Strategies to Mitigate Targeted Cyber Intrusions

Download Strategies to Mitigate Targeted Cyber Intrusions (PDF), updated February 2014

Introduction

  1. Australian computer networks are being targeted by adversaries seeking access to sensitive information.
  2. A commonly used technique is social engineering, where malicious 'spear phishing' emails are tailored to entice the reader to open them. Users may be tempted to open malicious email attachments or follow embedded links to malicious websites. Either action can compromise the network and disclose sensitive information.
  3. The Australian Signals Directorate (ASD) has developed a list of strategies to mitigate targeted cyber intrusions. The list is informed by ASD's experience in operational cyber security, including responding to serious cyber incidents and performing vulnerability assessments and penetration testing for Australian government agencies.

Mitigation strategies

  1. 2014 mitigation strategies table - click to enlargeASD's list of mitigation strategies, first published in February 2010, is revised for 2014 based on ASD's most recent analysis of incidents across the Australian Government. This document provides a summary of key changes for 2014.
  2. While no single strategy can prevent malicious activity, the effectiveness of implementing the Top 4 strategies remains very high. At least 85% of the cyber intrusions that ASD responds to involve adversaries using unsophisticated techniques that would have been mitigated by implementing the Top 4 mitigation strategies as a package.
  3. Implementing the Top 4 mitigation strategies can be achieved gradually, firstly on workstations of users who are most likely to be targeted by cyber intrusions, and then implementing them on all workstations and servers. Once this is achieved, organisations can selectively implement additional mitigation strategies to address security gaps until an acceptable level of residual risk is achieved.
  4. This document provides information about comparative mitigation implementation costs and user resistance levels to help organisations select the best set of strategies for their requirements.
  5. These strategies complement the guidance provided in the Australian Government Information Security Manual (ISM).

Summary of key changes for 2014

  1. The Strategies to Mitigate Targeted Cyber Intrusions – Mitigation Details document includes an annex of key changes for 2014. A summary of the most significant changes are as follows.
  2. Mitigation strategy #4 ‘Restrict administrative privileges’ has been amended to clarify that the goal of this strategy is to ensure that the only users who have administrative privileges to operating systems and applications such as databases, are those users who require such privileges based on their job role and duties.
  3. Mitigation strategy ‘User application configuration hardening’ has moved from #18 to #5 to address intrusions that exploit the prevalence of Java vulnerabilities or involve malicious macro code in Microsoft Office files. Additional technical guidance is provided to enable organisations to continue using Java for business purposes while minimising their risk.
  4. The newly introduced mitigation strategy #6 ‘Automated dynamic analysis’ extracts the behavioural analysis functionality from the existing two mitigation strategies ‘Email content filtering’ and ‘Web content filtering’. Additional technical guidance is provided to enable organisations to select an appropriate vendor product.
  5. Mitigation strategy ‘Operating system generic exploit mitigation’ has moved from #21 to #7 due to the increased support and proven effectiveness of Microsoft’s free Enhanced Mitigation Experience Toolkit (EMET) software tool at mitigating vulnerabilities that were not publicly known at the time.
  6. The previous ‘Antivirus software’ mitigation strategy has been divided into two separate mitigation strategies, to highlight the difference between less effective signature-based antivirus software and more effective heuristic/anomaly-based antivirus software.
  7. Mitigation strategy ‘User education’ has moved from #20 to #28 due to the increase in intrusions using techniques that an educated user would not detect.

