OSSN-0006: Keystone can allow user impersonation when using REMOTE_USER for external authentication¶
Summary¶
When external authentication is used with Keystone using the ExternalDefault plug-in, external usernames containing “@” characters are truncated at the “@” character before being mapped to a local Keystone user. This can result in separate external users mapping to the same local Keystone user, which could lead to user impersonation.
Affected Services / Software¶
Keystone, Havana
Discussion¶
When Keystone is run in the Apache HTTP Server, the webserver can handle authentication and pass the authenticated username to Keystone using the REMOTE_USER environment variable. External authentication behavior is handled by authentication plugins in Keystone. In the Havana release of OpenStack, if the external username provided in the REMOTE_USER environment variable contains an “@” character Keystone will only use the portion preceding the “@” character as the username when using the ExternalDefault authentication plugin. This results in the ability for multiple unique external usernames to map to the same single username in Keystone. For example, the external usernames “jdoe@example1.com” and “jdoe@example2.com” would both map to the Keystone user “jdoe”. This behavior could potentially be abused to allow one to impersonate another similarly named external user.
Keystone in OpenStack releases prior to Havana uses the entire value contained in the REMOTE_USER environment variable, so those versions are not vulnerable to this impersonation issue.
Recommended Actions¶
If the ExternalDefault plugin is being used for external authentication in
the Havana release, you should ensure that external usernames do not contain
“@” characters unless you want to collapse similarly named external users into
a single user on the Keystone side.
If your external usernames do contain “@” characters and you do not want to
collapse similarly named external users into a single user on the Keystone
side, you might be able to use the ExternalDomain plug-in. This plugin
considers the portion of the external username that follows an “@” character to
be the domain that the user belongs to in Keystone. This allows similarly named
external users to map to separate Keystone users if the portion of the external
username that follows an “@” character maps to a Keystone domain name. To
configure the ExternalDomain authentication plugin, set the external
parameter in the [auth] section of Keystone’s keystone.conf as follows:
[auth]
methods = external,password,token
external = keystone.auth.plugins.external.ExternalDomain
If neither of the above recommendations work for your deployment, a custom authentication plugin can be created that uses the external username that contains an “@” character as-is.
Contacts / References¶
Author: Nathan Kinder, Red Hat
This OSSN : https://bugs.launchpad.net/ossn/+bug/1254619
Original LaunchPad Bug : https://bugs.launchpad.net/keystone/+bug/1254619
OpenStack Security ML : openstack-security@lists.openstack.org
OpenStack Security Group : https://launchpad.net/~openstack-ossg
