named contains a feature which allows operators to issue commands to a running server by communicating with the server process over a control channel, using a utility program such as rndc. A regression introduced in a recent feature change has created a situation under which some versions of named can be caused to exit with a REQUIRE assertion failure if they are sent a null command string.

The BIND control channel is not configured by default, but when configured will accept commands from those IP addresses that are specified in its access control list and/or from clients which present the proper transaction key.  Using this defect, an attacker can cause a running server to stop if they can get it to accept control channel input from them.  In most instances this is not as bad as it sounds, because existing commands permitted over the control channel (i.e. "rndc stop") can already be given to cause the server to stop. However, BIND 9.11.0 introduced a new option to allow "read only" commands over the command channel.  Using this restriction, a server can be configured to limit specified clients to giving control channel commands which return information only (e.g. "rndc status") without affecting the operational state of the server.  The defect described in this advisory, however, is not properly stopped by the "read only" restriction, in essence permitting a privilege escalation allowing a client which should only be permitted the limited set of "read only" operations to cause the server to stop execution.

External References:

https://kb.isc.org/article/AA-01471