Introduction to logging for security purposes

Created:  05 Jul 2018
Updated:  09 Jul 2018
Laying the groundwork for incident readiness


Logging is the foundation on which security monitoring and situational awareness are built.

This guidance will help you devise an approach to logging that will help answer some of the typical questions asked during a cyber incident, such as:

  • What has happened?
  • What is the impact?
  • What should we do next?
  • Has any post-incident remediation been effective?
  • Are our security controls working?

If you can answer these questions, even partially, it will help you to recover quickly from a cyber incident and develop your defences. This will reassure your customers, suppliers, investors and regulators that you have taken all measures necessary to protect your data and systems.

In later guidance, the logging approach we design here will form the basis of a more richly featured capability, geared towards detecting attacks as they happen.


This guidance is for you if:

  • Your organisation currently has little or no logging capability OR You would like to assess the suitability of your current logging capability
  • You would like to be better prepared for a cyber incident
  • You want to understand the NCSC’s expectations regarding basic good practice for logging

Steps to Implementation

This guidance proposes a four step program for putting in place a simple but effective logging capability.

You will learn how to:

  1. Choose which logs to generate or retain 
  2. Decide how to retain logs
  3. Implement log storage and tooling for analysis (includes a list of useful logging tools)
  4. Validate that your logging capability is working as intended

1. Choose which logs to generate or retain: What questions do you need to answer?

Your first step is to generate a list of logs that could be used to determine whether your organisation’s IT has been compromised, and to what extent.

In the table below we've listed the common questions you're likely to be asked during the initial phases of a cyber incident. Working through these will help to focus your efforts and guide next steps.

Accessing logs

If you outsource much of your IT, as many UK public sector bodies do, it can be difficult to access logging data without making unforeseen demands on your suppliers. The table below is configured to help with this situation, giving you some flexibility in how you obtain your logging data.

For example, there are several ways to collect web browsing activity - through a web proxy, host-based agent or (in a limited form) from DNS. If one source is providing contractually difficult to obtain, try another. There are often many ways to capture the data you need.

Using the table

Work out which logs would give you visibility of the categories listed in the first column, and help to answer the questions in the second column. Identify where the relevant logs are generated, how, and for how long they are stored and secured.

This table covers the highest priority categories and questions, but is not an exhaustive list. You may identify other helpful data you can bring in to augment your overall picture.

You might be able to answer the questions without using the event sources listed - this is fine. Your answers, and how you find them, will depend on your unique architecture and technology choices.

Host-based logs

Host-based logs can include those already generated by applications and operating systems, as well as more detailed events captured by host-based agents.

Table: Typical incident questions and suggested event sources

Event category Incident questions Event sources

External network communications (and host based logging)

Visibility of communication between your network and other networks (i.e. the internet or 3rd party suppliers).

This information allows you to determine if an Indicator of Compromise has been seen on your systems.

Are any of our systems talking to a suspect external IP address? IP connections are usually made from network devices such as firewalls, routers, and switches. These generate netflow or firewall logs containing the desired information (which you can also obtain by passively analysing network traffic at the boundary). Note: Captured logs should take into account if (and where) Network Address Translation (NAT) is used on the network, to capture the internal and external IP details.
  Have any devices queried for a given specific domain? Web proxy or host based agent logs should be able to generate domain (and URL) lookup information. Domain Name System (DNS) logs can find which domains are being looked up (resolved), and will show you the domain part of the link the user has clicked (but not the full url).
  Has anyone in your organisation received an email with a similar subject to a given example? Has anyone clicked on the link?

Email events (including subjects and contents) can be generated from SMTP servers, MS Exchange, mail scanning appliances, cloud provider APIs or by passively parsing traffic.

Web proxy logs can provide information on malware command and control traffic, or initial stages of an infection – such as clicking a link or downloading a malware dropper. Devices need to be configured to use web proxies, and the logs may miss on-device infections if web traffic reaches the Internet without going through the proxy.


Has anyone in your organisation executed a given, suspect binary, or does a specified file exist on one of our systems?

Has any anomalous activity happened on any host? E.g. Out of hours crashes, etc

This data can be derived from centralising already generated local system logs (e.g. Microsoft EVTX, Syslog), deploying free Windows tools (like Microsoft Sysmon), right through to purchasing commercial host agent tools.

Host based agents and host logs (OS and application) are a rich source of information. They can generate process information, DNS lookup events, file system changes, file hashes, web requests, and so on. Their main limitation is that they must be consciously installed and configured onto operating systems, which leaves coverage gaps for unofficial ‘shadow’ IT, network devices, unsupported operating systems or where the malware has disabled the agent.


Event category Incident questions Event sources

Authentication and access

Capturing authentication and access attempts helps you to understand the impact of a compromise. 

