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CVE-2011-0997: dhclient Does Not Strip or Escape Shell Meta-characters
Author: Michael McNally Reference Number: AA-00455 Views: 1252 Created: 2011-09-09 23:15 Last Updated: 2012-06-08 14:01 0 Rating/ Voters

DHCP: dhclient does not strip or escape shell meta-characters

dhclient doesn't strip or escape certain shell meta-characters in dhcpd responses, allowing a rogue server or party with with escalated privileges on the server to cause remote code execution on the client.
CERT
VU#107886
Document version:
1.1
Posting date: 
05 Apr 2011
Program Impacted: 
DHCP
Versions affected: 
3.0.x-4.2.x
Severity: 
Medium
Exploitable: 
remotely
Description: 

ISC dhclient did not strip or escape certain shell meta-characters in responses from the dhcp server (like hostname) before passing the responses on to dhclient-script. Depending on the script and OS, this can result in execution of exploit code on the client.

CVSS Score: 6.8 (AV:A/AC:L/Au:N/C:P/I:N/A:C) 
For more information on CVSS scores, visit http://nvd.nist.gov/cvss.cfm?calculator&adv&version=2

Workarounds: 

On SUSE systems, it is possible to disable hostname update by setting DHCLIENT_SET_HOSTNAME="no" in /etc/sysconfig/network/dhcp.
Other systems may add following line to dhclient-script at the beginning of the set_hostname() function:

new_host_name=${new_host_name//[^-.a-zA-Z0-9]/}

In environments where filters/acls can be put into place to limit clients to accessing only legitimate dhcp servers, this will protect clients
from rogue dhcp servers deliberately trying to exploit this bug. However, this will not protect from compromised servers.

Active exploits: 
None known at this time.
Solution: 

Upgrade to 3.1-ESV-R1, 4.1-ESV-R2 or 4.2.1-P1.  https://www.isc.org/downloads/all

No patch is available for 4.0.x as it is EOL.  Anyone running 4.1.x should upgrade to 4.1-ESV-R2.

Acknowledgments:
Sebastian Krahmer and Marius Tomaschewski, SUSE Security Team

Revisions:
Mar29-Updated CVSS score
Apr6-Corrected workaround

Do you have Questions? Questions regarding this advisory should go to security-officer@isc.org.

This security advisory is a copy of the official document located on our website: https://www.isc.org/software/dhcp/advisories/cve-2011-0997

Do you need Software Support? Questions on ISC's Support services or other offerings should be sent to sales@isc.org. More information on ISC's support and other offerings are available at: http://www.isc.org/community/blog/201102/BIND-support

ISC Security Vulnerability Disclosure Policy: Details of our current security advisory policy and practice can be found here: https://www.isc.org/security-vulnerability-disclosure-policy  

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