CVE-2020-8616: BIND does not sufficiently limit the number of fetches performed when processing referrals

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

Description:

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.

Impact:

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:

1. The performance of the recursing server can potentially be degraded by the additional work required to perform these fetches, and
2. 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: https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:F/RL:O/RC:C&version=3.1.

Workarounds:

None

Active exploits:

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

Solution:

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

Acknowledgments: 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 http://www.nxnsattack.com

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

Do you still have questions? Questions regarding this advisory should go to security-officer@isc.org. To report a new issue, please encrypt your message using security-officer@isc.org's PGP key which can be found here: https://www.isc.org/pgpkey/. If you are unable to use encrypted email, you may also report new issues at: https://www.isc.org/reportbug/.

Note:

ISC patches only currently supported versions. When possible we indicate EOL versions affected. (For current information on which versions are actively supported, please see https://www.isc.org/download/.)

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 https://kb.isc.org/docs/aa-00861.

The Knowledgebase article https://kb.isc.org/docs/cve-2020-8616 is the complete and official security advisory document.

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