CVE:                 CVE-2020-8616
Document version:    2.0
Posting date:        19 May 2020
Program impacted:    BIND
Versions affected:   BIND 9.0.0 -> 9.11.18, 9.12.0 -> 9.12.4-P2,
                     9.14.0 -> 9.14.11, 9.16.0 -> 9.16.2, and releases
                     9.17.0 -> 9.17.1 of the 9.17 experimental development
                     branch. All releases in the obsolete 9.13 and 9.15
                     development branches. All releases of BIND Supported
                     Preview Edition from 9.9.3-S1 -> 9.11.18-S1.
Severity:            High
Exploitable:         Remotely


   In order for a server performing recursion to locate records in
   the DNS graph it must be capable of processing referrals, such
   as those received when it attempts to query an authoritative
   server for a record which is delegated elsewhere. In its original
   design BIND (as well as other nameservers) does not sufficiently
   limit the number of fetches which may be performed while processing
   a referral response.


   A malicious actor who intentionally exploits this lack of effective
   limitation on the number of fetches performed when processing
   referrals can, through the use of specially crafted referrals,
   cause a recursing server to issue a very large number of fetches
   in an attempt to process the referral.

   This has at least two potential effects:

   -  The performance of the recursing server can potentially be degraded
      by the additional work required to perform these fetches, and

   -  The attacker can exploit this behavior to use the recursing
      server as a reflector in a reflection attack with a high
      amplification factor.

CVSS Score:          8.0
CVSS Vector:         CVSS:3.1AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:F/RL:O/RC:C

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



Active exploits:

   We are not aware of any active exploits but an academic paper
   reporting on the defect was published in May 2020.


   Upgrade to the patched release most closely related to your
   current version of BIND:

   -  BIND 9.11.19
   -  BIND 9.14.12
   -  BIND 9.16.3

   BIND Supported Preview Edition is a special feature preview
   branch of BIND provided to eligible ISC support customers.

   -  BIND 9.11.19-S1


   ISC would like to thank Lior Shafir and Yehuda Afek of Tel Aviv
   University and Anat Bremler-Barr of Interdisciplinary Center
   (IDC) Herzliya for discovering and reporting this issue.

Document revision history:

   1.0 Delivery to early advance notification customers
   1.1 Added Acknowledgments section
   1.2 CVSS score adjusted, as delivery of fixed versions changes CVSSv3
       environmental subscore
   1.3 Corrected the academic affiliation of one of the submitters.
   1.4 Added link to submitter's paper on the exploit
   2.0 Public disclosure

Related documents:

   The discoverers/reporters of this vulnerability have prepared
   an academic paper presenting their findings, which they have
   made available at

   See our BIND 9 Security Vulnerability Matrix for a complete
   listing of security vulnerabilities and versions affected.

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   ISC patches only currently supported versions. When possible we
   indicate EOL versions affected. (For current information on which
   versions are actively supported, please see

ISC Security Vulnerability Disclosure Policy:

   Details of our current security advisory policy and practice can
   be found in the ISC Software Defect and Security Vulnerability
   Disclosure Policy at

The Knowledgebase article is
the complete and official security advisory document.

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