An attacker may execute a UDP Fragmentation attack against a target server in an attempt to consume resources such as bandwidth and CPU. IP fragmentation occurs when an IP datagram is larger than the MTU of the route the datagram has to traverse. Typically the attacker will use large UDP packets over 1500 bytes of data which forces fragmentation as ethernet MTU is 1500 bytes. This attack is a variation on a typical UDP flood but it enables more network bandwidth to be consumed with fewer packets. Additionally it has the potential to consume server CPU resources and fill memory buffers associated with the processing and reassembling of fragmented packets.
An adversary causes the target to allocate excessive resources to servicing the attackers' request, thereby reducing the resources available for legitimate services and degrading or denying services. Usually, this attack focuses on memory allocation, but any finite resource on the target could be the attacked, including bandwidth, processing cycles, or other resources. This attack does not attempt to force this allocation through a large number of requests (that would be Resource Depletion through Flooding) but instead uses one or a small number of requests that are carefully formatted to force the target to allocate excessive resources to service this request(s). Often this attack takes advantage of a bug in the target to cause the target to allocate resources vastly beyond what would be needed for a normal request.
An attacker may execute a TCP Fragmentation attack against a target with the intention of avoiding filtering rules. IP fragmentation occurs when an IP datagram is larger than the MTU of the route the datagram has to traverse. The attacker attempts to fragment the TCP packet such that the headers flag field is pushed into the second fragment which typically is not filtered. This behavior defeats some IPS and firewall filters who typically check the FLAGS in the header of the first packet since dropping this packet prevents the following fragments from being processed and assembled. Another variation is overlapping fragments thus that an innocuous first segment passes the filter and the second segment overwrites the TCP header data with the true payload which is malicious in nature. The malicious payload manipulated properly may lead to a DoS due to resource consumption or kernel crash. Additionally the fragmentation could be used in conjunction with sending fragments at a rate slightly slower than the timeout to cause a DoS condition by forcing resources that assemble the packet to wait an inordinate amount of time to complete the task. The fragmentation identification numbers could also be duplicated very easily as there are only 16 bits in IPv4 so only 65536 packets are needed.
Resource Leak Exposure
An adversary utilizes a resource leak on the target to deplete the quantity of the resource available to service legitimate requests. Resource leaks most often come in the form of memory leaks where memory is allocated but never released after it has served its purpose, however, theoretically, any other resource that can be reserved can be targeted if the target fails to release the reservation when the reserved resource block is no longer needed. In this attack, the adversary determines what activity results in leaked resources and then triggers that activity on the target. Since some leaks may be small, this may require a large number of requests by the adversary. However, this attack differs from a flooding attack in that the rate of requests is generally not significant. This is because the lost resources due to the leak accumulate until the target is reset, usually by restarting it. Thus, a resource-poor adversary who would be unable to flood the target can still utilize this attack. Resource depletion through leak differs from resource depletion through allocation in that, in the former, the adversary may not be able to control the size of each leaked allocation, but instead allows the leak to accumulate until it is large enough to affect the target's performance. When depleting resources through allocation, the allocated resource may eventually be released by the target so the attack relies on making sure that the allocation size itself is prohibitive of normal operations by the target.
An adversary consumes the resources of a target by rapidly engaging in a large number of interactions with the target. This type of attack generally exposes a weakness in rate limiting or flow. When successful this attack prevents legitimate users from accessing the service and can cause the target to crash. This attack differs from resource depletion through leaks or allocations in that the latter attacks do not rely on the volume of requests made to the target but instead focus on manipulation of the target's operations. The key factor in a flooding attack is the number of requests the adversary can make in a given period of time. The greater this number, the more likely an attack is to succeed against a given target.
An attacker may execute a ICMP Fragmentation attack against a target with the intention of consuming resources or causing a crash. The attacker crafts a large number of identical fragmented IP packets containing a portion of a fragmented ICMP message. The attacker these sends these messages to a target host which causes the host to become non-responsive. Another vector may be sending a fragmented ICMP message to a target host with incorrect sizes in the header which causes the host to hang.