Hijacking ASP.NET Sessions
So, you want to be able to access other user’s session state from the session id, right? Well, I don’t know if you should, but you definitely can do that! 
Here is an extension method for that purpose. It uses a bit of reflection, which means, it may not work with future versions of .NET (I tested it with .NET 4.0/4.5).
1: public static class HttpApplicationExtensions
2: {
3: private static readonly FieldInfo storeField = typeof(SessionStateModule).GetField("_store", BindingFlags.NonPublic | BindingFlags.Instance);
4:
5: public static ISessionStateItemCollection GetSessionById(this HttpApplication app, String sessionId)
6: {
7: var module = app.Modules["Session"] as SessionStateModule;
8:
9: if (module == null)
10: {
11: return (null);
12: }
13:
14: var provider = storeField.GetValue(module) as SessionStateStoreProviderBase;
15:
16: if (provider == null)
17: {
18: return (null);
19: }
20:
21: Boolean locked;
22: TimeSpan lockAge;
23: Object lockId;
24: SessionStateActions actions;
25:
26: var data = provider.GetItem(HttpContext.Current, sessionId.Trim(), out locked, out lockAge, out lockId, out actions);
27:
28: if (data == null)
29: {
30: return (null);
31: }
32:
33: return (data.Items);
34: }
35: }
As you can see, it extends the HttpApplication class, that is because we need to access the modules collection, for the Session module.
Use with care!