CVE-2022-0635: DNAME insist with synth-from-dnssec enabled
  • 16 Mar 2022
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CVE-2022-0635: DNAME insist with synth-from-dnssec enabled

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Article summary

CVE: CVE-2022-0635

Document version: 2.0

Posting date: 16 March 2022

Program impacted: BIND

Versions affected: BIND 9.18.0

Severity: High

Exploitable: Remotely


We refactored the RFC 8198 Aggressive Use of DNSSEC-Validated Cache feature (synth-from-dnssec) for the new BIND 9.18.0 stable release, and changed the default so that is now automatically enabled for dnssec-validating resolvers. Subsequently it was found that repeated patterns of specific queries to servers with this feature enabled could cause an INSIST failure in query.c:query_dname which causes named to terminate unexpectedly.

The vulnerability affects BIND resolvers running 9.18.0 that have both dnssec-validation and synth-from-dnssec enabled. (Note that dnssec-validation auto; is the default setting unless configured otherwise in named.conf and that enabling dnssec-validation automatically enables synth-from-dnssec unless explicitly disabled)


When a vulnerable version of named receives a series of specific queries, the named process will eventually terminate due to a failed assertion check.

CVSS Score: 7.0


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


The failure can be avoided by adding this option to named.conf:

synth-from-dnssec no;

However we do not recommend disabling this feature other than as a temporary workaround because it provides protection from pseudo-random-subdomain attacks against DNSSEC-signed zones.

Active exploits:

We are not aware of any active exploits.


  • Users of BIND 9.18.0 should upgrade to BIND 9.18.1

ISC would like to thank Vincent Levigneron of AFNIC for reporting this issue to us and for verifying the fix and workaround.

Document revision history:

1.0 Early Notification, 9 March 2022
2.0 Public disclosure, 16 March 2022
Related documents:

See Operational Notification: synth-from-dnssec may cause slow resolution on resolvers under certain cache conditions for more information regarding early BIND support for RFC 8198 Aggressive Use of DNSSEC-Validated Cache and our recommendation that this feature be disabled until versions of BIND were made available which contain an improvement to the synth-from-dnssec feature.

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.

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