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In this article by Gerard Johansen, author of the book Digital Forensics and Incident Response, explains that the traditional focus of digital forensics has been to locate evidence on the host hard drive. Law enforcement officers interested in criminal activity such as fraud or child exploitation can find the vast majority of evidence required for prosecution on a single hard drive. In the realm of Incident Response though, it is critical that the focus goes far beyond a suspected compromised system. There is a wealth of information to be obtained within the points along the flow of traffic from a compromised host to an external Command and Control server for example.

(For more resources related to this topic, see here.)

This article focuses on the preparation, identification and collection of evidence that is commonly found among network devices and along the traffic routes within an internal network. This collection is critical during an incident where an external threat sources is in the process of commanding internal systems or is in the process of pilfering data out of the network. Network based evidence is also useful when examining host evidence as it provides a second source of event corroboration which is extremely useful in determining the root cause of an incident.


The ability to acquire network-based evidence is largely dependent on the preparations that are untaken by an organization prior to an incident. Without some critical components of a proper infrastructure security program, key pieces of evidence will not be available for incident responders in a timely manner. The result is that evidence may be lost as the CSIRT members hunt down critical pieces of information. In terms of preparation, organizations can aid the CSIRT by having proper network documentation, up to date configurations of network devices and a central log management solution in place.

Aside from the technical preparation for network evidence collection, CSIRT personnel need to be aware of any legal or regulatory issues in regards to collecting network evidence. CSIRT personnel need to be aware that capturing network traffic can be considered an invasion of privacy absent any other policy. Therefore, the legal representative of the CSIRT should ensure that all employees of the organization understand that their use of the information system can be monitored. This should be expressly stated in policies prior to any evidence collection that may take place.

Network diagram

To identify potential sources of evidence, incident responders need to have a solid understanding of what the internal network infrastructure looks like. One method that can be employed by organizations is to create and maintain an up to date network diagram. This diagram should be detailed enough so that incident responders can identify individual network components such as switches, routers or wireless access points. This diagram should also contain internal IP addresses so that incident responders can immediately access those systems through remote methods. For instance, examine the below simple network diagram:

This diagram allows for a quick identification of potential evidence sources. In the above diagram, for example, suppose that the laptop connected to the switch at is identified as communicating with a known malware Command and Control server. A CSIRT analyst could examine the network diagram and ascertain that the C2 traffic would have to traverse several network hardware components on its way out of the internal network. For example, there would be traffic traversing the switch at, through the firewall at and finally the router out to the Internet.


Determining if an attacker has made modifications to a network device such as a switch or a router can be made easier if the CSIRT has a standard configuration immediately available. Organizations should already have configurations for network devices stored for Disaster Recovery purposes but should have these available for CSIRT members in the event that there is an incident.

Logs and log management

The lifeblood of a good incident investigation is evidence from a wide range of sources. Even something as a malware infection on a host system requires corroboration among a variety of sources. One common challenge with Incident Response, especially in smaller networks is how the organization handles log management. For a comprehensive investigation, incident response analysts need access to as much network data as possible. All to often, organizations do not dedicate the proper resources to enabling the comprehensive logs from network devices and other systems.

Prior to any incident, it is critical to clearly define the how and what an organization will log and as well as how it will maintain those logs. This should be established within a log management policy and associated procedure. The CSIRT personnel should be involved in any discussion as what logs are necessary or not as they will often have insight into the value of one log source over another.

NIST has published a short guide to log management available at: http://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-92.pdf.

Aside from the technical issues regarding log management, there are legal issues that must be addressed. The following are some issues that should be addressed by the CSIRT and its legal support prior to any incident.

  • Establish logging as a normal business practice: Depending on the type of business and the jurisdiction, users may have a reasonable expectation of privacy absent any expressly stated monitoring policy. In addition, if logs are enabled strictly to determine a user’s potential malicious activity, there may be legal issues. As a result, the logging policy should establish that logging of network activity is part of the normal business activity and that users do not have a reasonable expectation of privacy.
  • Logging as close to the event: This is not so much an issue with automated logging as they are often created almost as the event occurs. From an evidentiary standpoint, logs that are not created close to the event lose their value as evidence in a courtroom.
  • Knowledgeable Personnel: The value of logs is often dependent on who created the entry and whether or not they were knowledgeable about the event. In the case of logs from network devices, the logging software addresses this issue. As long as the software can be demonstrated to be functioning properly, there should be no issue.
  • Comprehensive Logging: Enterprise logging should be configured for as much of the enterprise as possible. In addition, logging should be consistent. A pattern of logging that is random will have less value in a court than a consistent patter of logging across the entire enterprise.
  • Qualified Custodian: The logging policy should name a Data Custodian. This individual would speak to the logging and the types of software utilized to create the logs. They would also be responsible for testifying to the accuracy of the logs and the logging software used.
  • Document Failures: Prolonged failures or a history of failures in the logging of events may diminish their value in a courtroom. It is imperative that any logging failure be documented and a reason is associated with such failure.
  • Log File Discovery: Organizations should be made aware that logs utilized within a courtroom proceeding are going to be made available to opposing legal counsel.
  • Logs from compromised systems: Logs that originate from a known compromised system are suspect. In the event that these logs are to be introduced as evidence, the custodian or incident responder will often have to testify at length concerning the veracity of the data contained within the logs.
  • Original copies are preferred: Log files can be copied from the log source to media. As a further step, any logs should be archived off the system as well. Incident responders should establish a chain of custody for each log file used throughout the incident and these logs maintained as part of the case until an order from the court is obtained allowing their destruction.

