Introduction

Two years ago I was facing a nice security problem when I start dealing to prevent to go back in my asp.net application and avoiding a refresh page already visited.

This request came from my boss. He was angry to see how the user after logging off from app can go back to my app just by heating the navigator's back button. This means that the logged user can go away from my app on typing in the address bar: www.codeproject.com and came back again to my app easily. My boss asked me how to prevent this because many of users can use the app on public computers and can forget to close the app before leaving.

To get this done, of course I started searching on the net and discussing this matter on many forums. At first I find an article on code project but it doesn't fit my needs. And I came to the point, with my colleague Mathieu that we should deal with this at our ends.

So this is the solution I came up with:

Our security solution reposes on a simple yet very effective method to track what pages have been seen so far by the user (based on each session individually, do not base this on the whole server).

Simply described

Pass in a GET parameter (or query string parameter) a random page number of your choice. This random number is generated by your server ( c# function) and each time a number is generated, it is kept in a mechanism on the server. You could use files, databases, caching or session variables. Its up to you. This list will grow adding all the random numbers already visited. When a page requested contains the same number as one used before, you may assume that the page is a FAVORITE link that has been used or a BACK in the browser history since the page numbers should never repeat. It is as simple as that and prevents any refresh of the current page but also prevents going BACK in the history of the browser.

Process is simple

Look for a parameter passed in the GET vars (Query string). If this variable doesn't exist, bump to the login screen.

If the variable is found, look it up in the tracking system to match the pages seen. If the page number was already used, bump to the login screen

Add the current page number to the tracking system (this number must not be used anymore)

Create a new page number from a random system avoiding numbers already used

Append this new page number to all links in the page

From this point on, any link will look at least like :

asdf.aspx?random_page_number=987

And the number should always change automatically on each page request. All the links in the page ask for the same number and it's normal. The point of adding this number to the links makes it impossible for a user to go back in its history of press back since he will ask for a URL containing the random variable number that was already used before when the page was asked.

Potential security flaws

It is possible for anyone to copy a URL and append whatever number he knows has not been used yet; therefore, the person can re-ask the page anew. A quick fix to that is to create a hash of the page number. (Get something complex that is very hard to crack, not just a sha1 or md5, that's too easy) That hash number must be based on the random number already generated. If you append this hash with the random_page_number you created, just add one more step to your initial verification to recreate the hash from the page number. If the new generated hash and the provided hash are the same, you can assume either the person has cracked your hash code algorithm therefore your security is cracked or that the hash provided and the page number have not been tempered with…

Problems concerning this method

This method obviously takes a lot of space in memory to work with depending on the way your implement the page tracking. It's certain that if you have 5 pages to visit max, a random number of 0 to 99 is great; this means the person can visit a total of 100 pages before touching the limit. This then bumps the user back to the login and the tracking system is reset for this user's session. Why put a limit? Simple, first of all random number generation has to have a limit; second, you want to limit the size of the tracking stack. Having too many items in the stack depending on your tracking method can be dangerous. For example, we chose a simple session based tracking with 0 to 999. This limits us to 1000 page navigated, but the problem is not there, imagine the numbers 0 to 999 lined together with commas. This makes the session data very heavy in the long run. Databases will handle this more gracefully but to the cost of adding several new SQL queries on each page just for that...

Conclusion

Good system, lots of alternative ways to build it, lots of alternative ways to secure it. Have fun with it…

prevent a goback to the previous page and refresh the current page in asp.net 1.0 app