CVE:                 CVE-2021-25216
Document version:    2.0
Posting date:        28 April 2021
Program impacted:    BIND
Versions affected:   BIND 9.5.0 -> 9.11.29, 9.12.0 -> 9.16.13, and versions
                     BIND 9.11.3-S1 -> 9.11.29-S1 and 9.16.8-S1 -> 9.16.13-S1
                     of BIND Supported Preview Edition, as well as release
                     versions 9.17.0 -> 9.17.1 of the BIND 9.17 development
Severity:            High
Exploitable:         Remotely


   GSS-TSIG is an extension to the TSIG protocol which is intended
   to support the secure exchange of keys for use in verifying the
   authenticity of communications between parties on a network.

   SPNEGO is a negotiation mechanism used by GSSAPI, the application
   protocol interface for GSS-TSIG.

   The SPNEGO implementation used by BIND has been found to be
   vulnerable to a buffer overflow attack.


   BIND servers are vulnerable if they are running an affected
   version and are configured to use GSS-TSIG features.

   In a configuration which uses BIND's default settings the
   vulnerable code path is not exposed, but a server can be rendered
   vulnerable by explicitly setting values for the tkey-gssapi-keytab
   or tkey-gssapi-credential configuration options.

   Although the default configuration is not vulnerable, GSS-TSIG
   is frequently used in networks where BIND is integrated with
   Samba, as well as in mixed-server environments that combine BIND
   servers with Active Directory domain controllers. For servers
   that meet these conditions, the ISC SPNEGO implementation is
   vulnerable to various attacks, depending on the CPU architecture
   for which BIND was built:

   -  For named binaries compiled for 64-bit platforms, this flaw
      can be used to trigger a buffer over-read, leading to a server

   -  For named binaries compiled for 32-bit platforms, this flaw
      can be used to trigger a server crash due to a buffer overflow
      and possibly also to achieve remote code execution.

   We have determined that standard SPNEGO implementations are
   available in the MIT and Heimdal Kerberos libraries, which support
   a broad range of operating systems, rendering the ISC implementation
   unnecessary and obsolete. Therefore, to reduce the attack surface
   for BIND users, we will be removing the ISC SPNEGO implementation
   in the April releases of BIND 9.11 and 9.16 (it had already been
   dropped from BIND 9.17).

   We would not normally remove something from a stable ESV (Extended
   Support Version) of BIND, but since system libraries can replace
   the ISC SPNEGO implementation, we have made an exception in this
   case for reasons of stability and security.

CVSS Score:          8.1 (on 32-bit platforms) or 7.4 (on 64-bit)
CVSS Vector:

   -  CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H (32-bit)
   -  CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H (64-bit)

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


   This vulnerability only affects servers configured to use GSS-TSIG,
   most often to sign dynamic updates. If another mechanism can be
   used to authenticate updates, the vulnerability can be avoided
   by choosing not to enable the use of GSS-TSIG features.

   Prior to the April 2021 BIND releases, on some platforms it was
   possible to build a working BIND installation that was not
   vulnerable to CVE-2021-25216 by providing the --disable-isc-spnego
   command-line argument when running the ./configure script in the
   top level of the BIND source directory, before compiling and
   linking named.

   After the April 2021 BIND releases, all supported branches have
   removed isc-spnego support. This corrects CVE-2021-25216, but
   requires that the system have other libraries and header files
   to support GSS-TSIG functionality, unless such functionality is
   completely disabled at build time by providing the --without-gssapi
   argument to the ./configurescript when selecting build options.

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 9.11.31
   -   BIND 9.16.15

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

    -  BIND 9.11.31-S1
    -  BIND 9.16.15-S1


   ISC would like to thank an anonymous party, working in conjunction
   with Trend Micro Zero Day Initiative, for reporting this issue to us.

Document revision history:

   1.0 Early Notification, 14 April 2021
   1.1 Disclosure schedule change, 15 April 2021
   2.0 Public disclosure, 28 April 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
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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

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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