By default Rich Text Editor will encode the value in the Hidden field automatically. You can turn it off by setting this property to "false". If this property is set to false, ValidateRequest="false" in the page directive is required.

public bool EncodeHiddenValue { get; set; }

Public Property EncodeHiddenValue As Boolean
	Get
	Set

public:
property bool EncodeHiddenValue {
	bool get ();
	void set (bool value);
}