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Using escaping functions correctly can be very tricky In this subsection, we provide an example in which a second order SQL injection attack can occur if escaping functions are not used carefully and consistently We now describe our example At some point, user o'connor might want to change her password to be stronger, and may enter a password such as SkYn3t The web server code that creates the SQL statement to update the password might be constructed as follows: new_passwd = requestgetParameter("new_passwd"); uname = sessiongetUsername(); sql = "UPDATE USERS SET passwd='"+ escape(new_passwd) + "' WHERE uname='" + uname + "'"; Note that the new password is escaped, but the username is not (since the username was escaped when it was first chosen and stored in the database).

The End method in the APM returns any data that comes out of the operation. In this case, there s a single return value of IPHostEntry, but some implementations may return more by having out or ref arguments. Example 16-16 then prints the results, and finally prints the AsyncState property of the IAsyncResult, which will be 42 this is where the value we passed as the final argument to BeginGetHostEntry pops out. This is not the only way to use the Asynchronous Programming Model you are allowed to pass null as the delegate argument. You have three other options, all revolving around the IAsyncResult object returned by the Begin call. You can poll the IsCompleted property to test for completion. You can call the End method at any time if the work is not finished this will block until it completes.* Or you can use the Asyn cWaitHandle property this returns an object that is a wrapper around a Win32 synchronization handle that will become signaled when the work is complete. (That last one is rarely used, and has some complications regarding ownership and lifetime of the handle, which are described in the MSDN documentation. We mention this technique only out of a pedantic sense of duty to completeness.)

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* This isn t always supported. For example, if you attempt such an early call on an End method for a networking operation on the UI thread in a Silverlight application, you ll get an exception.

You are required to call the End method at some point, no matter how you choose to wait for completion. Even if you don t care about the outcome of the operation you must still call the End method. If you don t, the operation might leak resources.

Asynchronous operations can throw exceptions. If the exception is the result of bad input, such as a null reference where an object is required, the Begin method will throw an exception. But it s possible that something failed while the operation was in progress perhaps we lost network connectivity partway through some work. In this case, the End method will throw an exception. The Asynchronous Programming Model is widely used in the .NET Framework class library, and while it is an efficient and flexible way to support asynchronous operations, it s slightly awkward to use in user interfaces. The completion callback typically happens on some random thread, so you can t update the UI in that callback. And the support for multiple simultaneous operations, possible because each operation is represented by a distinct IAsyncResult object, may be useful in server environments, but it s often just an unnecessary complication for client-side code. So there s an alternative pattern better suited to the UI.

The SQL statement that is executed is UPDATE USERS SET passwd='SkYn3t' WHERE uname='o'connor' This statement will most likely generate an error because the quote between the o and the c in o'connor is interpreted as the end of the username string, and the remaining characters do not satisfy grammar for a properly formed SQL statement But it could have been worse If an attacker chooses a username such as admin' --, and changes her password (eg, to cracked), the SQL statement that will be executed is UPDATE USERS SET passwd='cracked' WHERE uname='admin' --' The preceding statement executes successfully and changes the password for the database administrator s account to one of the attacker s choice! This type of attack, in which data stored in the database is later used to conduct SQL injection, is sometimes referred to as second order SQL injection (Anley 2002).

Some classes offer an alternative pattern for asynchronous programming. You start an operation by calling a method whose name typically ends in Async; for example, the WebClient class s DownloadDataAsync method. And unlike the APM, you do not pass a delegate to the method. Completion is indicated through an event, such as the DownloadDataCompleted event. Classes that implement this pattern are required to use the SynchronizationContext class (or the related AsyncOperationManager) to ensure that the event is raised in the same context in which the operation was started. So in a user interface, this means that completion events are raised on the UI thread. This is, in effect, a single-threaded asynchronous model. You have the responsiveness benefits of asynchronous handling of slow operations, with fewer complications than multithreaded code. So in scenarios where this pattern is an option, it s usually the best choice, as it is far simpler than the alternatives. It s not always available, because some classes offer only the APM. (And some don t offer any kind of asynchronous API, in which case you d need to use one of the other multithreading mechanisms in this chapter to maintain a responsive UI.)

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