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CVE-2018-5737: BIND 9.12's serve-stale implementation can cause an assertion failure in rbtdb.c or other undesirable behavior, even if serve-stale is not enabled.
Author: Michael McNally Reference Number: AA-01606 Views: 44331 Created: 2018-05-18 10:14 Last Updated: 2018-05-18 18:03 0 Rating/ Voters

Document Version: 
Posting date: 
18 May 2018
Program Impacted: 
Versions affected: 
9.12.0, 9.12.1


A problem with the implementation of the new serve-stale feature in BIND 9.12 can lead to an assertion failure in rbtdb.c, even when stale-answer-enable is off.  Additionally, problematic interaction between the serve-stale feature and NSEC aggressive negative caching can in some cases cause undesirable behavior from named, such as a recursion loop or excessive logging.

Deliberate exploitation of this condition could cause operational problems depending on the particular manifestation -- either degradation or denial of service.


Servers running a vulnerable version of BIND (9.12.0, 9.12.1) which permit recursion to clients and which have the max-stale-ttl parameter set to a non-zero value are at risk.

CVSS Score:  5.9

CVSS Vector:  CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

For more information on the Common Vulnerability Scoring System and to obtain your specific environmental score please visit:


Setting "max-stale-ttl 0;" in named.conf will prevent exploitation of this vulnerability (but will effectively disable the serve-stale feature.)

Setting "stale-answer enable off;" is not sufficient to prevent exploitation, max-stale-ttl needs to be set to zero.

Active exploits:

No known active exploits.


The error which can be exploited in this vulnerability is present in only two public release versions of BIND, 9.12.0 and 9.12.1.  If you are running an affected version then upgrade to BIND 9.12.1-P2


ISC would like to thank Tony Finch of the University of Cambridge for his assistance in discovering and analyzing this vulnerability.

Document Revision History:

1.0 Advance Notification, 09 May 2018
1.1 BIND 9.12.1-P1 was recalled before public announcement due to defect, the advisory language was re-written to be clearer about the exploit risk, and the public disclosure date was adjusted because of the problem with 9.12.1-P1, 17 May 2018
2.0 Public Disclosure, 18 May 2018

Related Documents:

See our BIND9 Security Vulnerability Matrix at for a complete listing of Security Vulnerabilities and versions affected.

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