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Session fixation

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Session fixation attacks attempt to exploit the vulnerability of a system which allows one person to fixate (set) another person's session identifier (SID).

Most session fixation attacks are web based, and most rely on session identifiers being accepted from URLs (query string) or POST data.

Attack scenarios

Alice is a nice girl who just wants to use her bank http://unsafe/. Unfortunately, Alice is not very security savvy.

Mallory is out to get Alice's money from the bank.

Alice has a reasonable level of trust in Mallory, and will visit links Mallory sends her.

A simple attack scenario

Straight forward scenario:

1. Mallory has determined that http://unsafe/ accepts any session identifier, accepts session identifiers from query string and has no security validation. http://unsafe/ is thus not secure.
2. Mallory sends Alice an e-mail: "Hey, check this out, there is a cool new account summary feature on our bank, http://unsafe/?SID=I_WILL_KNOW_THE_SID". Mallory is trying to fixate the SID to I_WILL_KNOW_THE_SID.
3. Alice is interested and visits http://unsafe/?SID=I_WILL_KNOW_THE_SID. The usual log-on screen pops up, and Alice logs on.
4. Mallory visits http://unsafe/?SID=I_WILL_KNOW_THE_SID and now has unlimited access to Alice's account.

Attack using server generated SID

A misconception is that servers which only accept server generated session identifiers are safe from fixation. This is false.

Scenario:

1. Mallory visits http://vulnerable/ and checks which SID is returned. For example, the server may respond: Set-Cookie: SID=0D6441FEA4496C2.
2. Mallory is now able to send Alice an e-mail: "Check out this new cool feature on our bank, http://vulnerable/?SID=0D6441FEA4496C2.
3. Alice logs on, with fixated session identifier SID=0D6441FEA4496C2.

Attacks using cross-site cooking

Another session fixation attack, cross-site cooking, exploits browser vulnerabilities. This allows a site http://evil/ to store cookies on browser in the cookie domain of another server http://good/, that is trusted. In order for this attack to succeed there is no need for any vulnerability within http://good/, as http://good/ may assume browser cookie management is secure.

Scenario:

1. Mallory sends Alice an e-mail: "Hey, check out this cool site, http://evil/".
2. Alice visits http://evil/, which will cookie SID with value I_WILL_KNOW_THE_SID into the domain of http://good/.
3. Alice then receives an e-mail from from Mallory, "hey check out your bank account at http://good/".
4. When Alice logs on Mallory may use her account using the fixated session identifier.

Counter-measures

Do not accept session identifiers from GET / POST variables

Session identifiers in URL (query string, GET variables) or POST variables are not recommended as it simplifies this attack - it is easy to make links or forms which sets GET / POST variables.

Additionally, session identifiers (SID) in query string enables other risk / attack scenarios as well;

• SID is leaked to others servers through the Referrer
• SID is leaked to other people as users cut & paste "interesting links" from the address bar into chat, forums, communities etc.
• SID is stored in many places (browser history log, web server log, proxy logs, ...)

Solution: Store session identifier in HTTP cookie

The session identifier is on most modern systems by default stored in an HTTP cookie which has a moderate level of security.

However, session identifiers are often accepted from GET/POST as well on these standard systems. Browser configuration must be modified in order to protect against this vulnerability.

For PHP, if you have access to PHP configuration, use this: php.ini:

; Whether to use cookies.
session.use_cookies = 1

; This option enables administrators to make their users invulnerable to
; attacks which involve passing session ids in URLs; defaults to 0.
session.use_only_cookies = 1

Solution: Utilize SSL / TLS Session identifier

When enabling HTTPS security, some system will allow applications to obtain the SSL / TLS session identifier. Use of the SSL/TLS session identifier is very secure, but many web development languages do not provide robust built-in functionality for this.

SSL/TLS session identifier may only be suitable for critical applications such as large financial sites due to the size of the system. It is however an issue rarely debated even in security forums.

Regenerate SID on each request

A countermeasure against session fixation is to generate a new session identifier (SID) on each request. Thus, although attacker may trick a user into accepting a known SID, the SID will be invalid when attacker attempts to re-use the SID. Implementation of such a system is simple, as demonstrated by the following:

• Get previous Session Identifier OLD_SID from HTTP request.
• If OLD_SID is null, empty, or no session with SID=OLD_SID exists, create a new session.
• Generate new session identifier NEW_SID with a secure random number generator.
• Let session be identified by SID=NEW_SID (and no longer by SID=OLD_SID)
• Transmit new SID to client.

Example:

If Mallory successfully tricks Alice into visiting http://victim/?SID=I_KNOW_THE_SID, this HTTP request is sent to victim:

GET /?SID=I_KNOW_THE_SID HTTP/1.1
Host: victim

victim accepts SID=I_KNOW_THE_SID, which is bad. However, victim is secure because it performs session regeneration. victim sends the following response:

HTTP/1.1 200 OK
Cookie: SID=3134998145AB331F

Alice will now use SID=3134998145AB331F which is unknown to Mallory, and SID=I_KNOW_THE_SID is invalid. Mallory is thus unsuccessful in the session fixation attempt.

