Download ACSC Protect: Drive-by Downloads (PDF), December 2012
First published December 2012
- Cyber adversaries are increasingly using drive-by download techniques to deliver malicious software that compromises agency networks. This document explains how drive-by downloads operate and how the risk of compromise from these techniques can be mitigated. It is intended to assist IT Security Advisors in protecting their networks.
What is a drive-by download?
- A drive-by download occurs when a user visits a legitimate website that has been temporarily compromised, enabling an adversary to install malicious software on the user’s computer. It occurs without the knowledge or authorisation of the user.
What happens in a drive-by download?
- A drive-by download starts when a user goes to a legitimate but compromised website. When the user accesses the website, the cyber adversary’s malicious code exploits weaknesses or other vulnerabilities in the user’s browser or browser plugins, allowing the download of malicious files to the user's computer. The downloaded files could enable the adversary to have full access and control of the user's computer, either to steal valuable information or to launch denial of service attacks against other users on the Internet.
- Another form of a drive-by download is 'malvertisement', which is commonly Flash Player-based and takes advantage of unpatched software. Disguised as legitimate advertisers, cyber adversaries implant their malicious software in an advertisement on a legitimate website. When the victim views the advertisement, the malware will start to infect the victim’s computer.
- Most drive-by downloads require scripts to be loaded from third party sites. Cyber adversaries inject inline frame codes into a legitimate website, which will load malicious software hosted on another website operated by the adversaries when the website is visited.
- Search engine optimisation (SEO) is another technique often used in conjunction with a drive-by download exploit. SEO increases a website’s visibility in a search engine. Generally, the higher or more often a website appears in the search result, the more traffic the website is likely to receive from the search engine’s users. Cyber adversaries use SEO to promote their malicious websites in search engines to increase the chance of getting traffic to their website for the exploit to occur.
- There are malware kits available which target specific browser or software flaws, including Adobe PDF, Microsoft's Internet Explorer, as well as other browser plug-ins. The server to which these kits are connected can use HTTP request headers from a browser to determine which specific exploits are most likely to work on the victim’s computer.
- As of late October 2011, drive-by download sites have started using new techniques where they use complex logic to limit their attacks to previously uninfected networks1 The technique also attempts to avoid monitoring tools which blacklist compromised websites. Consequently, you should adopt a defence-in-depth approach to network security, instead of relying mainly on anti-virus programs.
Minimise the risk
- To mitigate the risks of drive-by downloads, at least the top four strategies of the Strategies to Mitigate Cyber Incidents should be implemented in your environment. Other strategies may also be effective, depending on security gaps in your network.
- The top four strategies involve:
- Strategy 1: Implementing application whitelisting. In many cyber incidents witnessed by the Cyber Security Operations Centre, application whitelisting was the only strategy able to stop drive-by downloads from executing the malicious software.
- Strategies 2 and 3: Patching applications and operating system vulnerabilities, especially Java and Flash. Old versions of a product are more vulnerable to these exploits. Keep all applications up to date.
- Strategy 4: Minimising the number of users with domain or administrator privileges. Limit the ability for users with administrator privileges to have access to email and the Internet by employing separate unprivileged workstations or accounts for these purposes.
- Agencies should also consider implementing these strategies from the remainder of the list:
- Strategies 16 and 17: Implement domain whitelisting for all domains including HTTPS/SSL domains, to only enable trusted domains to be accessed by users. This will not prevent drive-by download attacks, but it will prevent secondary malicious websites from loading.
- Strategy 25: Install and maintain updated antivirus software capable of scanning Internet traffic and detecting exploits.
- The Australian Government Information Security Manual (ISM) assists in the protection of official government information that is processed, stored or communicated by Australian government systems.
- DSD’s Strategies to Mitigate Targeted Cyber Intrusions and other DSD products complement the advice in the ISM.
- Organisations or individuals with questions regarding this advice can contact the ACSC by emailing firstname.lastname@example.org or calling 1300 CYBER1 (1300 292 371).