I have viewstate logs every seconds :


User Agent:

Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/6.0)

It tries to hit every public page passing various parameters like __EVENTVALIDATION or __VIEWSTATE.

It's an IP from the United States while our website serves only Canadians. This really looks like an attack but we can't block it by IP because it is always from a new IP.

Any idea how to block them or what this type of attack is called?

  • 1
    You got me on this one. It will be interesting to see what people can come up with. Cheers! – closetnoc Feb 10 '16 at 18:43
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    Install Fail2Ban, or manually block them using IP Class C. – Simon Hayter Feb 10 '16 at 19:49

ASP.NET Event validation does provide some level of protection against some specific web security attacks.

Request Validation, a feature included in ASP.NET 1.1, may not adequately sanitize hostile user-supplied input. ASP scripts that depend on Request Validation to sanitize user-supplied input may still be prone to cross-site scripting or HTML injection attacks as a result. Request Validation may be bypassed by including a null byte (%00) in malicious user-supplied input.

Microsoft ASP.NET Request Validation Null Byte Filter Bypass Vulnerability

Microsoft has addressed this issue in a Hotfix rollup package and is available by contacting the vendor. Further information can be obtained from this link.

  • Thank you, At the moment, we use simple forms that never pass __Eventvalidation as a url parameter. So at the moment, if we detect a url that includes this parameter, we redirect the user to a 404 page. I think this will do the trick for the moment. – Yannick Richard Feb 11 '16 at 14:25

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