CVE-2021-25218: A too-strict assertion check could be triggered when responses in BIND 9.16.19 and 9.17.16 require UDP fragmentation if RRL is in use
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CVE-2021-25218: A too-strict assertion check could be triggered when responses in BIND 9.16.19 and 9.17.16 require UDP fragmentation if RRL is in use

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Article summary

CVE: CVE-2021-25218

Document version: 2.0

Posting date: 18 August 2021

Program impacted: BIND

Versions affected: BIND 9.16.19, 9.17.16. Also, version 9.16.19-S1 of BIND Supported Preview Edition

Severity: High

Exploitable: Remotely


If named attempts to respond over UDP with a response that is larger than the current effective interface maximum transmission unit (MTU), and if response-rate limiting (RRL) is active, an assertion failure is triggered (resulting in termination of the named server process).

There are two ways for named to exceed the interface MTU:

  • Direct configuration in named.conf setting max-udp-size to a value larger than the interface's MTU, or
  • Path MTU discovery (PMTUD) informing the IP stack that it should use a smaller MTU for the interface and destination than the default max-udp-size value of 1232. Some operating systems allow packets received via other protocols to affect PMTUD values for DNS over UDP.

While RRL is not enabled by default for user-defined views or the built-in default INTERNET (IN) class view, "_default", the built-in default CHAOS (CH) class view, "_bind", does have RRL enabled.

Note that while this defect can be triggered through misconfiguration or by deliberate exploitation, it can also arise during normal operating conditions, even with hardened PMTUD settings.


When a vulnerable version of named receives a query under the circumstances described above, the named process will terminate due to a failed assertion check.

The vulnerability affects only BIND 9 releases 9.16.19, 9.17.16, and release 9.16.19-S1 of the BIND Supported Preview Edition.

CVSS Score: 7.5


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


Disabling RRL in all views, including the built-in CHAOS class view "_bind", prevents the faulty assertion from being reached. This can be done by removing all existing rate-limit statements from named.conf, and defining a replacement for the default CHAOS view:

view override_bind chaos {
        recursion no;
        notify no;
        allow-new-zones no;
        max-cache-size 2M;

        zone "version.bind" chaos {
                type primary;
                database "_builtin version";
        zone "hostname.bind" chaos {
                type primary;
                database "_builtin hostname";
        zone "authors.bind" chaos {
                type primary;
                database "_builtin authors";
        zone "id.server" chaos {
                type primary;
                database "_builtin id";

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 9.16.20
  • BIND 9.17.17

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

  • BIND 9.16.20-S1

Document revision history:

1.0 Advance notification to customers, 11 August 2021

2.0 Public disclosure, 18 August 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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