An authenticated user can crash the DCE/RPC DNS with

Description

Some DNS records (such as MX and NS records) usually contain data in
the additional section. Samba’s dnsserver RPC pipe (which is an
administrative interface not used in the DNS server itself) made an
error in handling the case where there are no records present: instead
of noticing the lack of records, it dereferenced uninitialised memory,
causing the RPC server to crash. This RPC server, which also serves
protocols other than dnsserver, will be restarted after a short delay,
but it is easy for an authenticated non-admin attacker to crash it
again as soon as it returns. The Samba DNS server itself will continue
to operate, but many RPC services will not.

Patch Availability

Patches addressing both these issues have been posted to:

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

Additionally, Samba 4.11.15, 4.12.9 and 4.13.1 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

CVSSv3.1 AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (6.5)

Workaround

The dnsserver task can be stopped by setting

‘dcerpc endpoint servers = -dnsserver’

in the smb.conf and restarting Samba.

Credits

Originally reported by Francis Brosnan Blázquez of ASPL.es in 2017.

Patches first provided for Samba 4.6 by Francis Brosnan Blázquez, and
adapted for modern Samba by Douglas Bagnall of the Samba team.

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