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Externe Links: » Session fixation
Das Session-Modul bietet keine Garantie dafür, dass Informationen, die Sie in einer Session speichern, nur vom Benutzer gesehen werden können, der die Session erzeugt hat. Sie müssen zusätzliche Maßnahmen ergreifen, um die Integrität der Session ihrer Wichtigkeit entsprechend angemessen aktiv zu schützen.
Schätzen Sie die Wichtigkeit der Daten ab, die in Ihren Sessions transportiert werden und treffen Sie zusätzliche Schutzmaßnahmen - in der Regel bezahlen Sie dafür mit einer geringeren Benutzerfreundlichkeit. Wenn Sie z.B. Benutzer vor einfachen Social Engineering Tactics (Anm. des Übersetzers: Techniken der Ausnutzung menschlicher Schwächen) schützen wollen, müssen Sie session.use_only_cookies aktivieren. Cookies müssen dann benutzerseitig auf jeden Fall aktiviert sein, weil Sessions sonst nicht funktionieren.
Es gibt mehrere Wege, über die eine Session-ID an Dritte gelangen kann. Eine entführte Session-ID ermöglicht diesen, auf alle Daten zuzugreifen, die mit dieser Session-ID verbunden sind. Zum einen sind das URLs, die Session-IDs enthalten. Wenn Sie auf eine externe Site verweisen, könnte die URL inklusive Session-ID in den Referrer-Logs der externen Site gespeichert werden. Zum anderen kann ein aktiverer Angreifer Ihren Netzwerkverkehr abhören. Falls Ihr Netzwerkverkehr nicht verschlüsselt ist, werden Session-IDs im Klartext über das Netzwerk übertragen. Hier ist die Lösung, auf Ihrem Server SSL zu implementieren und die Verwendung für Ihre Benutzer obligatorisch zu machen.
One thing I do is encrypt everything server side that passes into my $_SESSION array. This might be overkill, but if anyone can see or gets access to my $_SESSION array it'll be encrypted gibberish and my system won't process anything coming from the $_SESSION array unless it matches my encryption key.
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.
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.
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
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;
}
}
?>
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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