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

[Cathy Almond]

CVE:                 CVE-2019-6477
Document version:    1.1
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

Description:

    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.

Impact:

    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
CVSS Vector: CVSS:3.1AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C

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/E:U/RL:O/RC:C&version=3.1

Workarounds:

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.

Solution:

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, 13 November 2019
1.1 Updated Solution, 19 November 2019

Related documents:

See our BIND 9 Security Vulnerability Matrix
( https://kb.isc.org/docs/aa-00913 ) for a complete listing of security
vulnerabilities and versions affected.

Do you still have questions? Questions regarding this advisory should go
to security-officer at isc.org. To report a new issue, please encrypt your
message using security-officer at 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 .

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