Unknown vulnerability in ip_nat_sack_adjust of Netfilter in Linux kernels 2.4.20 and some 2.5.x when CONFIG_IP_NF_NAT_FTP or CONFIG_IP_NF_NAT_IRC is enabled or the ip_nat_ftp or ip_nat_irc modules are loaded allows remote attackers to cause a denial of service (crash) in systems using NAT possibly due to an integer signedness error.