Unprivileged user can crash winbind

Description

winbind in version 3.6 and later implements a request to
translate multiple Windows SIDs into names in one request. This
was done for performance reasons: Active Directory domain
controllers can do multiple SID to name translations in one RPC
call. It was an obvious extension to also offer this batch
operation on the winbind unix domain stream socket that is
available to local processes on the Samba server to reduce
network round-trips to the domain controller.

Due to improper input validation a hand-crafted packet can make
winbind perform a NULL pointer dereference and thus crash.

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

CVSS 3.1: AV:L/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:H (5.0)

Workaround and mitigating factors

Any user with local shell access to the machine running winbind can
issue the winbind socket request. The only workaround is to disable
shell access to exposed machines.

Typical file servers don’t offer full local access, they are not
affected.

Credits

Originally reported by Bas Alberts of the GitHub Security Lab Team as
GHSL-2020-134.

Advisory written by Volker Lendecke of SerNet and the Samba Team.

Patches provided by Volker Lendecke of SerNet and the Samba Team.

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