ViewState

ViewState is a feature specific to [[ASP.NET]], a web application framework developed by Microsoft. It is used to persist the state of web pages and controls between HTTP requests. In the stateless nature of web browsing, ViewState provides a way to store values across postbacks (when a user interacts with a form and sends it back to the server).

When a user interacts with a web page, such as entering data into a form field, the state of the page (like the contents of the form fields) needs to be maintained when the form is submitted. ViewState helps in preserving this information.

ViewState is stored in a hidden form field with the name __VIEWSTATE. This field contains a [[Base64]]-encoded string, representing the state of the page when it was last processed on the server. The state of the controls on the page is serialized to a string and sent to the client as part of the HTML. When the page is posted back to the server, the server deserializes the string to restore the state of the controls.

ViewState can become large if a lot of data needs to be maintained, which can increase page load times and affect performance. If not properly encrypted or validated, ViewState can be a vector for attacks. Attackers might tamper with the data in the ViewState, leading to issues like [[Cross-Site Scripting]] (XSS) or [[ViewState Tampering|ViewState tampering]] attacks.

To secure ViewState, it's essential to encrypt the ViewState data and use [[Message Authentication Code (MAC)|message authentication codes (MACs)]] to ensure the data has not been tampered with. ASP.NET provides options for encrypting and validating ViewState.

ViewState is commonly used to maintain the state of controls on a web page between postbacks. For example, if a user enters text into a textbox and submits the form, ViewState can be used to ensure the text stays in the textbox when the page reloads. ViewState is suitable for storing small amounts of data that are specific to a particular page, like user inputs and selections.

ViewState is a Base64-encoded string that represents the state of the page's controls and is used to maintain this state across postbacks. It's placed in a hidden field in the HTML of the page. An example token may be:

<input
  type="hidden"
  name="__VIEWSTATE"
  id="__VIEWSTATE"
  value="/wEPDwUKLTk1MzY5NTQwNGRkx5bdB2CfJsF+8o5cFT+zeV8="
/>
  • The name and id are set to __VIEWSTATE, which is standard for ASP.NET applications.
  • The value attribute contains the ViewState data. This is a Base64-encoded string, which, when decoded, contains the state of the page's controls.

Important

The actual content of the ViewState value is specific to the state of the particular page and its controls. It can vary greatly in length and complexity.

In a secure ASP.NET application, this value should also be integrity-protected (with a MAC) and potentially encrypted, depending on the sensitivity of the data it contains and the application's security configuration. The example shown here is a simplified representation and does not include these security features.