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Strategies to Mitigate Cyber Security Incidents

Download Strategies to Mitigate Cyber Security Incidents (1.4MB PDF), February 2017

  1. The Australian Signals Directorate (ASD) has developed prioritised mitigation strategies to help technical cyber security professionals in all organisations mitigate cyber security incidents. This guidance addresses targeted cyber intrusions (e.g. 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.
  2. This guidance is informed by ASD's experience responding to cyber security incidents, performing vulnerability assessments and penetration testing Australian government organisations.
  3. Prior to implementing mitigation strategies, organisations need to identify their assets and perform a risk assessment to identify the level of protection required from various threats. Organisations require motivation to improve their cyber security posture, supportive executives, access to skilled cyber security professionals and adequate financial resources. Motivators include a detected 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.
  4. 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.
  5. 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.
  6. No single mitigation strategy is guaranteed to prevent cyber security incidents. Properly implementing application whitelisting, patching applications, patching operating systems and restricting administrative privileges (referred to as the Top 4) continues to mitigate over 85% of adversary techniques used in targeted cyber intrusions which ASD has visibility of.
  7. Incorporating the Top 4, the eight mitigation strategies with an 'essential' effectiveness rating are so effective at mitigating targeted cyber intrusions and ransomware that ASD considers them to be the cyber security baseline for all organisations. Any organisation that has been compromised despite properly implementing these mitigation strategies is encouraged to notify ASD.
  8. The companion Strategies to Mitigate Cyber Security Incidents – Mitigation Details document contains updated implementation guidance for the mitigation strategies, as well as new guidance to mitigate 'business email compromise' and threats to industrial control systems.
  9. ASD’s Australian Government Information Security Manual (ISM) provides supporting guidance. ASD also has separate and specific guidance for mitigating denial of service, securely using cloud computing and enterprise mobility, including personally-owned computing devices.

Mitigation strategies

Mitigation strategies summary

Relative security effectiveness rating Mitigation strategy Potential user resistance Upfront cost (staff, equipment, technical complexity) Ongoing maintenance cost (mainly staff)
Mitigation strategies to prevent malware delivery and execution
Essential Application whitelisting of approved/trusted programs 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' 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. Whitelist allowed 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. Whitelist allowed 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 or 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 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' 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 or 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 network 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 or 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 permitted and denied: computer events, authentication, file access and network activity. Low Very
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
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 for 2017

  1. 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, hereafter referred to as 'the Mitigation Details document,' contains new guidance for these threats as well as for 'business email compromise' and industrial control systems.
  2. The left-most 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.
  3. The right-most 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.
  4. Effectiveness ratings now include 'very good' , while 'average' has been changed to 'limited'.
  5. Mitigation strategy 'Application whitelisting' now mentions Windows Script Host, PowerShell and HTML Applications (HTA). Further guidance has been added to the Mitigation Details document.
  6. The two patching mitigation strategies now reference ASD's definition of 'extreme risk' vulnerabilities to reflect that the 48-hour (previously two-day) timeframe to apply patches doesn't apply to every vulnerability affecting every computer. The list of applications has been re-ordered since Flash, web browsers and Microsoft Office are exploited more than Java and PDF viewers.
  7. 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. ASD has witnessed our guidance mitigate attempts to compromise Australian organisations by adversaries working for a foreign intelligence service.
  8. 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.
  9. Mitigation strategy 'Multi-factor authentication' is now rated 'essential' to reflect the prevalence of passphrase theft and the abuse of remote access for network infiltration, data exfiltration and persistence.
  10. Mitigation strategy 'Enforce a strong passphrase policy' has been renamed to 'Protect authentication credentials', contains specific new guidance and is now rated 'excellent'.
  11. 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, with details added to the Mitigation Details document.
  12. 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.
  13. Mitigation strategy 'Block spoofed emails' now advises to configure DMARC DNS records.
  14. 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'.
  15. Mitigation strategies 'Restrict access to Server Message Block (SMB) and NetBIOS' and 'Workstation inspection of Microsoft Office files' have merged with existing mitigation strategies.


Australian government customers with questions regarding this advice should contact ACSC Advice and Assistance.

Australian businesses and other private sector organisations seeking further information should contact CERT Australia.

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