CVE-2019-6477: TCP-pipelined queries can bypass tcp-clients limit
  • 20 Nov 2019
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CVE-2019-6477: TCP-pipelined queries can bypass tcp-clients limit

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Article summary

CVE: CVE-2019-6477

Document version: 2.0

Posting date: 20 November 2019

Program impacted: BIND

Versions affected: BIND 9.11.6-P1 -> 9.11.12, 9.12.4-P1 -> 9.12.4-P2, 9.14.1 -> 9.14.7, and versions 9.11.5-S6 -> 9.11.12-S1 of BIND 9 Supported Preview Edition. Versions 9.15.0 -> 9.15.5 of the BIND 9.15 development branch are also affected. Versions prior to BIND 9.11.0 have not been evaluated for vulnerability to CVE-2019-6477.

Severity: Medium

Exploitable: Remotely


By design, BIND is intended to limit the number of TCP clients that can be connected at any given time. The update to this functionality introduced by CVE-2018-5743 changed how BIND calculates the number of concurrent TCP clients from counting the outstanding TCP queries to counting the TCP client connections. On a server with TCP-pipelining capability, it is possible for one TCP client to send a large number of DNS requests over a single connection. Each outstanding query will be handled internally as an independent client request, thus bypassing the new TCP clients limit.


With pipelining enabled each incoming query on a TCP connection requires a similar resource allocation to a query received via UDP or via TCP without pipelining enabled. A client using a TCP-pipelined connection to a server could consume more resources than the server has been provisioned to handle. When a TCP connection with a large number of pipelined queries is closed, the load on the server releasing these multiple resources can cause it to become unresponsive, even for queries that can be answered authoritatively or from cache. (This is most likely to be perceived as an intermittent server problem).

CVSS Score: 6.5


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


The vulnerability can be avoided by disabling server TCP-pipelining:

    keep-response-order { any; };

and then restarting BIND. The server restart is necessary because neither a 'reload' nor a 'reconfig' operation will properly reset currently pipelining TCP clients.

Active exploits:

We are not aware of any active exploits but we have received reports of servers accidentally affected by high-query-volume clients using TCP-pipelining.


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

  • BIND 9.11.13
  • BIND 9.14.8
  • BIND 9.15.6

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

  • BIND 9.11.13-S1

Note that the fix for CVE-2019-6477 addresses only the server memory leak issue. TCP-pipelining may still malfunction by dropping some responses on a TCP connection where a client query pattern generates excessive outstanding queries, but the malfunction will affect that TCP connection alone and will not cause any degradation of service to other clients. An affected client connection might also appear to hang, but will clear when either the client or the server initiates a close or reset and will not remain in that state indefinitely.

Disabling TCP-pipelining entirely is completely effective at mitigating the vulnerability with minimal impact to clients that use pipelined TCP connections and with no impact to clients that do not support TCP-pipelining.

The majority of Internet client DNS queries are transported over UDP or TCP without use of TCP-pipelining.

Document revision history:

1.0 Early Notification, 11 November 2019
1.1 Updated Solution, 19 November 2019
2.0 Public Disclosure, 20 November 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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