ifconfig, when used on the Linux kernel 2.2 and later, does not report when the network interface is in promiscuous mode if it was put in promiscuous mode using PACKET_MR_PROMISC, which could allow attackers to sniff the network without detection, as demonstrated using libpcap.
http://www.securityfocus.com/bid/5304
http://www.iss.net/security_center/static/9676.php
http://online.securityfocus.com/archive/1/284257
http://online.securityfocus.com/archive/1/284142
http://archives.neohapsis.com/archives/bugtraq/2002-07/0279.html