The average QA engineer typically will be much more thorough. He will mouse over every country on the map and check that the results match. He might even set up an automated test script that will mouse over every single pixel on the screen, and he will check to see if there are any errors in the Ajax programming or underlying page code. But, even this extreme level of thoroughness won’t be enough to find the SQL injection vulnerability. By using a web browser (or automated script recorded from a web browser) as his test tool, the tester has limited his potential requests to only those which the browser can send, and the browser is itself limited by the source code of the web page. In the example above, the browser would be limited to sending only valid country parameters to the GetProducts method, since only valid countries are present in the page code. In other words, no matter where the user (or QA engineer) navigates with the mouse, the only parameters that would be sent are “GBR”, “FRA”, “USA”, etc. Using only these valid, well-formed parameters will never reveal the SQL injection vulnerability. To do that, the QA team needs to expand their arsenal of test tools beyond browsers alone.

Since browsers are so limited, hackers generally don’t use them to break into web applications or execute their SQL injection attacks and other hacks. They use tools that operate at a much lower level, tools that are capable of sending raw HTTP requests to an address and displaying the raw HTTP response. Netcat is a popular tool for this purpose, as is telnet, which has the benefit of being installed by default with virtually every modern operating system. So, instead of opening a browser, navigating to a page, and viewing the rendered HTML response, the hacker types:

nc 172.16.60.250 80 GET / HTTP/1.0

and then receives the response:

HTTP/1.1 200 OK Server: Microsoft-IIS/5.0 Date: Wed, 10 May 2006 18:04:56 GMT …

While interacting with Ajax programming may not seem like it would generate a round-trip request to the server since it doesn’t refresh the entire page, under the covers it’s doing exactly that. Like programming in standard hyperlink navigation or form submission, Ajax programming actions always have an HTTP request and response. So, armed with his low-level HTTP requestor tool, the hacker is now free to make attacks on the application that could never be possible with a browser alone. Instead of sending “GBR” or “FRA”, he could send “XXX”, or “!@#$%”, or “x’ OR ‘1’ = ‘1”, which in this case would successfully exploit the SQL injection vulnerability.