Introduction

The Australian Cyber Security Centre (ACSC) has developed prioritised mitigation strategies to help cyber security professionals in all organisations mitigate cyber security incidents caused by various cyber threats. This guidance addresses targeted cyber intrusions (i.e. those executed by advanced persistent threats such as foreign intelligence services), ransomware and external adversaries with destructive intent, malicious insiders, ‘business email compromise’, and industrial control systems.

This guidance is informed by the ACSC’s experience in responding to cyber security incidents, performing vulnerability assessments and penetration testing Australian government organisations.

Prior to implementing any of the mitigation strategies, organisations need to identify their assets and perform a risk assessment to identify the level of protection required from various cyber threats. Furthermore, organisations require motivation to improve their cyber security posture, supportive executives, access to skilled cyber security professionals and adequate financial resources. Motivators can include a significant cyber security incident, a penetration test, mandatory data breach reporting, mandatory compliance, and evidence of a lower cyber security posture or higher threat exposure than previously realised.

The following page provides mitigation strategies and a suggested implementation order for:

targeted cyber intrusions and other external adversaries who steal data

ransomware denying access to data for monetary gain, and external adversaries who destroy data and prevent computers/networks from functioning

malicious insiders who steal data such as customer details or intellectual property

malicious insiders who destroy data and prevent computers/networks from functioning.

When implementing a mitigation strategy, first implement it for high risk users and computers such as those with access to important (sensitive or high-availability) data and exposed to untrustworthy internet content, and then implement it for all other users and computers. Organisations should perform hands-on testing to verify the effectiveness of their implementation of mitigation strategies.

No single mitigation strategy is guaranteed to prevent cyber security incidents. Properly implementing the eight mitigation strategies with an ‘essential’ effectiveness rating is so effective at mitigating targeted cyber intrusions and ransomware, that the ACSC considers these to be the new cyber security baseline for all organisations.

The companion Strategies to Mitigate Cyber Security Incidents – Mitigation Details document contains implementation guidance for the mitigation strategies, as well as guidance to mitigate ‘business email compromise’ and threats to Industrial Control Systems. Further, the companion Essential Eight Maturity Model document advises how to implement mitigation strategies in a phased approach and how to measure the maturity of their implementation. Finally, the ACSC’s website has supporting guidance in the Australian Government Information Security Manual, as well as separate guidance for mitigating denial of service, and securely using cloud computing and enterprise mobility.

Suggested mitigation strategy implementation order

Start with threats of most concern to the organisation.

Targeted cyber intrusions (advanced persistent threats) and other external adversaries who steal data:

Implement ‘essential’ mitigation strategies to:

prevent malware delivery and execution limit the extent of cyber security incidents detect cyber security incidents and respond. Repeat step 1 with ‘excellent’ mitigation strategies. Repeat step 1 with less effective mitigation strategies until an acceptable level of residual risk is reached.

Ransomware and external adversaries who destroy data and prevent computers/networks from functioning:

Implement ‘essential’ mitigation strategies to:

recover data and system availability prevent malware delivery and execution limit the extent of cyber security incidents detect cyber security incidents and respond. Repeat step 1 with ‘excellent’ mitigation strategies. Repeat step 1 with less effective mitigation strategies until an acceptable level of residual risk is reached.

Note that ‘Hunt to discover incidents’ is less relevant for ransomware that immediately makes itself visible.

Malicious insiders who steal data:

Implement ‘Control removable storage media and connected devices’ to mitigate data exfiltration. Implement ‘Outbound web and email data loss prevention’. Implement ‘essential’ mitigation strategies to:

limit the extent of cyber security incidents detect cyber security incidents and respond. Repeat step 3 with ‘excellent’ mitigation strategies. Implement ‘Personnel management’. If employees are likely to have hacking skills and tools, implement ‘essential’ mitigation strategies to prevent malware delivery and execution, and repeat step 3 with less effective mitigation strategies until an acceptable level of residual risk is reached.

