安全校验Session验证码并避免绕开验证码攻击

<%
If Request.Form("SecurityCode") = Session("SecurityCode") Then
' TODO : Database operations
Else
Response.Write "Security code incorrect!"
End If
%>

' str为要校验的验证码，len为验证码长度
Function IsSecurityCodeValid(str, len)
IsSecurityCodeValid = Not CBool( _
IsEmpty(str) Or CStr(str)="" Or Len(str)<len)
End Function
If IsSecurityCodeValid(Request.Form("SecurityCode"), 4) AND _
IsSecurityCodeValid(Session("SecurityCode"), 4) AND _
Request.Form("SecurityCode") = Session("SecurityCode") Then
' TODO : Database operations
Else
Response.Write "Security code incorrect!"
End If