Security Advisory 2023-041

Release Date:

Multiple Vulnerabilities in BIND 9 DNS System



  • 26/06/2023 --- v1.0 -- Initial publication


On June 22, The Internet Systems Consortium (ISC) has released security advisories that address high severity vulnerabilities affecting multiple versions of the ISC’s Berkeley Internet Name Domain (BIND) 9. A remote attacker could exploit these vulnerabilities to potentially cause denial-of-service conditions [1].

Technical Details

CVE-2023-2828 (CVSSv3 base score of 7.5)

Every named instance configured to run as a recursive resolver maintains a cache database holding the responses to the queries it has recently sent to authoritative servers. The size limit for that cache database can be configured using the max-cache-size statement in the configuration file; it defaults to 90% of the total amount of memory available on the host. When the size of the cache reaches 7/8 of the configured limit, a cache-cleaning algorithm starts to remove expired and/or least-recently used RRsets from the cache, to keep memory use below the configured limit.

It has been discovered that the effectiveness of the cache-cleaning algorithm used in named can be severely diminished by querying the resolver for specific RRsets in a certain order, effectively allowing the configured max-cache-size limit to be significantly exceeded. By exploiting this flaw, an attacker can cause the amount of memory used by a named resolver to go well beyond the configured max-cache-size limit [2].

CVE-2023-2829 (CVSSv3 base score of 7.5)

A named instance configured to run as a DNSSEC-validating recursive resolver with the Aggressive Use of DNSSEC-Validated Cache (RFC 8198) option (synth-from-dnssec) enabled can be remotely terminated using a zone with a malformed NSEC record. By sending specific queries to the resolver, an attacker can cause named to terminate unexpectedly [3].

CVE-2023-2911 (CVSSv3 base score of 7.5)

If the recursive-clients quota is reached on a BIND 9 resolver configured with both stale-answer-enable yes; and stale-answer-client-timeout 0;, a sequence of serve-stale-related lookups could cause named to loop and terminate unexpectedly due to a stack overflow. By sending specific queries to the resolver, an attacker can cause named to terminate unexpectedly [4].

ISC is not aware of any active exploits related to the aforementioned vulnerabilities [2,3,4].

Affected Products


BIND [2]:

  • 9.11.0 -> 9.16.41
  • 9.18.0 -> 9.18.15
  • 9.19.0 -> 9.19.13

BIND Supported Preview Edition (a special feature preview branch of BIND provided to eligible ISC support customers) [2]:

  • 9.11.3-S1 -> 9.16.41-S1
  • 9.18.11-S1 -> 9.18.15-S1

Versions prior to 9.11.37 & 9.11.37-S1 were not assessed, but we believe that all versions of BIND 9.11 are vulnerable. Some even older major branches may be vulnerable as well [2].


BIND Supported Preview Edition [3]:

  • 9.16.8-S1 -> 9.16.41-S1
  • 9.18.11-S1 -> 9.18.15-S1


BIND [4]:

  • 9.16.33 -> 9.16.41
  • 9.18.7 -> 9.18.15

BIND Supported Preview Edition [4]:

  • 9.16.33-S1 -> 9.16.41-S1
  • 9.18.11-S1 -> 9.18.15-S1

BIND 9.11-S versions that support the stale-answer-client-timeout option are not vulnerable[4].


CERT-EU highly recommends update the system to most closely related to your current version of BIND 9:

BIND [2,3,4]:

  • 9.16.42
  • 9.18.16
  • 9.19.14

BIND Supported Preview Edition [2,3,4]:

  • 9.16.42-S1
  • 9.18.16-S1


  • CVE-2023-2828 - No workarounds known [2].

  • CVE-2023-2829 - Setting synth-from-dnssec to no prevents the problem [3].

  • CVE-2023-2911 - Setting stale-answer-client-timeout to off or to a non-zero value prevents the issue. Users of versions 9.18.10, 9.16.36, 9.16.36-S1 or older who are unable to upgrade should set stale-answer-client-timeout to off; using a non-zero value with these older versions leaves named vulnerable to CVE-2022-3924. Although it is possible to set the recursive-clients limit to a high number to reduce the likelihood of this scenario, this is not recommended; the limit on recursive-clients is important for preventing exhaustion of server resources. The limit cannot be disabled entirely [4].






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