How to Structure VB.NET Class Securely and Correctly - asp.net

Question

I am having a really tough time with structuring a Class that is used throughout the application.
The problem is the exact same issue as described here:
Sitecore switches user session with another user
A session variable is being shared by any secondary user that logs in AFTER the first user.
My Class basically assigns a permission level to the session object, so that each page and or control can read it without doing a trip to the DB.
The Class Structure is as follows:
Public NotInheritable Class cPermissions
Public Shared Sub AssignPermissionToSession(ByVal UserID As Integer)
Private Shared Sub SetInSession(key As String, value As Dictionary(Of String, String))
Public Shared Function HasPerm(ByVal ControlName As String) as Boolean
End Class
So to explain the process: User Logs in. The login code calls cPermissions.AssignPermissionToSession(UserID). That Sub calls SetInSession which assigns the permission dictionary to a session object.
As a sidenote, I would prefer a NotInheritable class in this instance as it allows me to shorthand through the code, using the Shared Function HasPerm without instantiating the class. HasPerm reads the session object and returns true or false if the control has permission or not.
So back to the original problem, according to the previous link mentioned at the start, because the Sub is shared, I am experiencing session Hijacking.
But if I don't share the SetInSession sub, the shared sub AssignPermissionsToSession (which login needs to see) cant access it.
So basically I would really appreciate if someone could instruct me how to structure this class correctly without experiencing Accidental Session Hijacking.
Thanks in Advance!
The Code as requested is as follows (shrunk for brevity):
Public NotInheritable Class cPermissions
'local dictionary that gets created, and then assigned to session
Private Shared dPermissions As New Dictionary(Of String, String)
Public Shared Sub AssignPermissionToSession(ByVal UserID As Integer)
dPermissions.Clear()
'Here I open DB and get a list of Roles that each member may have
While ....
BuildPermissionArray()
End While
'Now dPermission should be created assign to session
SetInSession("Permissions", dPermissions)
End Sub
Private Shared Sub BuildPermissionArray()
'Here we create the local Dictionary ready for the session
'So for each role we get each permission for each control ie:
dPermissions.add(control,perm)
End Sub
Private Shared Sub SetInSession(ByVal key As String, value As Dictionary(Of String, String))
If value Is Nothing Then
HttpContext.Current.Session(key) = New Dictionary(Of String, Integer)
Else
HttpContext.Current.Session(key) = value
End If
End Sub
Public Shared Function HasPermission(ByVal PermissionType As Permission, ByVal ControlName As String) As Boolean
Dim obj As Object = HttpContext.Current.Session("Permissions")
Dim d As Dictionary(Of String, String) = DirectCast(obj, Dictionary(Of String, String))
'Here I search the dictionary and check relevant permission for the control name
Return result
End Function
End Class
Now the Login code calls:
cPermissions.AssignPermissionToSession(UserID)
And each page load reads the controls on the current page and calls:
cPermissions.HasPermission(View,"PageOrControlName")