Validation of viewstate MAC failed.

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

Hello Friend, I have face this issue many time on online live website project.

Solution : 

Set the machinekey in the web config file.

If you want to generate the machinkey as per .net framework or see the below generated key.

Click Hear

Framework version

Asp.net 1.1

<machineKey validationKey="4B6124F1A96C1D90C2EB5298094895EBB9B1AB2227971EAC82E46D6E9B3E9
76CE56A7629E8C4E6C1AEF7A3B4F47681B60119D792EA47613971729564072DEB2A"
decryptionKey="4DD6849D7B818F86F6566B4AEDF1A9BD134DA1994236F8B8"
validation="SHA1" /> 

Asp.net 2.0

<machineKey validationKey="713BBE182A80928A389952B90A305817D43D87DF110B8F1BF4CF3D5628013F
8F93460838C28092EE99106FA2056191A9E3D9D4412607ADB55FE61193CFE30112"
decryptionKey="146AEAA8D1D915B020764CAF11EBD91A4C63B7D8898D13BDA7915F2E14E
92B41" validation="SHA1" decryption="AES" />

Asp.net 4.0

<machineKey validationKey="9BDD1AED72B3669A61961D2E1140865B6737FD394BCA2F4923C90F23CDCF
EE4E522A6E63EF6DA2BCD2DF736C917BBD47EEDCC570B46A623906334757A53ACF21"
decryptionKey="628176C339754EFADF3449C40A400ED185EC0DEBE8290331E3BD70AF463B
BDFA" validation="SHA1" decryption="AES" />

Asp.net 4.5

<machineKey validationKey="4047C7FA412278CF4A5D19BC670739DC9526FFB72F96C29D37A58807E1E4D8
374B6A19C97F7F27C73DB8AC81F5FA389138A9EB3F4752A578035966D5763BB72D"
decryptionKey="93BD9A1F3D1281400E1F5A3CCFF831F1249DC86930226D6B987E00007B989
DD5" validation="SHA1" decryption="AES" />