Note that technical mitigation strategies provide incomplete security since data could be photographed or otherwise copied from computer screens or printouts, or memorised and written down outside of the workplace.

Malicious insiders who destroy data and prevent computers/networks from functioning:

Implement ‘essential’ mitigation strategies to:

recover data and system availability limit the extent of cyber security incidents detect cyber security incidents and respond. Repeat step 1 with ‘excellent’ mitigation strategies. Implement ‘Personnel management’.

If employees are likely to have hacking skills and tools, implement ‘essential’ mitigation strategies to prevent malware delivery and execution, and repeat step 1 with less effective mitigation strategies until an acceptable level of residual risk is reached.

Strategies to mitigate cyber security incidents

Relative Security Effectiveness Rating Mitigation Strategy Potential User

Resistance Upfront Cost

(staff, software and hardware) Ongoing

Maintenance

Cost Mitigation Strategies to Prevent Malware Delivery and Execution: Essential Application control to prevent execution of unapproved/malicious programs including .exe, DLL, scripts (e.g. Windows Script Host, PowerShell and HTA) and installers. Medium High Medium Essential Patch applications (e.g. Flash, web browsers, Microsoft Office, Java and PDF viewers). Patch/mitigate computers with ‘extreme risk’ security vulnerabilities within 48 hours. Use the latest version of applications. Low High High Essential Configure Microsoft Office macro settings to block macros from the internet, and only allow vetted macros either in ‘trusted locations’ with limited write access or digitally signed with a trusted certificate. Medium Medium Medium Essential User application hardening. Configure web browsers to block Flash (ideally uninstall it), ads and Java on the internet. Disable unneeded features in Microsoft Office (e.g. OLE), web browsers and PDF viewers. Medium Medium Medium Excellent Automated dynamic analysis of email and web content run in a sandbox, blocked if suspicious behaviour is identified (e.g. network traffic, new or modified files, or other system configuration changes). Low High Medium Excellent Email content filtering. Allow only approved attachment types (including in archives and nested archives). Analyse/sanitise hyperlinks, PDF and Microsoft Office attachments. Quarantine Microsoft Office macros. Medium Medium Medium Excellent Web content filtering. Allow only approved types of web content and websites with good reputation ratings. Block access to malicious domains and IP addresses, ads, anonymity networks and free domains. Medium Medium Medium Excellent Deny corporate computers direct internet connectivity. Use a gateway firewall to require use of a split DNS server, an email server and an authenticated web proxy server for outbound web connections. Medium Medium Low Excellent Operating system generic exploit mitigation e.g. Data Execution Prevention (DEP), Address Space Layout Randomisation (ASLR) and Enhanced Mitigation Experience Toolkit (EMET). Low Low Low Very Good Server application hardening especially internet-accessible web applications (sanitise input and use TLS not SSL) and databases, as well as applications that access important (sensitive/high-availability) data. Low Medium Medium Very Good Operating system hardening (including for network devices) based on a Standard Operating Environment, disabling unneeded functionality (e.g. RDP, AutoRun, LanMan, SMB/NetBIOS, LLMNR and WPAD). Medium Medium Low Very Good Antivirus software using heuristics and reputation ratings to check a file’s prevalence and digital signature prior to execution. Use antivirus software from different vendors for gateways versus computers. Low Low Low Very Good Control removable storage media and connected devices. Block unapproved CD/DVD/USB storage media. Block connectivity with unapproved smartphones, tablets and Bluetooth/Wi-Fi/3G/4G/5G devices. High High Medium Very Good Block spoofed emails. Use Sender Policy Framework (SPF) or Sender ID to check incoming emails. Use ‘hard fail’ SPF TXT and DMARC DNS records to mitigate emails that spoof the organisation’s domain. Low Low Low Good User education. Avoid phishing emails (e.g. with links to login to fake websites), weak passphrases, passphrase reuse, as well as unapproved: removable storage media, connected devices and cloud services. Medium High Medium Limited Antivirus software with up-to-date signatures to identify malware, from a vendor that rapidly adds signatures for new malware. Use antivirus software from different vendors for gateways versus computers. Low Low Low Limited TLS encryption between email servers to help prevent legitimate emails being intercepted and subsequently leveraged for social engineering. Perform