Further information

  1. Additional supporting advice is available on the ASD website at Strategies to Mitigate Targeted Cyber Intrusions.

Contact

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

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

Mitigation strategies summary

Mitigation strategy effectiveness ranking 2014 (2012) Mitigation strategy Overall security effectiveness User resistance Upfront cost (staff, equipment, technical complexity) Maintenance cost (mainly staff) Helps detect intrusions Helps mitigate intrusion stage 1: code execution Helps mitigate intrusion stage 2: network propagation Helps mitigate intrusion stage 3: data exfiltration
1 (1)
Application whitelisting of permitted/trusted programs, to prevent execution of malicious or unapproved programs including DLL files, scripts and installers. Essential Medium High Medium
Yes
Yes
Yes
Yes
2 (2)
Patch applications,eg, Java, PDF viewers, Flash, web browsers and Microsoft Office. Patch or mitigate systems with 'extreme risk' vulnerabilities within two days. Use the latest version of applications. Essential Low High High
No
Yes
Possible
No
3 (3)
Patch operating system vulnerabilities. Patch or mitigate systems with 'extreme risk' vulnerabilities within two days. Use the latest suitable operating system. Avoid Windows XP. Essential Low Medium Medium
No
Yes
Possible
No
4 (4)
Restrict administrative privileges to operating systems and applications based on user duties. Such users should use a separate unprivileged account for email and web browsing. Essential Medium Medium Low
No
Possible
Yes
No
Once organisations have implemented the Top 4 mitigation strategies, first on the computers of users who are most likely to be targeted by cyber intrusions and then on all computers and servers, additional mitigation strategies can be selected to address security gaps until an acceptable level of residual risk is reached.
5 (18)
User application configuration hardening, disabling the running of internet-based Java code, untrusted Microsoft Office macros, and undesired web browser and PDF viewer features. Excellent Medium Medium Medium
No
Yes
No
No
6 (new)
Automated dynamic analysis of email and web content run in a sandbox to detect suspicious behaviour including network traffic, new or modified files, or configuration changes. Excellent Low Medium Low
Yes
Yes
No
Possible
7 (21)
Operating system generic exploit mitigation mechanisms, eg, Data Execution Prevention (DEP), Address Space Layout Randomisation (ASLR) and Enhanced Mitigation Experience Toolkit (EMET). Excellent Low Medium Low
Possible
Yes
Possible
No
8 (11)
Host-based Intrusion Detection/Prevention System to identify anomalous behaviour such as process injection, keystroke logging, driver loading and persistence. Excellent Low Medium Medium
Yes
Yes
No
Possible
9 (5)
Disable local administrator accounts to prevent network propagation using compromised local administration credentials that are shared by several computers. Excellent Low Medium Low
No
No
Yes
No
10 (7)
Network segmentation and segregation into security zones to protect sensitive information and critical services such as user authentication by Microsoft Active Directory. Excellent Low High Medium
Yes
No
Yes
Possible
11 (6)
Multi-factor authentication especially implemented for remote access or when the user is about to perform a privileged action or access a sensitive information repository. Excellent Medium High Medium
No
No
Possible
No
12 (8)
Software-based application firewall, blocking incoming network traffic that is malicious or otherwise unauthorised, and denying network traffic by default. Excellent Low Medium Medium
Yes
Yes
Yes
No
13 (9)
Software-based application firewall, blocking outgoing network traffic that is not generated by whitelisted applications, and denying network traffic by default. Excellent Medium Medium Medium
Yes
No
Yes
Yes
14 (10)
Non-persistent virtualised sandboxed trusted operating environment, hosted outside the organisation's internal network, for risk activities such as web browsing. Excellent High High Medium
Possible
No
Yes
Possible
15 (12)
Centralised and time-synchronised logging of successful and failed computer events with automated immediate log analysis, storing logs for at least 18 months. Excellent Low High High
Yes
No
Possible
Possible
16 (13)
Centralised and time-synchronised logging of allowed and blocked network events with automated immediate log analysis, storing logs for at least 18 months. Excellent Low High High
Yes
No
Possible
Possible
17 (14)
Email content filtering allowing only business-related attachment types. Preferably analyse/convert/sanitise links, PDF and Microsoft Office attachments. Excellent High High Medium
Yes
Yes
No
Possible
18 (15)
Web content filtering of incoming and outgoing traffic, whitelisting allowed types of web content and using behavioural analysis, cloud-based reputation ratings, heuristics and signatures. Excellent Medium Medium Medium
Yes
Yes
No
Possible
19 (16)
Web domain whitelisting for all domains, since this approach is more proactive and thorough than blacklisting a tiny percentage of malicious domains. Excellent High High Medium
Yes
Yes
No
Yes
20 (19)
Block spoofed emails using Sender ID or Sender Policy Framework (SPF) to check incoming emails, and a 'hard fail' SPF record to help prevent spoofing of your organisation's domain. Excellent Low Low Low
Possible
Yes
No
No
21 (22)
Workstation and server configuration management based on a hardened Standard Operating Environment with unrequired functionality disabled e.g. IPv6, autorun and LanMan. Good Medium Medium Low
Possible
Yes
Yes
Possible
22 (25)
Antivirus software using heuristics and automated internet-based reputation ratings to check a program's prevalence and its digital signature's trustworthiness prior to execution. Good Low Low Low
Yes
Yes
No
No
23 (24)
Deny direct internet access from workstations by using an IPv6-capable firewall to force traffic through a split DNS server, an email server or an authenticated web proxy server. Good Low Low Low
Yes
Possible
No
Yes
24 (23)
Server application security configuration hardening e.g. databases, web applications, customer relationship management, finance, human resources and other data storage systems. Good Low High Medium
Possible
Yes
No
Possible
25 (27)
Enforce a strong passphrase policy covering complexity, length and expiry, and avoiding both passphrase re-use and the use of a single dictionary word. Good Medium Medium Low
Possible
No
Yes
No
26 (29)
Removable and portable media control as part of a data loss prevention strategy, including storage, handling, whitelisting allowed USB devices, encryption and destruction. Good High Medium Medium
No
Yes
Possible
Yes
27 (28)
Restrict access to Server Message Block (SMB) and NetBIOS services running on workstations and on servers where possible. Good Low Medium Low
No
Yes
Yes
No
28 (20)
User education, eg, internet threats and spear-phishing socially-engineered emails. Avoid weak passphrases, passphrase re-use, exposing email addresses and unapproved USB devices. Good Medium High Medium
Possible
Possible
No
No
29 (26)
Workstation inspection of Microsoft Office files for potentially malicious abnormalities, eg, using the Microsoft Office File Validation or Protected View features. Good Low Low Low
Possible
Yes
No
No
30 (25)
Signature-based antivirus software that primarily relies on up-to-date signatures to identify malware. Use gateway and desktop antivirus software from different vendors. Good Low Low Low
Possible
Possible
No
No
31 (30)
TLS encryption between email servers to help prevent legitimate emails being intercepted and used for social engineering. Perform content scanning after email traffic is decrypted. Good Low Low Low
No
No
No
No
32 (32)
Block attempts to access web sites by their IP address instead of by their domain name, eg, implemented using a web proxy server, to force cyber adversaries to obtain a domain name. Average Low Low Low
Yes
Yes
No
Yes
33 (33)
Network-based Intrusion Detection/Prevention System using signatures and heuristics to identify anomalous traffic both internally and crossing network perimeter boundaries. Average Low High High
Possible
Possible
Possible
Possible
34 (34)
Gateway blacklisting to block access to known malicious domains and IP addresses, including dynamic and other domains provided free to anonymous internet users. Average Low Low High
Possible
Yes
No
Yes
35 (35)
Capture network traffic to/from internal critical-asset workstations and servers, as well as traffic traversing the network perimeter, to perform post-intrusion analysis. Average Low High Low
No
No
No
No