Security question: Using ajax and events to keep session alive

I know a few sites (suc开发者_如何学Ch as my bank and my school) that kill a session after their has been idle for a set amount of time. It is my understanding that session activity is determined by users following links or at the very least from some kind of active interaction, like updating a form via ajax. Basically the server gets a request to do something during the session and goes ahead and extends the session time another 15 minutes.

But on some occasions I have lost major amounts of time and info while filling out a text box or reading some long set of instructions along the way.

So why not have an ajax script that listens for keyboard activity and mouse movement and lets the server know that the user is still there and active, even if they aren't clicking a submit button or following a link?

I was wondering if anyone knew of respectable sites that already do this, or if I was overlooking some major security hazard with this idea.

The only thing I can imagine would be risky are the random acts of cats, vibrating electronics nearby, or a hyper child.

But in all of the above, the user is most likely at home and -- unless they are trying to get exploited -- have probably minimized the window and thus these things are very unlikely to trigger as an event.

Does anybody see any other major risks?

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Typical AJAX sites are making posts back to the server anyway. These events are renewing the users session already.

If you put these events on keyboard or mouse clicks, how many times are you going to be posting to the server? If I am typing in a form field like I am now, that means you could potentially have a ping to the server for each letter I type; not a very efficient solution. On the other end, what if your user is just sitting their reading or using an external text editor to type their text into and will copy it into your form later.

I think the more typical solution to provide a friendly UI so that long posts do not get dropped because of a session expiration is to use an auto-save feature. Google Docs does this. Every few seconds/minutes, they post the contents of the editor back to the server without the user actually clicking save/submit. The other option is to inform the user that their session is about to expire (could be done with a javascript timeout). Provide a link to ping the server to renew the session.

Your solution lends itself to the same problem: you are relying on user behavior. In the first case, navigating between pages and in the second, mouse clicks.

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I used to work for a company that did a lot of online contests where users would have to enter essay content as well as shorter blocks of data; we used to modify the session time out "session.setMaxInactiveInterval()" for the user's session when they hit the "long-winded" page so that they would have more time to edit, and then we would set it back to normal after the submit.

Later at that company and a couple others I worked at I proposed a solution similar to what you are describing, but for various reasons it was never accepted. It was never considered a bad idea, just not one we chose. Basically it was going to be an ajax call on a timer so that just before the session timed out, it would fire off a light-weight ajax "ping" and keep the session alive as long as that page was open. I have never had the chance to implement it in the "real world" so perhaps there are negatives that I have not thought of.

Good luck.