Sessions et sécurité

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Utiliser les sessions ne signifie pas que les données de session ne pourront être vues que par un seul utilisateur. Il est important de garder cela en tête lorsque vous stockez et affichez des données importantes. Lorsque vous stockez des données dans une session, il faut se demander quels seront les problèmes posés si quelqu'un d'autre accède à cette information, ou comment votre application est affectée si la session est en fait celle d'un autre.

Par exemple, si quelqu'un usurpe une session, il peut alors poster un message dans un forum sous une fausse identité. Quelle est la gravité de ce problème ? Ou bien, il peut accéder aux commandes d'un client, et même, modifier son panier d'achat. À priori, c'est moins problématique pour un fleuriste que pour un pharmacien. Si vous voulez résoudre ce souci de façon simple, il peut être utile d'activer session.use_only_cookies. Dans ce cas, les cookies devront être activés par le client, sinon, les sessions ne fonctionneront pas.

Les sessions reposent sur un identifiant de session, ce qui signifie que quelqu'un peut voler cet identifiant, rien qu'en volant l'ID. Ce vol peut être rendu très difficile, comme en utilisant les cookies, mais en aucun cas cela sera impossible. Les sessions dépendent aussi de la discipline de l'utilisateur qui referme son navigateur à la fin de la session pour tout clore proprement. De plus, même les cookies de session peuvent être surveillés sur un réseau, ou bien notés par un proxy car ils transitent en clair sur le réseau. Pour remédier à cela, vous devriez implémenter un chiffrage SSL sur votre plate-forme.

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34
Anurag Jain
5 years ago
Websites which have sensitive information need to be patched to ensure its not exploited because of session issues.

In earlier versions of apache cookie reliability was not assumed and hence the default method was always using url-rewrite which meant every url link, every form submission etc would have a PHPSESSID=<sessionid> passed along to inform the server about the active session. New versions have turned this off using

session.use_trans_sid = 0

in the /etc/php5/apache2/php.ini file.

Reasons?
Well one might safe the offline page as a bookmark or pass the link across to others not realizing that the session id information is also sent. So someone who quickly accesses these pages could possible get logged on, this was also true wrt search engines, and I guess in some cases it being seen as duplicate content as the same page will have a different session id every time the robots scan the website.

But having this set does not mean you are protected. Let me explain.
What prevents me from presetting the session id! Assume there is a banking site www.example.com which has a login screen at www.example.com/login.php
I can send you can email with a link to the bank site as http://www.example.com/login.php?PHPSESSID=12345
When you click on the link it presents the session id as 12345 rather then asking the server to generate a new one. This is called session fixation. Keep in mind even with session.use_trans_sid = 0 this will work as this sets it only not to use url-rewrite. To prevent this altogether set session.use_only_cookies = 1 which ensures that only cookies will be used, but this could cause problems when dealing with transaction which involve switch sites, i.e. siteA forwards to site B for payment which forwards to siteA for thank you, in which case a phpsessid inform might be used to revive the old session.

A good approach would always be to at the login screen and immediately post login to force a new session id generated using random numbers

session_start();
$newsessid = somerandomnumberfunction();
session_id($newsessid);

you can also use session_regenerate_id() function to generate a new id

session_start();
session_regenerate_id();

Also its always good to ensure every valid session is checked against an ip. One good method is to store the session id and remote ip information in a table, or better store the ip as a session variable itself, once the user logs in and ensure that this is continued for remaining pages for security. This ofcourse wont work when users use the same office or shared network as the ip to the outside world is the same.

https is always a good idea for sensitive sites, but keeping it persistent for all pages which use session is important if you really want a foolproof system else anyone can always sniff your packets.

So to quickly go through the bits

- set session.use_trans_sid = 0 in /etc/php5/apache2/php.ini file.
- Ensure you always use a new self generated session id on successful login attempt.
- Try setting session.use_only_cookies = 1 and check if all works fine.
- Use https throughout to ensure no one can sniff your session id.
- Store session id, remote IP information and compare for successive pages
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bmearns at ieee dot org
5 years ago
In addition to ip-address binding not always being effective, it can also prevent users connecting through a proxy-pool from even being able to use your site. In such a scenario, a person's IP address may very well change with every access.

If you're handling anything remotely secure, the only safe option is HTTPS. If the data doesn't need to be that secure, than you should not allow a high-jacked session to do too much damage. Basically, don't assume that a person really is who they pretend to be just because the session says a person authenticated with a username and password: it may have been that person who logged in, but that doesn't mean it's still that person. So if you're going to do something like change passwords or something, require them to authenticate again on the same form.

Of course this needs to be done in some secure way, as well, so that the password is not just floating over the network. A good way to do this is sending a nonce (number-used-once) along with the form and some javascript to concatenate the nonce to the password, then perform a cryptographic hash of the combined string. The resulting valid-once "password" should then be sent instead of the plain text password. To be effective, you also need to prevent people from using the form if they don't have JavaScript enabled. You can do this by disabling the form fields in HTML, then re-enabling them in JavaScript.
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2
justin at fatbird dot ca
5 years ago
IP checking is a sometimes useful feature with two limitations that are important to be aware of:

1. Anyone surfing behind a proxy (e.g., someone at work) will provide the proxy's IP, not his own.  Session ID replay attacks will not be prevented by IP checking for an attacker on the user's side of the proxy.

2. If the PHP application is behind a reverse proxy, the reverse proxy's IP address will be the only request IP seen by PHP, so IP checking is useless.
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JonathanFeller at NOSPAMgmx dot ch
5 years ago
Perhaps, you would also like to timeout a session after some idle time. I noticed that session.gc_maxlifetime is not suitable for this. So I used this code to do the job:

<?php
if (!isset($_SESSION['timeout_idle'])) {
   
$_SESSION['timeout_idle'] = time() + MAX_IDLE_TIME;
} else {
    if (
$_SESSION['timeout_idle'] < time()) {   
       
//destroy session
   
} else {
       
$_SESSION['timeout_idle'] = time() + MAX_IDLE_TIME;
    }
}
?>
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Olle Bergkvist
5 years ago
It is also quite important to (somehow) make sure that the cookies you're setting (including the session cookie) is only visible to the site that created it (or to other trusted sites only).

If the cookie's path is set to '/' (the whole domain), then any website on the same domain (might be lots of websites) _will_ get the cookie through HTTP headers and could possibly hijack your session.

One slightly acceptable protection would be to lock a session to one IP adress.
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