Network device evidence

There are a number of log sources that can provide CSIRT personnel and incident responders with good information. A range of manufacturers provides each of these network devices. As a preparation task, CSIRT personnel should become familiar on how to access these devices and obtain the necessary evidence:

  • Switches: These are spread throughout a network through a combination of core switches that handle traffic from a range of network segments to edge switches which handle the traffic for individual segments. As a result, traffic that originates on a host and travels out the internal network will traverse a number of switches. Switches have two key points of evidence that should be addressed by incident responders. First is the Content Addressable Memory (CAM) table. This CAM table maps the physical ports on the switch to the Network Interface Card (NIC) on each device connected to the switch. Incident responders in tracing connections to specific network jacks can utilize this information. This can aid in the identification of possible rogue devices. The second way switches can aid in an incident investigation is through facilitating network traffic capture.
  • Routers: Routers allow organizations to connect multiple LANs into either Metropolitan Area Networks or Wide Area Networks. As a result, the handled an extensive amount of traffic. The key piece of evidentiary information that routers contain is the routing table. This table holds the information for specific physical ports that map to the networks. Routers can also be configured to deny specific traffic between networks and maintain logs on allowed traffic and data flow.
  • Firewalls: Firewalls have changed significantly since the days when they were considered just a different type of router. Next generation firewalls contain a wide variety of features such as Intrusion Detection and Prevention, Web filtering, Data Loss Prevention and detailed logs about allowed and denied traffic. Firewalls often times serve as the detection mechanism that alerts security personnel to potential incidents. Incident responders should have as much visibility into how their organization’s firewalls function and what data can be obtained prior to an incident.
  • Network Intrusion Detection and Prevention systems: These systems were purposefully designed to provide security personnel and incident responders with information concerning potential malicious activity on the network infrastructure. These systems utilize a combination of network monitoring and rule sets to determine if there is malicious activity. Intrusion Detection Systems are often configured to alert to specific malicious activity while Intrusion Prevention Systems can detection but also block potential malicious activity. In either case, both types of platforms logs are an excellent place for incident responders to locate specific evidence on malicious activity.
  • Web Proxy Servers: Organization often utilize Web Proxy Servers to control how users interact with websites and other internet based resources. As a result, these devices can give an enterprise wide picture of web traffic that both originates and is destined for internal hosts. Web proxies also have the additional feature set of alerting to connections to known malware Command and Control (C2) servers or websites that serve up malware. A review of web proxy logs in conjunction with a possible compromised host may identify a source of malicious traffic or a C2 server exerting control over the host.
  • Domain Controllers / Authentication Servers: Serving the entire network domain, authentication servers are the primary location that incident responders can leverage for details on successful or unsuccessful logins, credential manipulation or other credential use.
  • DHCP Server: Maintaining a list of assigned IP addresses to workstations or laptops within the organization requires an inordinate amount of upkeep. The use of Dynamic Host Configuration Protocol allows for the dynamic assignment of IP addresses to systems on the LAN. The DHCP servers often contain logs on the assignment of IP addresses mapped to the MAC address of the hosts NIC. This becomes important if an incident responder has to track down a specific workstation or laptop that was connected to the network at a specific data and time.
  • Application Servers: A wide range of applications from Email to Web Applications is housed on network servers. Each of these can provide logs specific to the type of application.

Network devices such as switches, routers and firewalls also have their own internal logs that maintain data on access and changes. Incident responders should become familiar with the types of network devices on their organization’s network and also be able to access these logs in the event of an incident.

Security information and Event management system

A significant challenge that a great many organizations has is the nature of logging on network devices. With limited space, log files are often rolled over where the new log files are written over older log files. The result is that in some cases, an organization may only have a few days or even a few hours of important logs. If a potential incident happened several weeks ago, the incident response personnel will be without critical pieces of evidence.

