Samba AD users with permission to write to

Description

The Samba AD DC includes checks when adding service principals names
(SPNs) to an account to ensure that SPNs do not alias with those
already in the database. Some of these checks are able to be bypassed
if an account modification re-adds an SPN that was previously present
on that account, such as one added when a computer is joined to a
domain.

An attacker who has the ability to write to an account can exploit
this to perform a denial-of-service attack by adding an SPN that
matches an existing service. Additionally, an attacker who can
intercept traffic can impersonate existing services, resulting in a
loss of confidentiality and integrity.

Patch Availability

Patches addressing both these issues have been posted to:

https://www.samba.org/samba/security/

Additionally, Samba 4.13.17, 4.14.12, and 4.15.4 have been issued
as security releases to correct the defect. Samba administrators are
advised to upgrade to these releases or apply the patch as soon
as possible.

CVSSv3 calculation

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (8.8)

Workaround

None.

Credits

Originally reported by Kees van Vloten.

Analysis, patches, and this advisory by Joseph Sutton of Catalyst
and the Samba Team.

== Our Code, Our Bugs, Our Responsibility.
== The Samba Team