Bypassing Inherited Rights Filters in Novell Directory Services.

[FogHorn Security <info () FOGHORNSECURITY COM>]

                             FOGHORN SECURITY ADVISORY

                                      issued

                                 September 7, 2000

                http://www.foghornsecurity.com/advisories/20000907

          Bypassing Inherited Rights Filters in Novell Directory Services.




SUMMARY

A design weakness in NDS as shipped with Novell v5.0 and later
can allow certain users to bypass IRF's, and gain escalation of privileges.

SEVERITY
Serious.  Even in a well designed tree IRF's are sometimes needed to protect
more sensitive objects.  This issue, if not carefully considered, can easily
render IRF's ineffective, and expose sensitive information.

BACKGROUND

In NDS, rights are assigned in three ways:

Rights to an object [Object Rights]
Rights to all properties of an object [All Properties Rights]
Rights to selected properties of an object [Selected Properties Rights]

By default, rights granted at one level of a directory tree automatically
flow down to lower levels in the tree.  This inheritance of rights can be
blocked by using Inherited Rights Filters [IRFs].  IRFs can be set for
any of the three types of rights mentioned above.

THE PROBLEM

In previous versions of NDS, only Object Rights, and All Properties Rights
could be inherited.  In Netware 5.0, Novell added the ability to make
Selected Property Rights inheritable.  These rights are not blocked by
IRFs set for Object Rights or All Properties Rights.  They can only be
blocked by the creation of an explicit IRF for each property you need to
protect.

Obviously, this is unworkable in the real world.  Setting individual IRFs
for every property in the schema is tedious and prone to error, and it is
extremely difficult to anticipate all possible exploits for all properties.
Additionally, if the schema is extended, the new properties would be
unprotected until IRFs were updated.

This also presents a problem for sites upgrading from Novell 4.11.  If this
issue is not addressed in the upgrade process, IRF's which were previously
valid, could be rendered ineffective.

EXPLOIT

Active exploitation of this feature requires Write rights to the Object
Trustees ACL property of a container at or above the level of the object being
attacked.

Here's an example. An administrator, .BOB.ACME, has Supervisor [S] rights to
the .ACME container. There is a container, .SECRET.ACME, which BOB should not
have any access to.

Joe, .JOE.SECRET.ACME, is the administrator of .SECRET.ACME.  Joe has
been given S rights to SECRET, and an IRF has been put in place on SECRET
blocking all Object Rights, and all All Properties Rights.  This scenario is in
line with standard practice, and Novell's own documentation [See TID# 10011973]
Unfortunately, Bob can still gain full control of secret.

1] Bob modifies the trustees list of .ACME granting himself the Write [W]
right to the Object Trustees ACL property and designates this right as
inheritable.

2] Since Selected Properties Rights are not explictly blocked by the IRF
at .SECRET.ACME, Bob can now add himself to the trustee list of the
SECRET container and obtain full privileges.


OTHER CONSIDERATIONS

In addition to active exploits, this issue could result in administrators
inadvertently granting rights to objects they believe to be protected by IRF's.
For instance, Help Desk staff may be granted password reset rights by granting
Selected Properties Rights at a high level in the tree, and making those rights
inheritable.  Those rights can only be blocked with an explicit Selected
Properties IRF at the containers or objects you need protected.

WORKAROUND

It is impossible to anticipate all the scenarios where this *feature*
could be exploited.  Administrators should carefully evaluate their tree
and permission structures with this problem in mind.

At a minimum, where IRFs are used to protect objects or containers, the
following properties should be protected by explicit IRFs:

Object Trustees [ACL]
Members
Security Equal to Me
Password Required
Password Management
Incorrect Login Attempts

There are certainly others that would need to be protected as well.
Finding additional exploits is left as an exercise for the reader.
[Hint: Audit Objects]

FIX

We believe that Novell should either,

a) Make IRFs set for All Properties Rights apply to Selected Properties
   Rights as well

or

b) Provide a method whereby all Selected Properties Rights could be
filtered with a single IRF.

COMMENTS

This is a classic example of adding functionality without fully considering the
implications.  While we do not consider this to be a *bug*, it is clearly a
poorly designed *feature*.  We could not find any reference to this on Novell's
support website.  In fact, as referenced above, Novell's own documentation
[TID# 10011973 - last updated on June 29, 2000] does not address this issue.

In that document, they answer the question, "How do I create an admin for a
container that cuts off the main admin?"  They do not specify the filtering of
Selected Properties Rights, and thus leave readers open to this vulnerability.

Safe Computing,

-FogHorn Security Staff