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AUSCERT External Security Bulletin Redistribution
Operational Notification: A party that is allowed control over zone data
can overwhelm a server by transferring huge quantities of data
2 September 2016
AusCERT Security Bulletin Summary
Operating System: Windows
UNIX variants (UNIX, Linux, OSX)
Impact/Access: Denial of Service -- Existing Account
CVE Names: CVE-2016-6170
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Operational Notification: A party that is allowed control over zone data can
overwhelm a server by transferring huge quantities of data.
Author: Michael McNally
Reference Number: AA-01390
Created: 2016-07-07 20:44
Last Updated: 2016-07-18 23:47
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.
07 July 2016
9.0.x -> 9.9.9-P2, 9.10.0 -> 9.10.4-P2, 9.11.0a1 -> 9.11.0b2
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.
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.
Document Revision History:
1.0 Public Disclosure, 07 July 2016
1.1 Updated Versions affected to include 9.9.9-P2, 9.10.4-P2, and 9.11.0-b2
Do you still have questions? Questions regarding this advisory should go to
ISC Disclosure Policies: Additional information on our Operational
Notifications can be found at: https://www.isc.org/software/notifications, and
Phased Disclosure Process at:
This Knowledge Base article https://kb.isc.org/editArticle/AA-01390 is the
complete and official operational notification document.
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