CVE-2021-25216: A second vulnerability in BIND's GSSAPI security policy negotiation can be targeted by a buffer overflow attack
  • 28 Apr 2021
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CVE-2021-25216: A second vulnerability in BIND's GSSAPI security policy negotiation can be targeted by a buffer overflow attack

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Article summary

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 branch

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 crash.
  • 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

Acknowledgments: 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 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.

Legal Disclaimer:

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