content scanning after email traffic is decrypted. Low Low Low Mitigation Strategies to Limit the Extent of Cyber Security Incidents: Essential Restrict administrative privileges to operating systems and applications based on user duties. Regularly revalidate the need for privileges. Don’t use privileged accounts for reading email and web browsing. Medium High Medium Essential Patch operating systems. Patch/mitigate computers (including network devices) with ‘extreme risk’ security vulnerabilities within 48 hours. Use the latest operating system version. Don’t use unsupported versions. Low Medium Medium Essential Multi-factor authentication including for VPNs, RDP, SSH and other remote access, and for all users when they perform a privileged action or access an important (sensitive/high-availability) data repository. Medium High Medium Excellent Disable local administrator accounts or assign passphrases that are random and unique for each computer’s local administrator account to prevent propagation using shared local administrator credentials. Low Medium Low Excellent Network segmentation. Deny traffic between computers unless required. Constrain devices with low assurance (e.g. BYOD and IoT). Restrict access to network drives and data repositories based on user duties. Low High Medium Excellent Protect authentication credentials. Remove CPassword values (MS14-025). Configure WDigest (KB2871997). Use Credential Guard. Change default passphrases. Require long complex passphrases. Medium Medium Low Very Good Non-persistent virtualised sandboxed environment, denying access to important (sensitive/high-availability) data, for risky activities (e.g. web browsing, and viewing untrusted Microsoft Office and PDF files). Medium Medium Medium Very Good Software-based application firewall, blocking incoming network traffic that is malicious/unauthorised, and denying network traffic by default (e.g. unneeded/unauthorised RDP and SMB/NetBIOS traffic). Low Medium Medium Very Good Software-based application firewall, blocking outgoing network traffic that is not generated by approved/trusted programs, and denying network traffic by default. Medium Medium Medium Very Good Outbound web and email data loss prevention. Block unapproved cloud computing services. Log recipient, size and frequency of outbound emails. Block and log emails with sensitive words or data patterns. Medium Medium Medium Mitigation Strategies to Detect Cyber Security Incidents and Respond: Excellent Continuous incident detection and response with automated immediate analysis of centralised time-synchronised logs of allowed and denied computer events, authentication, file access and network activity. Low Very High Very High Very Good Host-based intrusion detection/prevention system to identify anomalous behaviour during program execution (e.g. process injection, keystroke logging, driver loading and persistence). Low Medium Medium Very Good Endpoint detection and response software on all computers to centrally log system behaviour and facilitate incident response. Microsoft’s free SysMon tool is an entry level option. Low Medium Medium Very Good Hunt to discover incidents based on knowledge of adversary tradecraft. Leverage threat intelligence consisting of analysed threat data with context enabling mitigating action, not just indicators of compromise. Low Very High Very High Limited Network-based intrusion detection/prevention system using signatures and heuristics to identify anomalous traffic both internally and crossing network perimeter boundaries. Low High Medium Limited Capture network traffic to and from corporate computers storing important data or considered as critical assets, and network traffic traversing the network perimeter, to perform incident detection and analysis. Low High Medium Mitigation Strategies to Recover Data and System Availability: Essential Daily backups of important new/changed data, software and configuration settings, stored disconnected, retained for at least three months. Test restoration initially, annually and when IT infrastructure changes. Low High High Very Good Business continuity and disaster recovery plans which are tested, documented and printed in hardcopy with a softcopy stored offline. Focus on the highest priority systems and data to recover. Low High Medium Very Good System recovery capabilities e.g. virtualisation with snapshot backups, remotely installing operating systems and applications on computers, approved enterprise mobility, and onsite vendor support contracts. Low High Medium Mitigation Strategy Specific to Preventing Malicious Insiders: Very Good Personnel management e.g. ongoing vetting especially for users with privileged access, immediately disable all accounts of departing users, and remind users of their security obligations and penalties. High High High

