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

Michael McNally mcnally at
Wed Aug 18 17:45:20 UTC 2021

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

    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 ONLY affects 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
CVSS Vector:         CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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 2020
    2.0 Public disclosure, 18 August 2020

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 security-officer at To report a new issue, please
encrypt your message using security-officer at's PGP key which
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    ISC patches only currently supported versions. When possible we
    indicate EOL versions affected.  (For current information on
    which versions are actively supported, please see

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    Details of our current security advisory policy and practice can
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The Knowledgebase article
is the complete and official security advisory document.

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