ESB-2018.2068 - [Linux][BSD][Solaris] ISC Kea: Denial of service - Remote/unauthenticated 2018-07-17

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             AUSCERT External Security Bulletin Redistribution

        CVE-2018-5739: ISC Kea 1.4.0 failure to release memory may
                         exhaust system resources
                               17 July 2018


        AusCERT Security Bulletin Summary

Product:           ISC Kea
Publisher:         ISC
Operating System:  Linux variants
                   BSD variants
Impact/Access:     Denial of Service -- Remote/Unauthenticated
Resolution:        Patch/Upgrade
CVE Names:         CVE-2018-5739  

Original Bulletin:

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CVE-2018-5739: ISC Kea 1.4.0 failure to release memory may exhaust system

Author: Cathy Almond 
Reference Number: AA-01626 
Created: 2018-06-29 14:32 
Last Updated: 2018-07-11 22:25

Kea DHCP 1.4.0 may fail to release memory after temporarily storing client
network packets.  This causes a constant increase in memory consumption
that can cause server resources to become exhausted, leading to loss of
DHCP server functionality.

CVE: CVE-2018-5739
Document Version: 2.0
Posting date: 11 July 2018
Program Impacted: Kea DHCP
Versions affected: 1.4.0
Severity: Medium
Exploitable: From adjacent networks permitted to relay DHCP traffic to the Kea server


An extension to hooks capabilities which debuted in Kea 1.4.0 introduced a
memory leak for operators who are using certain hooks library facilities. In
order to support multiple requests simultaneously, Kea 1.4 added a callout
handle store but unfortunately the initial implementation of this store
does not properly free memory in every case.  Hooks which make use of
query4 or query6 parameters in their callouts can leak memory, resulting
in the eventual exhaustion of available memory and subsequent failure of
the server process.


Only servers using hooks which make use of the callout handle store are
affected.  A Kea server which is using one or more hooks libraries that
exhibit this problem will increase its memory use over time, with the rate
of increase being proportional to the amount of DHCP traffic processed.
Eventually, due to uncontrolled growth, the server will either exhaust all
system memory or, if the administrator has set a per-process memory limit,
will hit that limit, after which point further memory allocations will
fail and the Kea server will crash.

An attacker who is within the broadcast domain of the Kea server or in
a network which is permitted to relay DHCP traffic to the Kea server can
hasten the arrival of this outcome by deliberately sending a large volume
of requests to the Kea server.

Ability to deliberately trigger this vulnerability depends on the hooks
libraries used and the hook points used for callouts.  Our scoring for
this vulnerability is based on the hook points used for hook libraries
distributed by ISC and also based on the assumption that the Kea server does
not accept arbitrary traffic from the internet (but is protected, e.g. by
firewall, and only accepts DHCP traffic from the local broadcast domain and
from nearby networks via authorized DHCP relay agents.)  We cannot score
every combination, but the risk could be higher to custom-developed hook
libraries using other hook points or to servers which accept arbitrary
DHCP traffic without restriction.

CVSS Score:  6.5, or 4.3 if a supervising process will restart the Kea
server if it terminates.

CVSS Vector:  CVSS:3.0/AV:A/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:


Monitoring and routinely restarting ISC Kea DHCPv4 and DHCPv6 services
may be an effective mitigation for some production environments
Running a new build of Kea without any hook libraries that use the
callout store is another option, though it may not be a viable option
where the production environment is dependent on the other hooks that
need to be omitted to avoid these symptoms.  These hooks distributed by
ISC do not use the callout store and are safe to use:  Lease Commands,
Stat Commands, Host Commands (a Kea Premium hook) and Subnet Commands
(a subscriber-only hook provided to Kea support customers).
Reverting to Kea DHCP 1.3.0 may be possible for some production
environments but because of differences in the database schema operators
should check carefully before attempting rollback:

If using memfile storage entirely, there should not be any
compatibility issues

If using a database solution for hosts or leases, the 1.4.0 schema
will be incompatible with ISC Kea 1.3.0; the database therefore
must be restored from a pre-upgrade backup for this to be successful.

If you are unsure whether or not you can roll back to 1.3.0 without
restoring a previous version of your database, you may send an
e-mail to describing your storage setup
and we will advise.

Active exploits:

ISC are not aware of any deliberate exploits of this condition but even
without deliberate exploitation the memory allocations of affected servers
will grow over time until memory exhaustion becomes a problem.


Upgrade to Kea 1.4.0-P1, available via

Acknowledgements:  ISC would like to thank Shawn Routhier of Infoblox for
making us aware of this issue.

Document Revision History:

1.0 Advance Notification, 29 June 2018
1.1 Corrected description of Subnet Commands hook, 02 July 2018
2.0 Public disclosure, 11 July 2018

If you'd like more information on ISC Subscription Support and Advance
Security Notifications, please visit

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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

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