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Introduction to ASP.Net Viewstate

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Introduction to ASP.Net Viewstate by Dhiraj Ranka @ null Mumbai Meet, November, 2010

Introduction to ASP.Net Viewstate by Dhiraj Ranka @ null Mumbai Meet, November, 2010

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  • 1. Introduction to ASP.NET ViewState
    - By DhirajRanka (NII Consulting)
  • 2. HTTP Protocol
    Stateless 
    Uses cookies, sessions to maintain state
    Where is ViewState in picture?
  • 3. ASP.NET Page life cycle
    verifying file access rights
    resurrecting the user's session state
    (HTTP modules)
  • 4. Page Life Cycle events
  • 5. View State
    persist state across post backs
    store name and value pairs
    “What sort of state needs to be persisted?”
    in instantiation stage, control hierarchy is created
    For example,
    <asp:Labelrunat="server" Font-Name="Verdana" Text="Hello, World!"></asp:Label>
  • 6. The View State Property
    Every control can stores its state
    Example
    public string NavigateUrl
    {
    get {
    string text = (string) ViewState["NavigateUrl"];
    if (text != null)
    return text;
    else
    return string.Empty;
    }
    set {
    ViewState["NavigateUrl"] = value;
    }
    }
    whenever a control's property is read, the control's ViewState is consulted
  • 7. Storing Information in the Page's ViewStateProperty
    persist page-specific and user-specific information across postbacks
    Code would look like
    ViewState[keyName] = value
    example - creating a pageable, sortable DataGrid
    sort expression must be persisted across postbacks
  • 8. The Cost of View State
    Nothing comes for free, and view state is no exception
    two performance hits for every request
    In save view state control hierarchy is saved in base-64 encoded string which is emitted in “__VIEWSTATE” hidden form field and in load view state the same is decoded and control hierarchy is updated
    extra size to the Web page. Some times for view state-heavy pages can be tens of kilobytes 
  • 9. Enabling/Disabling ViewState
    At control level
    <asp:LabelEnableViewState=“false”…></asp:Label>
    At individual page level
    <%@Page EnableViewState="False" %>
    Page.EnableViewState = false;
    At site/application level (in web.config)
    <pages enableViewState="true">
  • 10. View State and Security Implications
    Its nothing but base-64 encoded string
    Attacker can easily decode it, manipulate and encode back before sending it
  • 11. Prevention
    Tamper proofing
    Encryption
    The ViewStateUserKey Property
  • 12. Tamper Proofing
    Use a machine authentication check, or MAC
    It ensure that the data received by a computer is the same data that it transmitted
    hashing the view state data and appending this hash to the end of the view state
    When post back occurs it checks to ensure that the appended hash matches up with the hashed value
    default hashing algorithm used is SHA1 (MD5)
  • 13. Generic Error
  • 14. Encryption
    To encrypt the view state, set the <machineKey> element's validation attribute in the machine.config file to 3DES
    Can be SHA1, MD5, AES
    the <machineKey> element contains validationKey and decryptionKeyattributes
    validationKeyattribute for MAC
    decryptionKeyattribute for 3DES
    Default values are “AutoGenerate,IsolateApp”
  • 15. Concerns with Encryption
    Previous settings works fine with single web server
    In web farm, it's vital that all Web servers use the same keys for MAC and/or encryption and decryption
    Use shared key among all web servers
  • 16. The ViewStateUserKeyProperty
    Must be assigned a string value in initialization stage(Page_Init event)
    User-specific value as username, but can be guessable
    Recommended value is SessionID
    Code would look this (Page_Init event)
    Page.ViewStateUserKey = Session.SessionID;
  • 17. Attacker(Evil Bob) requests a page
    Server salting ViewState hash using attacker’s username(Evil Bob)
    Attacker tricks Normal user(Alice) to request server passing in his view state
    Web server notices that Alice’s ViewStateUserKey(“Alice”) doesn’t match up with the appended key (“Evil bob”) – Exception is thrown
  • 18. Thank you