We have a semi-weird issue for our site.

Consistently, from 1 anonymous\* user the following error occurs:

> 
> Exception type: System.FormatException
> 
> 
> Exception message: Invalid length for
> a Base-64 char array.
> 
> 
> 

After some investigation, it appears the user is using some form of load-balancing firewall, as the IIS logs shows the requests originating from 2 different (but successive) IP's.

From what I can determine, disabling 'ViewStateMAC' should solve this issue.

I am not however sure, and without any way to test this with the user, I feel a bit reluctant to go ahead with that.

Has anyone experienced similar issues? How did you deal with them?

Server details:

Single server (Win2003) running from a single IP.

Update:

From what I can determine, ViewStateMAC is only for the server-side. My issue is due to a client posting back a single page with multiple IP's.

\* But from the same 2 IP's as determined from IIS logs. The user is also not malicious.