This is particularly useful in understanding the ‘Affect’ phase of the kill chain – uncovering lateral movement and data exfiltration (amongst other objectives)

Who has viewed or downloaded a specific document?

Document repository stores can provide a source of audit information for which documents have been accessed.

Database and/or application logs for your sensitive assets (which could be bulk personal data repositories, design documents and so on) would be helpful if an attacker has successfully compromised your back end systems.

  Have any incorrect authentication attempts occurred on an Internet exposed service (email, VPNs, web services, remote desktop)? Email services and authentication servers (LDAP, Active Directory, ADFS or other) can provide metrics on authentication attempts and access information.
  How have your corporate domain and end user devices been accessed and used? Active Directory and Windows Logs. Windows can be configured to perform ‘Windows Event Forwarding’, allowing a secure way of forwarding logs from clients to a central store. This document from Microsoft contains some useful information.

Who has logged in remotely out of hours recently?

Have authentication events happened at unexpected times or from unexpected locations?

VPN servers, remote desktop and VDI logs can capture IP address information and user logs, to show who has accessed the corporate network remotely.


Event category Incident questions Event sources

IT asset and configuration information

This helps an organisation gain context of assets when investigating (or confirming) a cyber attack. Without accurate asset  information, organisations will struggle to delve into more detailed root cause analysis, or even have the right information to hand for recovery.

Which device used a specified internal IP address? Usually a DHCP server will provide information to tie an IP address to a MAC address. The MAC address can then be used to query network hardware and find the physical location of a device. Some products combine these two functions.
  Where is a specified device physically located? Managed network switches should allow an administrator to identify (in combination with DHCP logs) a MAC address to a physical port. WiFi networking hardware can sometimes identify the approximate location of the device.
  When did a network, server or end user device administrative configuration change occur? e.g. new local administrator account created on EUD, firewall rule change, etc

Capturing logs from network appliances (routers, firewalls, switches, etc) and operating systems should record events where administrative changes have been made. This document from Microsoftshows how to capture when a new local administrative user account is created on Windows.

For more advanced and detailed analysis, a privileged access tool would help tie change request tickets to configuration changes.

  What does a given server ‘do’? What is its business purpose, expected behaviour and system owner? Who should you call if there are issues?

An accurate Configuration Management Database (CMDB) is useful to record what a given server does on your estate. How this is captured will vary vastly between organisations, ranging from automatically recorded through orchestration and automation, all the way down to individual systems administrators storing this information in their heads.

Cloud provider consoles, APIs or on-premise virtualisation consoles can provide a source of information about virtualised assets. Some providers allow users to add tags and descriptions to assets, which can be part of the engineering process and useful in incident investigations (as well as understanding spending).

2. Decide how to retain logs

Two primary decisions shape the approach you will take to logging. 

Firstly you need to look carefully at each of the log sources you identified in step 1 to see how these should be properly collected, stored and secured.

Armed with this information, the next consideration is structural. Should you pull your logs into a central store, or are they best left where you found them?

Once you have settled these two matters, you will have outlined the work needed to prime your logging system.

Assessing your log sources

For each log source identified in step1, ask these four questions.

  • Do you have easy access to searchable logs?

Logs should be readily available, and you should know where they are stored before any investigation starts. You should make querying logs as easy as possible, so it's important the service that holds the logs can perform searches. It is bad practice to have logs distributed across devices with no easy way to access them, other than a manual login or physical action. Also, having prohibitive contracts in place would cause issues if access is required rapidly - so it's best to work on these situations now.

  • Are the logs safe from tampering and unauthorised access?

An attacker may well target the logging service in a bid to remove evidence of their actions. Is access to logs limited to individuals who need to perform log analysis? Are write permissions limited, and can changes be detected? As with any other management interface, logging solutions should be designed with good security practice in mind - it should be impossible for logs to be accessed or modified inappropriately.

  • Are logs held for long enough to answer incident questions?

For each log source you hold, you need to decide how long to store the data. This will depend on a number of factors including the cost and availability of storage, and the volume and usefulness of different data types (see Logging source section below). In general, we recommend that you hold logs which allow you to answer the incident questions from step 2 for a minimum of 6 months. The M-Trends 2018 report suggests that the average time to detect a cyber attack is 101 days and it's not uncommon for this figure to be significantly longer, so you may wish to store for longer if budget allows. Review and fine-tune as necessary.

A logging source might only capture minimal information in its default configuration, giving you insufficient detail to accurately answer the incident questions. You should attempt to answer the incident question with the logs that are being captured. If you find yourself short of data, change configuration so you capture more detail.

Structuring the system: Centralised versus decentralised logging

Once you begin planning how to collect your logs, you'll very likely have to decide between pulling them into a central store, and keeping them where they are.

A centralised solution can be used to provide a standalone logging service. The more sources that feed into a centralised store, the more useful it will be, and the better the return on your investment. Centralising logging will also mean you don’t have to physically go to each machine when investigating an incident. This will create a more responsive system, requiring minimal resources to operate it.

