Description:

In many cases, the ISC DHCP server does not effectively limit the number
of simultaneous open TCP connections to the ports the server uses for
inter-process communications and control.  Because of this, a malicious
party could interfere with server operation by opening (and never
closing) a large number of TCP connections to the server.

Impact:

By exploiting this vulnerability an attacker can interfere with DHCP
server operation.  Exact results are difficult to summarize concisely
because the effect of an attack varies depending on server version, the
channel being attacked, and in some operating systems on environment
settings inherited from the launching shell (e.g. "ulimit" settings on
per-process open file descriptors) but depending on the combination
potential undesirable outcomes include (but are not necessarily limited to):

The server may deliberately exit after encountering an INSIST failure
(server version dependent).
The server may become unresponsive and stop answering client requests.
The server may continue operating but not be able to accept further
connections from OMAPI clients or failover peers.
If no limits are inherited from the environment, the server may consume
all available sockets, potentially interfering with other services
running on the same machine.
Risk of exploitation is highest on the OMAPI port (if OMAPI is
configured).  The failover code will close incoming connections if they
are not received from a peer (making it more difficult but not
impossible to attack a server using failover channels).  OMAPI, however,
has no logic in the server limiting addresses from which it will accept
connections.  A firewall is recommended as an industry-standard
precaution against accepting connections from untrusted hosts.

CVSS Score:  5.7

CVSS Vector:  (AV:A/AC:M/Au:N/C:N/I:N/A:C)

For more information on the Common Vulnerability Scoring System and to
obtain your specific environmental score please visit:
http://nvd.nist.gov/cvss.cfm?calculator&adv&version=2&vector=(AV:A/AC:M/Au:N/C:N/I:N/A:C)

Workarounds:

ISC recommends that server operators restrict the hosts allowed to make
connections to DHCP server inter-process communication channels to
trusted hosts, blocking connections to the OMAPI control port and the
failover communications ports from all other hosts.

If OMAPI and/or failover are not being actively used, they can be
disabled. The ISC Knowledge Base contains some information that
operators may find relevant:

 +
https://kb.isc.org/article/AA-01355/56/Securing-dhcpd-against-unauthorised-OMAPI-control-connections.html
 + https://kb.isc.org/article/AA-01356/56/What-is-DHCP-Failover.html
 +
https://kb.isc.org/article/AA-00502/31/A-Basic-Guide-to-Configuring-DHCP-Failover.html

Additionally, in environments where per-process file descriptor limits
can be inherited from the shell used to launch dhcpd, using ulimit to
set a reasonable limit on simultaneous socket connections can prevent
the INSIST assertion failure outcome but may still allow interference
with legitimate interprocess communication traffic.

Active exploits:

No known active exploits, but a public mention of the issue has occurred
on an open mailing list.

Solution:

Mitigation code which will make this vulnerability harder to exploit
will be added to the upcoming DHCP maintenance releases (DHCP
4.1-ESV-R13, DHCP 4.3.4, due to be released in March 2016).

However, the strategies described in the "Workarounds" section of this
document are effective and can prevent exploitation of the
vulnerability.  Unless server operators have identified operational
needs unique to their environment which conflict with this advice, ISC
recommends blocking incoming TCP connections from untrusted hosts as a
preferred strategy.

Document Revision History:

1.0 Advance Notification 04 March 2016

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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 http://www.isc.org/downloads/).

ISC Security Vulnerability Disclosure Policy:  Details of our current
security advisory policy and practice can be found here:
https://kb.isc.org/article/AA-00861/164/ISC-Software-Defect-and-Security-Vulnerability-Disclosure-Policy.html

This Knowledge Base article https://kb.isc.org/article/AA-01354 is the
complete and official security advisory document.

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