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CVE-2016-2774: An attacker who is allowed to connect to DHCP inter-server communications and control channels can exhaust server resources
Author: Michael McNally Reference Number: AA-01354 Views: 24461 Created: 2016-03-07 11:34 Last Updated: 2016-03-31 15:10 0 Rating/ Voters

Document Version:          
2.1
Posting date: 
07 March 2016
Program Impacted:
ISC DHCP
Versions affected: 
4.1.0->4.1-ESV-R12-P1, 4.2.0->4.2.8, 4.3.0->4.3.3-P1.  Older versions may also be affected but are well beyond their end-of-life (EOL).  Releases prior to 4.1.0 have not been tested.
Severity:
Medium
Exploitable: 
Remotely, if remote network connections to the DHCP server's control ports (e.g. OMAPI and failover) are permitted.

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:

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) - see [ISC-Bugs #41845].

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.

Acknowledgements: ISC would like to thank Konstantin Orekhov for discovering this vulnerability.

Document Revision History:

1.0 Advance Notification 04 March 2016
2.0 Public Disclosure 07 March 2016
2.1 Added reference to ISC Bug ticket #41845, 31 March 2016

If you'd like more information on ISC Subscription Support and Advance Security Notifications, please visit http://www.isc.org/support/.

Do you still have questions?  Questions regarding this advisory should go to security-officer@isc.orgTo report a new issue, please encrypt your message using security-officer@isc.org's PGP key which can be found here: https://www.isc.org/downloads/software-support-policy/openpgp-key/.  If you are unable to use encrypted email, you may also report new issues at: https://www.isc.org/community/report-bug/.

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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