One tool that has been embraced by a number of enterprises is a Security Information and Event Management (SIEM) System. These appliances have the ability to aggregate log and event data from network sources and combine them into a single location. This allows the CSIRT and other security personnel to observe activity across the entire network without having to examine individual systems.

The diagram below illustrates how a SIEM integrates into the overall network:

A variety of sources from security controls to SQL databases are configured to send logs to the SIEM. In this case, the SQL database located at indicates that the user account USSalesSyncAcct was utilized to copy a database to the remote host located at The SIEM allows for quick examination of this type of activity. For example, if it is determined that the account USSalesSyncAcct had been compromised, CSIRT analysts can quickly query the SIEM for any usage of that account. From there, they would be able to see the log entry that indicated a copy of a database to the remote host. Without that SIEM, CSIRT analysts would have to search each individual system that might have been accessed, a process that may be prohibitive.

From the SIEM platform, security and network analysts have the ability to perform a number of different tasks related to Incident Response:

  • Log Aggregation: Typical enterprises have several thousand devices within the internal network, each with their own logs; the SIEM can be deployed to aggregate these logs in a central location.
  • Log Retention: Another key feature that SIEM platforms provide is a platform to retain logs. Compliance frameworks such as the Payment Card Industry Data Security Standard (PCI-DSS) stipulate that logs should be maintained for a period of one year with 90 days immediately available. SIEM platforms can aid with log management by providing a system that archives logs in an orderly fashion and allows for the immediate retrieval.
  • Routine Analysis: It is advisable with a SIEM platform to conduct period reviews of the information. SIEM platforms often provide a dashboard that highlights key elements such as the number of connections, data flow, and any critical alerts. SIEMs also allow for reporting so that stakeholders can keep informed of activity.
  • Alerting: SIEM platforms have the ability to alert to specific conditions that may indicate malicious activity. This can include alerting from security controls such as anti-virus, Intrusion Prevention or Detection Systems. Another key feature of SIEM platforms is event correlation. This technique examines the log files and determines if there is a link or any commonality in the events. The SIEM then has the capability to alert on these types of events. For example, if a user account attempts multiple logins across a number of systems in the enterprise, the SIEM can identify that activity and alert to it.
  • Incident Response: As the SIEM becomes the single point for log aggregation and analysis; CSIRT analysts will often make use of the SIEM during an incident. CSIRT analysis will often make queries on the platform as well as download logs for offline analysis. Because of the centralization of log files, the time to conduct searches and event collection is significantly reduced. For example, a CSIRT analysis has indicated a user account has been compromised. Without a SIEM, the CSIRT analyst would have to check various systems for any activity pertaining to that user account. With a SIEM in place, the analyst simply conducts a search of that user account on the SIEM platform, which has aggregated user account activity, logs from systems all over the enterprise. The result is the analyst has a clear idea of the user account activity in a fraction of the time it would have taken to examine logs from various systems throughout the enterprise.

SIEM platforms do entail a good deal of time and money to purchase and implement. Adding to that cost is the constant upkeep, maintenance and modification to rules that is necessary. From an Incident Response perspective though, a properly configured and maintained SIEM is vital to gathering network-based evidence in a timely manner. In addition, the features and capability of SIEM platforms can significantly reduce the time it takes to determine a root cause of an incident once it has been detected.

The following article has an excellent breakdown and use cases of SIEM platforms in enterprise environments: https://gbhackers.com/security-information-and-event-management-siem-a-detailed-explanation/.

Security onion

Full-featured SIEM platforms may be cost prohibitive for some organizations. One option that is available is the open source platform Security Onion. The Security Onion ties a wide range of security tools such as OSSEC, Suricata, and Snort into a single platform. Security Onion also has features such as dashboards and tools for deep analysis of log files. For example, the following screenshot shows the level of detail available:

Although installing and deploying the Security Onion may require some resources in time, it is a powerful low cost alternative providing a solution to organizations that cannot deploy a full-featured SIEM solution. (The Security Onion platform and associated documentation is available at https://securityonion.net/).


Evidence that is pertinent to incident responders is not just located on the hard drive of a compromised host. There is a wealth of information available from network devices spread throughout the environment. With proper preparation, a CSIRT may be able to leverage the evidence provided by these devices through solutions such as a SIEM. CSIRT personnel also have the ability to capture the network traffic for later analysis through a variety of methods and tools. Behind all of these techniques though, is the legal and policy implications that CSIRT personnel and the organization at large needs to navigate. By preparing for the legal and technical challenges of network evidence collection, CSIRT members can leverage this evidence and move closer to the goal of determining the root cause of an incident and bringing the organization back up to operations.

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