title: 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.
description: 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. Affects BIND 9.12.0 and 9.12.1.
problem types: 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.
affected:
{
product: BIND 9
vendor: ISC
versions:
status: affected, version: 9.12.0 and 9.12.1
}
metrics:
cvssV3_0: [
attackComplexity: HIGH
attackVector: NETWORK
availabilityImpact: HIGH
baseScore: 5.9
baseSeverity: MEDIUM
confidentialityImpact: NONE
integrityImpact: NONE
privilegesRequired: NONE
scope: UNCHANGED
userInteraction: NONE
vectorString: CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
version: 3.0
]
solutions: 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
workarounds: 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.