CVE-2021-25220: DNS forwarders - cache poisoning vulnerability
  • 16 Mar 2022
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CVE-2021-25220: DNS forwarders - cache poisoning vulnerability

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Article summary

CVE: CVE-2021-25220

Document version: 2.0

Posting date: 16 March 2022

Program impacted: BIND

Versions affected:


  • 9.11.0 -> 9.11.36
  • 9.12.0 -> 9.16.26
  • 9.17.0 -> 9.18.0

BIND Supported Preview Editions:

  • 9.11.4-S1 -> 9.11.36-S1
  • 9.16.8-S1 -> 9.16.26-S1

Versions of BIND 9 earlier than those shown - back to 9.1.0, including Supported Preview Editions - are also believed to be affected but have not been tested as they are EOL.

Severity: Medium

Exploitable: Remotely


When using forwarders, bogus NS records supplied by, or via, those forwarders may be cached and used by named if it needs to recurse for any reason, causing it to obtain and pass on potentially incorrect answers.

Some examples of configurations that will be vulnerable are:

  • Resolvers using per zone or global forwarding with forward first (forward first is the default).
  • Resolvers not using global forwarding, but with per-zone forwarding with either forward first (the default) or forward only.
  • Resolvers configured with global forwarding along with zone statements that disable forwarding for part of the DNS namespace.

Authoritative-only BIND 9 servers are not vulnerable to this flaw.


The cache could become poisoned with incorrect records leading to queries being made to the wrong servers, which might also result in false information being returned to clients.

CVSS Score: 6.2


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


If applicable, modify your configuration to either remove all forwarding or all possibility of recursion. Depending on your use-case, it may be possible to use other zone types to replace forward zones.

Active exploits:

We are not aware of any active exploits.


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

  • BIND 9.11.37
  • BIND 9.16.27
  • BIND 9.18.1

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

  • BIND 9.11.37-S1
  • BIND 9.16.27-S1


ISC would like to thank Xiang Li, Baojun Liu, and Chaoyi Lu from Network and Information Security Lab, Tsinghua University and Changgen Zou from Qi An Xin Group Corp. for discovering and reporting this issue.

Document revision history:

1.0 Early Notification, 9 March 2022
1.1 Correction to official document link, 16 March 2022
2.0 Public disclosure, 16 March 2022

Related documents:

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 To report a new issue, please encrypt your message using's PGP key which can be found here: If you are unable to use encrypted email, you may also report new issues at:


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.

Legal Disclaimer:

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