Why is ASP.NET accepting externally created session identifiers?

Question

I have an ASP.NET 3.5 Web Site using the standard SQL Membership Provider.

The application has to pass the IBM Rational AppScan before we can push to production.

I am getting the error:
Severity: High
Test Type: Application
Vulnerable URL: http://mytestserver/myapp/login.aspx
Remediation Tasks: Do not accept externally created session identifiers

What can I do to fix this?

I am using SQL Membership Provider. Is this related? I am using the standard login controls too. I have the "Remember Me" turned off, and hidden.

Thanks.

Answer 1

This isn't a vulnerability (and I really don't like AppScan because of its false positives - the number of times I've had to explain CSRF cookies need not be linked to a session on my little open source project is getting annoying).

All that will happen in this case is the first time anything is stored in session state with a created session identifier a new session will be opened on the server, with nothing in it. If you're worried about session fixation then you can clear the cookie after authentication.

Session.Abandon();
Response.Cookies.Add(new HttpCookie("ASP.NET_SessionId", ""));

But with forms authentication the authentication details are not held in the session and so fixation is not a problem at all.

Frankly if you must pass security scans without anyone evaluating if the results are not false positives then that's a whole different problem.

Answer 2

You might need to change the default cookie settings to be unique to you app

Try setting a unique cookie path:

<forms name="YourAppName"
       path="/FormsAuth" ... />

http://msdn.microsoft.com/en-us/library/ms998310.aspx#paght000012_additionalconsiderations

More reading... http://msdn.microsoft.com/en-us/library/ms998258.aspx