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.


CVE: CVE-2011-0997
CERT: VU#107886
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

Questions regarding this advisory or ISC's Support services should be
sent to dhcp-bugs@isc.org