CVE-2023-2911: Exceeding the recursive-clients quota may cause named to terminate unexpectedly when stale-answer-client-timeout is set to 0

CVE: CVE-2023-2911

Document version: 2.0

Posting date: 21 June 2023

Program impacted: BIND 9

Versions affected:

BIND

• 9.16.33 -> 9.16.41
• 9.18.7 -> 9.18.15

BIND Supported Preview Edition

• 9.16.33-S1 -> 9.16.41-S1
• 9.18.11-S1 -> 9.18.15-S1

(BIND 9.11-S versions that support the stale-answer-client-timeout option are not vulnerable.)

Severity: High

Exploitable: Remotely

Description:

If the recursive-clients quota is reached on a BIND 9 resolver configured with both stale-answer-enable yes; and stale-answer-client-timeout 0;, a sequence of serve-stale-related lookups could cause named to loop and terminate unexpectedly due to a stack overflow.

Impact:

By sending specific queries to the resolver, an attacker can cause named to terminate unexpectedly.

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: https://nvd.nist.gov/vuln-metrics/cvss/v3-calculator?vector=AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H&version=3.1.

Workarounds:

Setting stale-answer-client-timeout to off or to a non-zero value prevents the issue.

Users of versions 9.18.10, 9.16.36, 9.16.36-S1 or older who are unable to upgrade should set stale-answer-client-timeout to off; using a non-zero value with these older versions leaves named vulnerable to CVE-2022-3924.

Although it is possible to set the recursive-clients limit to a high number to reduce the likelihood of this scenario, this is not recommended; the limit on recursive-clients is important for preventing exhaustion of server resources. The limit cannot be disabled entirely.

Active exploits:

This flaw was discovered in internal testing. We are not aware of any active exploits.

Solution:

Upgrade to the patched release most closely related to your current version of BIND 9:

• 9.16.42
• 9.18.16

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

• 9.16.42-S1
• 9.18.16-S1

Document revision history:

• 1.0 Early Notification, 14 June 2023
• 2.0 Public disclosure, 21 June 2023

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 be mailed 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-2023-2911 is the complete and official security advisory document.

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