CVE-2021-25219: Lame cache can be abused to severely degrade resolver performance
  • 27 Oct 2021
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CVE-2021-25219: Lame cache can be abused to severely degrade resolver performance

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Article summary

CVE: CVE-2021-25219

Document version: 2.0

Posting date: 27 October 2021

Program impacted: BIND

Versions affected: BIND 9.3.0 -> 9.11.35, 9.12.0 -> 9.16.21, and versions 9.9.3-S1 -> 9.11.35-S1 and 9.16.8-S1 -> 9.16.21-S1 of BIND Supported Preview Edition, as well as release versions 9.17.0 -> 9.17.18 of the BIND 9.17 development branch

Severity: Medium

Exploitable: Remotely


Exploitation of broken authoritative servers using a flaw in response processing can cause degradation in BIND resolver performance. The way the lame cache is currently designed makes it possible for its internal data structures to grow almost infinitely, which may cause significant delays in client query processing.

The purpose of a resolver's lame cache is to ensure that if an authoritative server responds to a resolver's query in a specific broken way, subsequent client queries for the same <QNAME, QTYPE> tuple do not trigger further queries to the same server for a configurable amount of time. The lame cache is enabled by setting the lame-ttl option in named.conf to a value greater than 0. That option is set to lame-ttl 600; in the default configuration, which means the lame cache is enabled by default.


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

A successful attack exploiting this flaw causes a named resolver to spend most of its CPU time on managing and checking the lame cache. This results in client queries being responded to with large delays, and increased likelihood of DNS timeouts on client hosts.

CVSS Score: 4.9


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


Setting lame-ttl 0; disables the lame cache and prevents the performance issue. Our research and testing indicate that in the current Internet there is almost no downside to disabling the lame cache.

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 9.11.36
  • BIND 9.16.22
  • BIND 9.17.19

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

  • BIND 9.11.36-S1
  • BIND 9.16.22-S1

Acknowledgments: ISC would like to thank Kishore Kumar Kothapalli of Infoblox for bringing this vulnerability to our attention.

Document revision history:

1.0 Advance notification to customers, 20 October 2021
1.1 Revised Versions Affected (now includes all older versions of BIND Supported Preview Edition), 26 October 2021
2.0 Public disclosure, 27 October 2021

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.

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