CVE-2020-8625: A vulnerability in BIND's GSSAPI security policy negotiation can be targeted by a buffer overflow attack

CVE: CVE-2020-8625

Document version: 2.0

Posting date: 17 February 2021

Program impacted: BIND

Versions affected: BIND 9.5.0 -> 9.11.27, 9.12.0 -> 9.16.11, and versions BIND 9.11.3-S1 -> 9.11.27-S1 and 9.16.8-S1 -> 9.16.11-S1 of BIND Supported Preview Edition. Also release versions 9.17.0 -> 9.17.1 of the BIND 9.17 development branch

Severity: High

Exploitable: Remotely

Description:

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.

Impact:

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 valid values for the tkey-gssapi-keytab or tkey-gssapi-credentialconfiguration 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.

The most likely outcome of a successful exploitation of the vulnerability is a crash of the named process. However, remote code execution, while unproven, is theoretically possible.

CVSS Score: 8.1

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

For more information on the Common Vulnerability Scoring System and to obtain your specific environmental score, please visit: https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H&version=3.1.

Workarounds:

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.

On some platforms it may be possible to build a working BIND installation that is not vulnerable to CVE-2020-8625 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.

Choosing to configure and build BIND without the ISC SPNEGO implementation does not produce a vulnerable BIND on any platform, but on platforms where GSSAPI support in the system is lacking, building without the ISC SPNEGO implementation may result in unusable GSSAPI features (such as an inability to use GSS-TSIG-signed DDNS updates).

Active exploits:

We are not aware of any active exploits.

Solution:

Upgrade to the patched release most closely related to your current version of BIND:

• BIND 9.11.28
• BIND 9.16.12

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

• BIND 9.11.28-S1
• BIND 9.16.12-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, 9 February 2021
2.0 Public Disclosure, 17 February 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 security-officer@isc.org. To report a new issue, please encrypt your message using security-officer@isc.org's PGP key which can be found here: https://www.isc.org/pgpkey/. If you are unable to use encrypted email, you may also report new issues at: https://www.isc.org/reportbug/.

Note:

ISC patches only currently supported versions. When possible we indicate EOL versions affected. (For current information on which versions are actively supported, please see https://www.isc.org/download/.)

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 https://kb.isc.org/docs/aa-00861.

The Knowledgebase article https://kb.isc.org/docs/cve-2020-8625 is the complete and official security advisory document.

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