Event Log - Viewstate verification failed

Recently we upgraded our system from .net 1.1 to .net 2.0. Since doing so we have been getting errors in our event logs with the following error about every minute. It's weird but all client ips or user host address seems to be pointing to eastern European countries like Russia or Belarus. Is it a logging problem or is somebody legitimately trying to hack or something? -

Information 8/2/2011 15:02  ASP.NET 2.0.50727.0 1316    Web Event   Event code: 4009 
Event message: Viewstate verification failed. Reason: Viewstate was invalid.                    
Event time: 8/2/2011 3:02:36 PM                     
Event time (UTC): 8/2/2011 7:02:36 PM                   
Event ID: e25e0918f9e34bda98abcafadc61a0b6                  
Event sequence: 144401                  
Event occurrence: 5595                  
Event detail code: 50204                    

Application information:                    
    Application domain: OMMITED-OMMITED             
    Trust level: Full                   
    Application Virtual Path: /DirID                    
    Application Path: W:\SITE\DirID\                    
    Machine name: OMMITED-OMMITED                       

Process information: 
    Process ID: 1740 
    Process name: w3wp.exe 
    Account name: NT AUTHORITY\SYSTEM 

Request information: 
    Request URL: http://www.mysite.com/DirID/Default.aspx 
    Request path: /DirID/Default.aspx 
    User host address: 176.14.136.181 
    User:  
    Is authenticated: False 
    Authentication Type:  
    Thread account name: NT AUTHORITY\SYSTEM 

ViewStateException information: 
    Exception message: Invalid viewstate.                   
    Client IP: 176.14.136.181                   
    Port: 63815                     
    User-Agent: TrackChecker                    
    PersistedState: [KEY1]                  
    Referer: http://www.mysite.com/DirID/Default.aspx                   
    Path: /DirID/Default.aspx                   
-------------------------
Information 8/2/2011 14:57  ASP.NET 2.0.50727.0 1316    Web Event   Event code: 4009 
Event message: Viewstate verification failed. Reason: Viewstate was invalid.                    
Event time: 8/2/2011 2:57:11 PM                     
Event time (UTC): 8/2/2011 6:57:11 PM                   
Event ID: 4d814be560f64258b2c926814fdb10c6                  
Event sequence: 142726                  
Event occurrence: 5536                  
Event detail code: 50204                    

Application information:                    
    Application domain: OMMITED-OMMITED                     
    Trust level: Full                   
    Application Virtual Path: /DirID                    
    Application Path: W:\SITE\DirID\                    
    Machine name: OMMITED-OMMITED    

Process information: 
    Process ID: 1740 
    Process name: w3wp.exe 
    Account name: NT AUTHORITY\SYSTEM 

Request information: 
    Request URL: http://www.mysite.com/DirID/Default.aspx 
    Request path: /DirID/Default.aspx 
    User host address: 213.87.131.86 
    User:  
    Is authenticated: False 
    Authentication Type:  
    Thread account name: NT AUTHORITY\SYSTEM 

ViewStateException information:                     
    Exception message: Invalid viewstate.                   
    Client IP: 213.87.131.86                    
    Port: 21441                     
    User-Agent:                     
    PersistedState: [KEY1]                  
    Referer: http://www.mysite.com/DirID/Default.aspx                   
    Path: /DirID/Default.aspx                   
-----------
Information 8/2/2011 14:56  ASP.NET 2.0.50727.0 1316    Web Event   Event code: 4009 
Event message: Viewstate verification failed. Reason: The viewstate supplied failed integrity check.                    
Event time: 8/2/2011 2:56:10 PM                     
Event time (UTC): 8/2/2011 6:56:10 PM                   
Event ID: e20e446446374000bf9ad9c6863192e8 
Event sequence: 142476 
Event occurrence: 5534 
Event detail code: 50203 

Application information: 
    Application domain: OMMITED-OMMITED 
    Trust level: Full 
    Application Virtual Path: /DirID 
    Application Path: W:\SITE\DirID\ 
    Machine name: OMMITED-OMMITED   

Process information: 
    Process ID: 1740 
    Process name: w3wp.exe 
    Account name: NT AUTHORITY\SYSTEM 

Request information: 
    Request URL: http://www.mysite.com/DirID/Default.aspx 
    Request path: /DirID/Default.aspx 
    User host address: 85.174.246.134 
    User:  
    Is authenticated: False 
    Authentication Type:  
    Thread account name: NT AUTHORITY\SYSTEM 

ViewStateException information: 
    Exception message: Invalid viewstate. 
    Client IP: 85.174.246.134 
    Port: 3957 
    User-Agent: TrackChecker 
    PersistedState: dDwxNTA2NDg4MjAzO3Q8O2w8aTwzPjs+O2w8dDw7bDxpPDE+O2k8OT47aTwxMT47aTwxMz47aTwxNT47aTwxNz47aTwxOT47aTwyMT47aTwyMz47aTwyND47aTwyNT47aTwyNj47aTwzMj47aTwzND47PjtsPHQ8O2w8aTwzPjtpPDE5PjtpPDQxPjs+O2w8dDxwPH开发者_JAVA百科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 
    Referer: http://www.mysite.com/DirID/Default.aspx                   
    Path: /DirID/Default.aspx   

The first 2 requests have caused the viewstate verification/validation issues because of this: PersistedState: [KEY1] - this is a validation error straight away.

Also - you say you've upgraded from .Net 1.1 to 2.0 But the viewstate supplied in the 3rd request starts with "dDw" - this is a .Net 1.1 viewstate (for .Net 2.0 it starts with "/wE")

Seeing "TrackChecker" in the user agent tells me that some kind of bot/crawler saved older versions of your pages (when they were generated by .Net 1.1 - including the viewstate) and now it is re-checks your content and submits invalid viewstates (.Net 1.1 viewstates will fail validation on .Net 2.0 for obvious reasons)

I get a lot of these Viewstate errors on one of my sites, and it's normally a bot trying its luck to post something nefarious.

I suspect the same here - unless you have a lot of users from Belarus?

If you amend your logs to also capture the query string and other request params, that can give you some clues as to what the (alleged) attacker - or unfortunate user - was trying to achieve.