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.