Document version: 2.0
Posting date: 16 March 2022
Program impacted: BIND
Versions affected: BIND 9.18.0
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
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/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:
The failure can be avoided by adding this option to named.conf:
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.
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
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 security-offi...@isc.org. To report a new issue, please encrypt your
message using security-offi...@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/.
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
The Knowledgebase article https://kb.isc.org/docs/cve-2022-0635 is
the complete and official security advisory document.
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