Denial of Service Attack on external print server.

Description

All versions of Samba from 3.6.0 onwards are vulnerable to a denial of
service attack when the RPC spoolss service is configured to be run as
an external daemon. Missing input sanitization checks on some of the
input parameters to spoolss RPC calls could cause the print spooler
service to crash.

There is no known vulnerability associated with this error, merely a
denial of service. If the RPC spoolss service is left by default as an
internal service, all a client can do is crash its own authenticated
connection.

Patch Availability

A patch addressing this defect has been posted to

http://www.samba.org/samba/security/

Additionally, Samba 4.7.6, 4.6.14 and 4.5.16 have been issued as
security releases to correct the defect. Patches against older Samba
versions are available at http://samba.org/samba/patches/. Samba
vendors and administrators running affected versions are advised to
upgrade or apply the patch as soon as possible.

Workaround

Ensure the parameter:

rpc_server:spoolss = external

is not set in the [global] section of your smb.conf.

Credits

This problem was found by the Synopsys Defensics intelligent fuzz
testing tool. Jeremy Allison of Google and the Samba Team provided
the fix.