dhclient in ISC DHCP 3.0.x through 4.2.x before 4.2.1-P1 3.1-ESV before 3.1-ESV-R1 and 4.1-ESV before 4.1-ESV-R2 allows remote attackers to execute arbitrary commands via shell metacharacters in a hostname obtained from a DHCP message as demonstrated by a hostname that is provided to dhclient-script.