Containment strategies are the actions taken during an incident to limit damage to specific systems or areas of the network. It is critical for organizations to have prepared these in the event of an incident. The rise of ransomware that combines elements of viruses and worms that can quickly spread through an organization highlights the need to rapidly contain an outbreak before it impacts a great many systems. Compounding the challenge with containment is that many enterprise IT systems utilize a "flat" topology, whereby the bulk of systems can communicate with each other. In this type of environment, ransomware and other worms can quickly propagate via legitimate protocols, such as Remote Desktop Services (RDS) or through the Server Message Block (SMB), that were popular during the WannaCry ransomware campaign, which leveraged the EternalBlue vulnerability in the Windows OS SMB installation. For more information, visit https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-0144.

In order to address containment, an organization should have a clear idea of the network topology. This type of network awareness can be achieved through outputs of network discovery tools, up-to-date network diagrams, system inventories, and vulnerability scans. This data should be shared with the CSIRT so that an overall view of the network can be achieved. From here, the CSIRT should coordinate containment plans with network operations personnel so that an overall containment strategy can be crafted, and the potential damage of an incident limited. Having network operations personnel as part of the technical support personnel goes a long way in ensuring this process is streamlined and that containment is achieved as quickly as possible.

One other aspect of how infrastructure is managed that has a direct impact on incident management is that of change management. Mature IT infrastructures usually have a well-documented and governed change management process in place. During an incident, though, the CSIRT and support personnel cannot wait for a change management authorization and a proper change window to implement changes. When exercising containment strategies, IT and organizational leadership should fully understand that changes are going to be made based on the incident. This does not absolve the CSIRT and IT personnel from exercising due care and ensuring that changes are well documented.

In terms of containing a malware outbreak such as a ransomware attack, there are several strategies that can be employed. Ideally, organizations should have some ability to isolate segments of the network from each other, but in the event that this is not possible, CSIRT and IT personnel can take one or more of the following measures:

• Physical containment: In this case, the physical connection to the network is removed from the system. This can be as simple as unplugging the network cable, disabling wireless access, or disabling the connection through the operating system. While this sounds simple, there are several factors that may make this strategy challenging for even the smallest organization. First, is the ability to physically locate the systems impacted. This may be a simpler task inside a data center where the impacted systems are in the same rack, but attempting to physically locate 20 to 30 desktops in a fairly corporate environment takes a great deal of effort. In the time that it may take to remove 20 systems from the network, the malware could have easily spread across to other systems. Further compounding the difficulty of physical containment is the challenge of addressing geographically diverse systems. Having a data center or other operation an hour's drive away would necessitate having an individual on that site to perform the physical containment. As can be seen, physically containing a malware outbreak or another incident can be very difficult if the scope of the incident is beyond the capability of the CSIRT. Physical containment should be reserved for those incidents where the scope is limited and where the CSIRT personnel can immediately remove the systems from the network.

• Network containment: The network containment strategy relies heavily on the expertise of network engineers or architects. It is for this reason that they are often included as part of the technical support personnel within the CSIRT, and should be involved in any containment strategy planning. With this containment strategy, the network administrator(s) will be tasked with modifying switch configurations, to limit the traffic from infected systems on a subnet to other portions of the network. This containment strategy may require modification of configurations on individual switches or through the use of the management console. One aspect of this approach that needs to be addressed is how the organization handles change control. In many organizations, it is common practice to review any switch configuration changes as part of the normal change control process. There needs to be an exception written into that process to facilitate the rapid deployment of switch configuration changes during a declared incident. Network administrators should also ensure that any changes that are made are properly documented so that they can be reversed or otherwise modified during the recovery phase of an incident.
• Perimeter containment: The perimeter firewall is an asset well suited for containment. In some circumstances, the perimeter firewall can be utilized in conjunction with network containment in a Russian nesting-doll approach, where the CSIRT contains network traffic at the perimeter and works its way to the specific subnets containing the impacted systems. For example, malware will often download additional code or other packages via tools such as PowerShell. In the event that the CSIRT has identified the external IP address that is utilized by the malware to download additional packages, it can be blocked at the firewall, thereby preventing additional damage. From here, the CSIRT can then work backward from the perimeter to the impacted systems. The organization can then leave the rule in place until such time that it is deemed no longer necessary. As with network containment, it is important to address any change control issues that may arise from making changes to the firewall ruleset.
• Virtual containment: With the advent of cloud computing and virtualization, many organizations have at least partially moved systems such as servers from physical systems to virtualized systems. Virtualization provides a great deal of flexibility to organizations during normal operations but also has some advantages, in the event that an incident may need to be contained. First, hypervisor software such as VMware's ESXi platform can be utilized to remove the network connection to multiple systems at once. Organizations may also make use of virtual switching in much the same way as physical switches, in terms of containment. Finally, virtualization software allows for the pausing of systems during an incident. This is the preferred method, as suspending or pausing a virtual machine during an incident preserves a good deal of evidence that can be examined later.