It seems that server_name from DHCP response is passed to environment variable without sanitising.

  3437		if (check_option_values(NULL, DHO_HOST_NAME,
  3438					lease->server_name,
  3439					strlen(lease->server_name)) == 0 ) {
  3440			client_envadd (client, prefix, "server_name",
  3441				       "%s", lease->server_name);