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We now examine the security properties of the action token scheme introduced in the previous section.

Web forms are event-driven. An event represents the idea that something happened (see 5 for a full discussion of events). An event is raised when the user clicks a button, or selects from a listbox, or otherwise interacts with the UI. Of course, in web applications these user interactions happen on the client s machine in the web browser, but ASP.NET events are handled on the server. For this to work, user interactions require a round trip the browser needs to send a

message to the server, and the server needs to respond to handle the event completely. This can take a while, so our hands are somewhat tied compared to classic Windows application event handling it s just not practical for ASP.NET to offer server-side event handlers for things like mouse move events. So ASP.NET offers only a limited set of events, such as button clicks and text changes. These are events that the user might expect to cause a significant change, and for which it s reasonable to perform a round trip to the server.

Postback events are those that cause the form to be posted back to the server immediately. These include click-type events, such as the button Click event. In contrast, many events are considered nonpostback, meaning that the form isn t posted back to the server immediately.

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You can force controls with nonpostback events to behave in a postback manner by setting their AutoPostBack property to true.

Nonpostback events are raised at the point at which ASP.NET discovers it needs to raise them, which may be some considerable time after the user performed the actions to which the events relate. For example, the TextBox web control has a TextChanged event. You wouldn t expect a web page to submit a form automatically the moment you typed into a text box, and so this is a nonpostback event. If the user fills in several text fields in a form, the server knows nothing about that this change in state happens on the client side, and it s only when the user clicks a button to submit the form that ASP.NET discovers the changes. So this is when it will raise TextChanged events for all the text boxes that changed. Consequently, you can expect to see multiple events during the handling of a single submission.

We have chosen the value of the token such that it is effectively unguessable. The token value is the output of a cryptographically strong MAC computation, which means that the attacker would need billions of attempts to guess the correct value with any reasonable probability (the specific numbers depend on the MAC function and the quality of the key material). Note that each guess has to be carried out from within the user s browser, (i.e., the malicious page would have to implement a loop in JavaScript that iterates over the guessed values of the token, which would likely reduce the attack rate to a few tens per second far slower than the rate usually assumed for offline attacks against cryptographic algorithms).

Users tend to expect controls in user interfaces to remember their state it s disconcerting when text boxes lose their content, or listboxes forget which item was selected. Sadly, the Web is inherently a stateless environment.* This means that every post to the server loses the state from previous posts, unless the developer takes great pains to preserve this session knowledge. The Web is rife with sites where you fill in a form, only for it to lose all of your data if anything goes wrong. Developers have to do a lot of extra work to prevent this. ASP.NET, however, provides support for maintaining some of the state automatically.

Whenever a web form is posted to the server, the server re-creates it from scratch before it is returned to the browser. ASP.NET provides a mechanism that automatically maintains state for server controls (ViewState). Thus, if you provide a list and the user has made a selection, that selection is preserved after the page is posted back to the server and redrawn on the client.

Every request for a page made to a web server causes a chain of events at the server. These events, from beginning to end, constitute the life cycle of the page and all its components. The life cycle begins with a request for the page, which causes the server to load it. When the request is complete, the page is unloaded. From one end of the life cycle to the other, the goal is to render appropriate HTML output back to the requesting browser.

Since ASP.NET is a server-side technology, its view of the lifetime of the page is quite different from the user s view. By the time the user sees the page, the server has already finished with it. Once the HTML has reached the browser, you can switch off and unplug the web server and the user will be none the wiser for as long as she s looking at that page.

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