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Session-fixation vulnerability in Horizon when using the default signed cookie sessions


The default setting in Horizon is to use signed cookies to store session state on the client side. This creates the possibility that if an attacker is able to capture a user's cookie, they may perform all actions as that user, even if the user has logged out.

Affected Services / Software

Horizon, Folsom, Grizzly, Havana, Icehouse


When configured to use client side sessions, the server isn't aware of the user's login state. The OpenStack authorization tokens are stored in the session ID in the cookie. If an attacker can steal the cookie, they can perform all actions as the target user, even after the user has logged out.

There are several ways attackers can steal the cookie. One example is by intercepting it over the wire if Horizon is not configured to use SSL. The attacker may also access the cookie from the filesystem if they have access to the machine. There are also other ways to steal cookies that are beyond the scope of this note.

By enabling a server side session tracking solution such as memcache, the session is terminated when the user logs out. This prevents an attacker from using cookies from terminated sessions.

It should be noted that Horizon does request that Keystone invalidate the token upon user logout, but this has not been implemented for the Identity API v3. Token invalidation may also fail if the Keystone service is unavailable. Therefore, to ensure that sessions are not usable after the user logs out, it is recommended to use server side session tracking.

Recommended Actions

It is recommended that you configure Horizon to use a different session backend rather than signed cookies. One possible alternative is to use memcache sessions. To check if you are using signed cookies, look for this line in Horizon's local_settings.py

 SESSION_ENGINE = 'django.contrib.sessions.backends.signed_cookies'

If the SESSION_ENGINE is set to value other than 'django.contrib.sessions.backends.signed_cookies' this vulnerability is not present. If SESSION_ENGINE is not set in local_settings.py, check for it in settings.py.

Here are the steps to configure memcache sessions:

1. Ensure the memcached service is running on your system

2. Ensure that python-memcached is installed

3. Configure memcached cache backend in local_settings.py

      CACHES = {
          'default': {
              'BACKEND': 'django.core.cache.backends.memcached.MemcachedCache',
              'LOCATION': '',

Make sure to use the actual IP and port of the memcached service.

4. Add a line in local_settings.py to use the cache backend:

 SESSION_ENGINE = 'django.contrib.sessions.backends.cache'

5. Restart Horizon's webserver service (typically 'apache2' or httpd')

Furthermore, you should always enable SSL for Horizon to help mitigate such attack scenarios.

Please note that regardless of which session backend is used, if the cookie is compromised, an attacker may assume all privileges of the user for as long as their session is valid.

Contacts / References