GEN:SID 1:272
Message DOS IGMP dos attack
Summary This event is generated when a remote attacker transmits fragmented IGMP packets with malformed headers to the internal network, indicating an IGMP Denial of Service (DoS) attack.
Impact Denial of service.
Detailed Information If an IGMP packet with a malformed header is transmitted to an unpatched Microsoft Windows computer, the computer may crash when it attempts to process the packet.  
Affected Systems Microsoft Windows 95
Microsoft Windows 98
Microsoft Windows 98 SE
Microsoft Windows NT 4
Attack Scenarios An attacker sends fragmented IGMP packets with malformed headers to a target computer. If the computer is running an unpatched version of Windows, it may crash.
Ease of Attack Simple. Exploits exist.
Corrective Action Install the latest patches available for your operating system. See http://www.microsoft.com/technet/security/bulletin/ms99-034.asp for more information.

Implement a packet-filtering firewall to block inappropriate traffic to the network.
Additional References Bugtraq
http://www.securityfocus.com/bid/514

Microsoft
http://www.microsoft.com/technet/security/bulletin/ms99-034.asp
Rule References url: www.microsoft.com/technet/security/bulletin/MS99-034.mspx
cve: 1999-0918
bugtraq: 514