Unfortunately session regeneration is not always possible. Known cases where session regeneration may cause problems include when third party software such as ActiveX, Java Applets, or browser plugins communicate with the server as well. Third party software could cause logouts, or the session could be split into two separate sessions.

Accept only server generated SID

One way to improve security is to not accept session identifiers not generated by server.

if (!isset($_SESSION['SERVER_GENERATED_SID'])) {
   session_destroy(); // destroy all data in session
}
session_regenerate_id(); // generate a new session identifier
$_SESSION['SERVER_GENERATED_SID'] = true;

xcv

Logout function

A logout function is useful as it allows users to indicate that a session should not allow further requests. Thus attacks can only be effective while a session is active.

if (isset($_GET['LOGOUT']))
   session_destroy(); // destroy all data in session

Time-out old SIDs

This defense is simple to implement and has the advantage of providing a measure of protection against unauthorized users accessing an authorized user's account by using a machine they may have left unattended.

Store a session variable containing a time stamp of the last access made by that SID. When that SID is next used compare the current timestamp (in PHP you can get this by using the time() function call) with the one stored in the session. If the difference is greater than a predefined number, say 5 minutes, destroy the session. Otherwise, update the session variable with the current timestamp.

Destroy session if Referrer is suspicious

When visiting a page, most browsers will set the Referrer - from which page did you click to get to this page?

When using a site which you are logged into, if the site should rarely be linked to (e.g., banking websites, webmails), and/or the session should not last too long as the user might run into links to the site, it is usually highly suspicious if the Referrer is not from within the site.

For example, http://vulnerable/ may employ the following security check:

if (strpos($_SERVER['HTTP_REFERER'], 'http://vulnerable/') !== 0) {
   session_destroy(); // destroy all data in session
}
session_regenerate_id(); // generate a new session identifier

Verify that additional information is consistent throughout session

One way to further improve security is to ensure that it appears to be the same end user (client). This makes it a bit harder to perform session fixation and other attacks.

IP address

As more and more network begin to conform to RFC 3704 and other anti-spoofing practices, IP numbers are more reliable as a "same source" identifier. Therefore, by verifying that the source IP is consistent throughout a session, the security of a web site can be improved.

This could be performed in this manner:

if($_SERVER['REMOTE_ADDR'] != $_SESSION['PREV_REMOTEADDR']) {
   session_destroy(); // destroy all data in session
}
session_regenerate_id(); // generate a new session identifier
$_SESSION['PREV_REMOTEADDR'] = $_SERVER['REMOTE_ADDR'];

However, there are some things to consider before employing this approach.

• Several users may share one IP. It is not uncommon that an entire building shares one IP using NAT.
• One user may have an inconsistent IP. This is true for users behind some proxies (such as AOL customers). It is also true for some mobile / roaming users.

For some sites, the added security outweighs the lack of convenience, and, for others, it does not.

User Agent

Browsers identify themselves by "User-Agent" HTTP headers. This header does not normally change during use; it would be extremely suspicious if that were to happen. A web application might make use of User-Agent detection in attempt to prevent malicious users from stealing sessions. This is Security through obscurity, which has its uses, but should not be used as a single defence.

if ($_SERVER['HTTP_USER_AGENT'] !== $_SESSION['PREV_USERAGENT']) {
   session_destroy(); // destroy all data in session
}
session_regenerate_id(); // generate a new session identifier
$_SESSION['PREV_USERAGENT'] = $_SERVER['HTTP_USER_AGENT'];

Defense in Depth

Defense in depth is to combine several countermeasures. The idea is simple: if one obstacle is hard to overcome, several obstacles could be very hard to overcome.

A DiD strategy could involve:

• Enable HTTPS (to protect against other problems)
• Correcting configuration (do not accept external SID, set time-out etc)
• Perform session_regeneration, support log-out, reject illegal referrers, etc.

The following PHP script demonstrates some such countermeasures put together in a Defence in Depth manner:

if (strpos($_SERVER['HTTP_REFERER'], 'https://DiD/') !== 0)
  session_destroy();

if (isset($_GET['LOGOUT']))
  session_destroy();

if ($_SERVER['REMOTE_ADDR'] !== $_SESSION['PREV_REMOTEADDR'])
  session_destroy();

if ($_SERVER['HTTP_USER_AGENT'] !== $_SESSION['PREV_USERAGENT'])
  session_destroy();

session_regenerate_id(); // generate a new session identifier

$_SESSION['PREV_USERAGENT'] = $_SERVER['HTTP_USER_AGENT'];
$_SESSION['PREV_REMOTEADDR'] = $_SERVER['REMOTE_ADDR'];

See also

• Session poisoning

External links

• Security Corner: Session Fixation
• Session Fixation Vulnerability in Web-based Applications (PDF)

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