A decentralised setup involves leaving the logs in-situ because, without any further work on your part, they allow you to answer 'yes' to the questions listed above. Cloud-based email services, for example, may already be in an easy to search interface and securely held for a suitable length of time. You could extract these logs to a centralised service, but for most common scenarios this won't give enough extra benefit to justify the costs.

A determining factor is how you want to use the data in future. For example, if you want to combine it with other data sets (to query across both), moving these logs to your central storage system will be worthwhile. The other consideration is the ongoing maintenance and assurance that logs are being captured. A decentralised model could slowly change over time, becoming less effective.

In most cases, a centralised store is used in combination with other vendor dashboards/APIs. Taking all logs to a centralised source is a never-ending (and resource intensive) job. In practice, you will cherry pick what you need to hold centrally (so you can run multi-dataset queries) and call out to other services that already hold the logs in a suitable manner (e.g. cloud services, network panel).

3. Implement log storage and tooling for analysis

What you do at this stage will depend on the decisions you made in Step 2. If you chose a centralised solution, you will need to consider how to implement the following components:

Components of a logging system

Some products combine multiple components, so the individual components may not always be obvious. This is particularly true of commercial products.

Logging source

You will have established this in Step 1. Log sources may generate large volumes of traffic, so it's likely that you will need to refine what events (and context) you collect. Start with the default settings, then remove the information that is least likely to answer the incident questions until you achieve your desired cost/benefit balance. 

It is important to configure log sources properly, including synchronisation to an accurate time source and a level of verbosity which captures the fields needed to answer the incident questions.

Log transport

This is dictated by the logging source and the service that ingests logs, although network overlays could be added. The NCSC recommends using transport encryption where possible. The NCSC has not examined and does not endorse particular protocols, but common choices include Syslog, SNMP traps, and Windows Event Forwarding. When sending logs across trust boundaries, they should be sent across a one-way flow control (e.g. UDP or a data diode) to make it harder for an attacker to modify stored logs.

Processing and storage

Accepts logs pushed (or pulled) from device sources, cleans them up (formatting), normalises and then loads them into a data store. Plan for storage to roll-over, avoiding disks filling and the service failing.

Querying and analytics

Authenticates users and allows searches to be performed on the data set.

Configuring log sources

Some logs at their default settings may not provide all the information you require to pinpoint activity in an incident, so some configuration will be required.

There is no single right way to do this, but consider some of the following points that the NCSC has found useful in real world scenarios.

You should ensure that:

  • Your local operating system log cache is large enough to account for any network interruption between uploads to a central service.
  • The fields you are expecting are actually logged, including:
    • Source and destination IP address/host name to identify the machine
    • Account Names are available to identify the user
  • Times are accurate and synchronised
  • Firewall and proxy logs identify the originating IPs, and not just the IP of the proxy or gateway
  • Detail on process execution or crashes provides context as fully as possible (Parent process, user, machine, failure codes, etc)

Microsoft Windows event configuration guidance can be found here.

Open source logging tools

Here are some open source tools we’ve seen used to good effect. The NCSC has not formally tested these products, and does not recommend a particular one. They are a starting point to help you establish what could work for your business. Commercial products are also available.

Log ingestion, processing, dashboard and analysis tools:





Security Onion:


Log processing and transport tools:




Host based agents:



Other guides and interesting reads:

Cyber Wardog: Powershell and Sysmon logging.

Windows event forwarding: WEFFLES.

4.Validate that your logging capability is working as intended

As your technology and use of IT changes, you may find your logs become incomplete or incorrect. We recommend you review and validate your logging strategy every 6-12 months, to capture any relevant changes that have happened in the meantime. This essentially means repeating Step 1 to see if your log sources should have changed.

You also need to ensure that existing logs are still being captured as expected. If you can automate a way of alerting when log messages stop arriving centrally, then do this. Otherwise, you could automate a way of initiating a test event that should be captured, and validating that it has been captured by your logging service.

You may not need to query large portions of the data set for long periods of time, so check that any storage mechanism works as intended. Otherwise, you may discover that your logging data is unavailable during an incident.

In Summary...

Once you have a logging strategy in place, you will be better prepared for the most pressing questions put to you by incident investigators should you suffer a cyber attack. This will give you the best chance of recovering swiftly, and learning how to defend your systems better against future incursions.

Logging has benefits outside of security too. Logging data can be used to investigate performance issues, provide administrative alerts (such as a storage disk being near capacity) and help verify that organisational IT policy is working as intended. The costs associated with establishing a logging capability should be viewed in light of these benefits.

The next step is to proactively search your collected information for known threats, which requires further investment in people, skills and business processes. Following the advice given in this guidance will mean that you start that process on a sound technical footing.

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