Summary of key changes since previous version

The title and scope of the document have been updated to mitigate additional threats. Three new mitigation strategies to recover data and system availability help mitigate ransomware. The new mitigation strategies ‘Personnel management’ and ‘Outbound web and email data loss prevention’ help mitigate malicious insiders. The Strategies to Mitigate Cyber Security Incidents – Mitigation Details document has new guidance for these threats as well as for business email compromise and industrial control systems.

The leftmost numerical ranking column was being misinterpreted by some readers, and has been converted into a suggested mitigation strategy implementation order for each threat, providing a principles-based approach to building a defence-in-depth cyber security posture.

The rightmost four columns (e.g. ‘Helps Prevent Intrusion Stage 1: Code Execution’) have been converted into category headings (e.g. ‘Mitigation Strategies to Prevent Malware Delivery and Execution’). Mitigation strategies have been categorised based on their primary security outcome.

Effectiveness ratings now include ‘very good’, while ‘average’ has been changed to ‘limited’.

Mitigation strategy ‘Application control’ now mentions Windows Script Host, PowerShell and HTML Applications (HTA). Further guidance has been added to the Mitigation Details document.

The two patching mitigation strategies now reference the ACSC’s definition of ‘extreme risk’ security vulnerabilities to reflect that the 48 hour (previously two day) timeframe to apply patches doesn’t apply to every security vulnerability affecting every computer. The list of applications has been reordered since Flash, web browsers and Microsoft Office are exploited more than Java and PDF viewers.

New mitigation strategy ‘Configure Microsoft Office macro settings’ has been extracted from mitigation strategy ‘User application hardening’ to reflect the prevalence of malicious Microsoft Office macros. The ACSC has seen our guidance mitigate attempts to compromise Australian organisations by adversaries working for a foreign intelligence service.

Mitigation strategy ‘User application hardening’ is now rated ‘essential’ and advises to uninstall Adobe Flash if possible, disable Microsoft Office OLE packages, and block internet ads due to malicious advertising (malvertising). Some organisations might choose to support selected websites that rely on ads for revenue by enabling just their ads and potentially risking compromise.

Mitigation strategy ‘Multi-factor authentication’ is now rated ‘essential’ to reflect the prevalence of passphrase theft and the abuse of remote access for infiltration, data exfiltration and persistence.

Mitigation strategy ‘Enforce a strong passphrase policy’ has been renamed to ‘Protect authentication credentials’, contains specific new guidance and is now rated ‘excellent’.

The two logging mitigation strategies have been combined into mitigation strategy ‘Continuous incident detection and response’. Also, while the key goal remains to identify and protect assets to prevent cyber security incidents, two new mitigation strategies reduce the time to detect and respond to such incidents – ‘Endpoint detection and response software’ and ‘Hunt to discover incidents’ leveraging threat intelligence. Details are in the Mitigation Details document.

Mitigation strategy ‘Server application hardening’ is now rated ‘very good’ to reflect an increase in cyber security incidents involving web servers compromised with web shells.

Mitigation strategy ‘Block spoofed emails’ now advises to configure DMARC DNS records.

Mitigation strategies ‘Web domain whitelisting for all domains’, ‘Block attempts to access websites by their IP address’ and ‘Gateway blacklisting’ have merged into ‘Web content filtering’.

Mitigation strategies ‘Restrict access to Server Message Block (SMB) and NetBIOS’ and ‘Workstation inspection of Microsoft Office files’ have merged with existing mitigation strategies.

Contact details

If you have any questions regarding this guidance you can contact us via 1300 CYBER1 (1300 292 371) or https://www.cyber.gov.au/acsc/contact.