window.clipboardData.getData("Text") returns 0 in IE8

I'm trying to implement maxlength on a textarea. In IE7, window.clipboardData.getData("Text") returns the correct number of characters copied. in IE8, the same call returns 0. What's wrong?

here is the js

var someRule= {
  "textarea" : function(element) {
    element.onpast开发者_如何学Pythone = function() {
      var copied = window.clipboardData.getData("Text");
      alert('copied length = '+copied.length);
    }
  }
};

Behaviour.register(someRule);

There is a security setting in IE8:

To prevent a web site from reading your clipboard, take the following steps:

Go to Tools->Internet Options. Click on the Security Tab. Click on "Custom Level." Scroll down to the Scripting section under Settings. Set "Allow paste operations via script" to Disable or Prompt. Press the OK buttons to close the dialog boxes.

In your case, this setting is probably disabled.