Validation of viewstate MAC failed - Error Occurs for 1 User Alone
One user of a website that I support receives the following error whenever performing a particular web based task:

    System.Web.HttpException: Validation of viewstate MAC failed

This error only ever occurs for a single user browsing from an IE7 Browser. 

I have crossed off the following usual suspects:

- The web-server is not a part of a web farm
- I have added code to ensure that the ViewState from being rendered at the beginning of the form to prevent the problem where the user performs a second post the Validate ViewState tag is rendered.

This form contains a drop-down box where the contents are modified client side via JavaScript. Could this cause the error for a particular user?

Is it possible that the there is something malicious running on the client side for that PC that is playing with the ViewState?
Validation of viewstate MAC failed - Error Occurs for 1 User Alone
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One user of a website that I support receives the following error whenever performing a particular web based task:

    System.Web.HttpException: Validation of viewstate MAC failed

This error only ever occurs for a single user browsing from an IE7 Browser. 

I have crossed off the following usual suspects:

- The web-server is not a part of a web farm
- I have added code to ensure that the ViewState from being rendered at the beginning of the form to prevent the problem where the user performs a second post the Validate ViewState tag is rendered.

This form contains a drop-down box where the contents are modified client side via JavaScript. Could this cause the error for a particular user?

Is it possible that the there is something malicious running on the client side for that PC that is playing with the ViewState?