DDoS Protection Requires Looking Both Ways
Service availability is a key component of the user experience. Customers expect services to be constantly available and fast-responding, and any downtime can result in disappointed users, abandoned shopping carts, and lost customers.
Consequently, DDoS attacks are increasing in complexity, size and duration. Radware’s 2018 Global Application and Network Security Report found that over the course of a year, sophisticated DDoS attacks, such as burst attacks, increased by 15%, HTTPS floods grew by 20%, and over 64% of customers were hit by application-layer (L7) DDoS attacks.
Some Attacks are a Two-Way Street
As DDoS attacks become more complex, organizations require more elaborate protections to mitigate such attacks. However, in order to guarantee complete protection, many types of attacks – particularly the more sophisticated ones – require visibility into both inbound and outbound channels.
Some examples of such attacks include:
Out of State Protocol Attacks: Some DDoS attacks exploit weaknesses in protocol communication processes, such as TCP’s three-way handshake sequence, to create ‘out-of-state’ connection requests, thereby drawing-out connection requests in order to exhaust server resources. While some attacks of this type, such as a SYN flood, can be stopped by examining the inbound channel only, others require visibility into the outbound channel, as well.
An example of this is an ACK flood, whereby attackers continuously send forged TCP ACK packets towards the victim host. The target host then tries to associate the ACK reply to an existing TCP connection, and if none such exists, it will drop the packet. However, this process consumes server resources, and large numbers of such requests can deplete system resources. In order to correctly identify and mitigate such attacks, defenses need visibility to both inbound SYN and outbound SYN/ACK replies, so that they can verify whether the ACK packet is associated with any legitimate connection request.
Reflection/Amplification Attacks: Such attacks exploit asymmetric responses between the connection requests and replies of certain protocols or applications. Again, some types of such attacks require visibility into both the inbound and outbound traffic channels.
An example of such attack is a large-file outbound pipe saturation attack. In such attacks, the attackers identify a very large file on the target network, and send a connection request to fetch it. The connection request itself can be only a few bytes in size, but the ensuing reply could be extremely large. Large amounts of such requests can clog-up the outbound pipe.
Another example are memcached amplification attacks. Although such attacks are most frequently used to overwhelm a third-party target via reflection, they can also be used to saturate the outbound channel of the targeted network.
Scanning Attacks: Large-scale network scanning attempts are not just a security risk, but also frequently bear the hallmark of a DDoS attack, flooding the network with malicious traffic. Such scan attempts are based on sending large numbers of connection requests to host ports, and seeing which ports answer back (thereby indicating that they are open). However, this also leads to high volumes of error responses by closed ports. Mitigation of such attacks requires visibility into return traffic in order to identify the error response rate relative to actual traffic, in order for defenses to conclude that an attack is taking place.
Server Cracking: Similar to scanning attacks, server cracking attacks involve sending large amounts of requests in order to brute-force system passwords. Similarly, this leads to a high error reply rate, which requires visibility into both the inbound and outbound channels in order to identify the attack.
Stateful Application-Layer DDoS Attacks: Certain types of application-layer (L7) DDoS attacks exploit known protocol weaknesses or order to create large amounts of spoofed requests which exhaust server resources. Mitigating such attacks requires state-aware bi-directional visibility in order to identify attack patterns, so that the relevant attack signature can be applied to block it. Examples of such attacks are low-and-slow and application-layer (L7) SYN floods, which draw-out HTTP and TCP connections in order to continuously consume server resources.
Two-Way Attacks Require Bi-Directional Defenses
As online service availability becomes ever-more important, hackers are coming up with more sophisticated attacks than ever in order to overwhelm defenses. Many such attack vectors – frequently the more sophisticated and potent ones – either target or take advantages of the outbound communication channel.
Therefore, in order for organizations to fully protect themselves, they must deploy protections that allow bi-directional inspection of traffic in order to identify and neutralize such threats.