CVE-2018-5743: Limiting simultaneous TCP clients is ineffective
  • 24 Apr 2019
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CVE-2018-5743: Limiting simultaneous TCP clients is ineffective

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Article Summary

CVE: CVE-2018-5743

Document version: 2.0

Posting date: 24 April 2019

Program impacted: BIND

Versions affected: BIND 9.9.0 -> 9.10.8-P1, 9.11.0 -> 9.11.6, 9.12.0 -> 9.12.4, 9.14.0. BIND 9 Supported Preview Edition versions 9.9.3-S1 -> 9.11.5-S3, and 9.11.5-S5. Versions 9.13.0 -> 9.13.7 of the 9.13 development branch are also affected. Versions prior to BIND 9.9.0 have not been evaluated for vulnerability to CVE-2018-5743.

Severity: High

Exploitable: Remotely


By design, BIND is intended to limit the number of TCP clients that can be connected at any given time. The number of allowed connections is a tunable parameter which, if unset, defaults to a conservative value for most servers. Unfortunately, the code which was intended to limit the number of simultaneous connections contains an error which can be exploited to grow the number of simultaneous connections beyond this limit.


By exploiting the failure to limit simultaneous TCP connections, an attacker can deliberately exhaust the pool of file descriptors available to named, potentially affecting network connections and the management of files such as log files or zone journal files.

In cases where the named process is not limited by OS-enforced per-process limits, this could additionally potentially lead to exhaustion of all available free file descriptors on that system.

CVSS Score: 7.5


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



Active exploits:

No known deliberate exploits, but the situation may occur accidentally on busy servers.

It is possible for operators to mistakenly believe that their configured (or default) limit is sufficient for their typical operations, when in fact it is not. Following an upgrade to a version that effectively applies limits, named may deny connections which were previously improperly permitted. Operators can monitor their logs for rejected connections, keep an eye on "rndc status" reports of simultaneous connections, or use other tools to monitor whether the now-effective limits are causing problems for legitimate clients. Should this be the case, increasing the value of the tcp-clients setting in named.conf to an appropriate value would be recommended.


Upgrade to a version of BIND containing a fix for the ineffective limits.

  • BIND 9.11.6-P1
  • BIND 9.12.4-P1
  • BIND 9.14.1

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

  • BIND 9.11.5-S6
  • BIND 9.11.6-S1

Acknowledgements: ISC would like to thank AT&T for helping us to discover this issue.

Document revision history:

1.0 Advance Notification, 16 January 2019
1.1 Recall due to error in original fix, 17 January 2019
1.3 Replacement fix delivered to Advance Notification customers, 15 April 2019
1.4 Corrected Versions affected and Solution, 16 April 2019
1.5 Added reference to BIND 9.11.6-S1
2.0 Public disclosure, 24 April 2019

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.

This Knowledgebase article is the complete and official security advisory document.

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