BIND Operational Notification: A party that is allowed control over zone data can overwhelm a server by transferring huge quantities of data [CVE-2016-6710]

Michael McNally mcnally at
Thu Jul 7 22:58:35 UTC 2016


   DNS protocols were designed with the assumption that a certain
   amount of trust could be presumed between the operators of primary
   and secondary servers for a given zone.  However, in current
   practice some organizations have scenarios which require them
   to accept zone data from sources that are not fully trusted (for
   example: providers of secondary name service).  A party who is
   allowed to feed data into a zone (e.g. by AXFR, IXFR, or Dynamic
   DNS updates) can overwhelm the server which is accepting data
   by intentionally or accidentally exhausting that server's memory.

CVE:                   CVE-2016-6170
Document Version:      1.0
Posting date:          07 July 2016
Program Impacted:      BIND
Versions affected:     9.0.x -> 9.9.9-P1, 9.10.0 -> 9.10.4-P1, 9.11.0a1 -> 9.11.0b1


   A server is potentially vulnerable if it accepts zone data from
   another source, as no limit is currently placed on zone data
   size.  A master server can therefore feed excessive data to a
   slave server, exhausting its memory.  Similarly a client allowed
   to insert records into a zone using dynamic updates can also
   cause a zone to grow without limit until memory is exhausted.
   In all cases a trust relationship allowing the attacker to insert
   zone data must exist between the two parties for an attack to
   occur using this vector.


   A server which is successfully attacked using this method can
   have its memory exhausted, causing unpredictable behavior or
   termination by the OS when it runs out of memory.


   In a typical case where zone data is accepted only from trusted
   sources under the control of the same organization, servers are
   at little risk.  The chief risk from this attack vector is to
   parties who operate secondary name servers which accept zone
   data from not fully trusted sources.

   Operators of servers which accept untrusted zone data can mitigate
   their risk by operating an intermediary server whose role it is
   to receive zone data and then (if successful) re-distribute it
   to client-facing servers.  Successful exploitation of the attack
   against the intermediary server may still occur but denial of
   service against the client-facing servers is significantly more
   difficult to achieve in this scenario.

Active exploits: 

   No known active exploits, but a public discussion of the issue
   has taken place on a public mailing list and a CVE assignment
   has been requested by a party other than ISC.

   In practice this vulnerability has existed for as long as slave
   servers have taken data from master servers and has no history
   (of which we are aware) of being exploited as an attack vector
   because it requires an existing trust relationship as a prerequisite
   and identification of the attacking party is very easy (at which
   point the trust relationship can be revoked).

   However, it is a possible attack vector and recent public
   discussion and a CVE assignment requested by an outside party
   have prompted us to issue a statement on the subject in this
   Operational Notification.


   ISC wish to stress that the behavior in question is not a failure
   of BIND to implement DNS protocols correctly, but is if anything
   an oversight in the protocol.  However, for the convenience of
   operators who take zone data from untrusted sources (such as
   secondary name service providers) we have committed to delivering
   a feature in upcoming maintenance releases of BIND which will
   address the issue by allowing operators to set limits on the
   maximum zone size BIND will accept.

Do you still have questions?  Questions regarding this advisory
should go to security-officer at

ISC Disclosure Policies: 

   Additional information on our Operational Notifications can be
   found at:, and Phased Disclosure
   Process at:

This Knowledge Base article:
is the complete and official operational notification document.

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