Thursday, February 24, 2005

ViewState in ASP.NET 1.x

ViewState is the mechanism ASP.NET uses to keep track of server control state values that don't otherwise post back as part of the HTTP form.

Read Susan Warren's excellent article Taking a Bite Out of ASP.NET ViewState in more detail about ViewState. I list facts about ViewState related to security:

  • ViewState is merely base64-encoded
  • ASP.NET to append a hashcode to the ViewState field if the EnableViewStateMAC is enabled
  • By default, ASP.NET generates the ViewState hashcode using the SHA1 algorithm. You can select the MD5 algorithm by setting in the machine.config file
  • Encryption First, set EnableViewStatMAC="true". Then, set the machineKey validation type to 3DES.
  • ViewState Security on a Web Farm By default, ASP.NET creates a random validation key and stores it in each server's Local Security Authority (LSA). In order to validate a ViewState field created on another server, the validationKey for both servers must be set to the same value. The validation key is a string of 20 to 64 random, cryptographically-strong bytes, represented as 40 to 128 hexadecimal characters. A 128-character key is recommended for machines that support it.

When MAC checking is enabled, the serialized view state is appended a hash value that results from some server-side values and the view state user key, if any. Assuming a hacker has the skills to crack and rebuild the view state, he/she needs to know server-stored values to come up with a valid hash. Specifically, the hacker needs to know the machine key referenced in the machineKey entry of machine.config. So the view state is not at risk of tampering.

By default, the machineKey entry is autogenerated and physically stored in the Windows Local Security Authority (LSA). Only in case of Web farms—when the view state's machine keys must be the same on all machines—should you specify it as clear text in the machine.config file.

With the discussion above, ViewStateUserKey makes it much harder for hackers to use the content of the client-side view state to prepare malicious posts against the site.

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