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Unfortunately, there are a number of existing exploitation strategies for installing malware on a user's computer. One common technique for doing so is by remotely exploiting vulnerable network services. However, lately, this attack strategy has become less successful (and presumably, less profitable). Arguably, the proliferation of technologies such as Network Address Translators (NATs) and firewalls make it difficult to remotely connect and exploit services running on users' computers. This, in turn, has lead attackers to seek other avenues of exploitation. An equally potent alternative is to simply lure web users to connect to (compromised) malicious servers that subsequently deliver exploits targeting vulnerabilities of web browsers or their plugins.

Adversaries use a number of techniques to inject content under their control into benign websites. In many cases, adversaries exploit web servers via vulnerable scripting applications. Typically, these vulnerabilities (e.g., in phpBB2 or InvisionBoard) allow an adversary to gain direct access to the underlying operating system. That access can often be escalated to super-user privileges which in turn can be used to compromise any web server running on the compromised host. In general, upon successful exploitation of a web server the adversary injects new content to the compromised website. In most cases, the injected content is a link that redirects the visitors of these websites to a URL that hosts a script crafted to exploit the browser. To avoid visual detection by website owners, adversaries normally use invisible HTML components (e.g., zero pixel IFRAMEs) to hide the injected content.

Another common content injection technique is to use websites that allow users to contribute their own content, for example, via postings to forums or blogs. Depending on the site's configuration, user contributed content may be restricted to text but often can also contain HTML such as links to images or other external content. This is particularly dangerous, as without proper filtering in place, the adversary can simply inject the exploit URL without the need to compromise the web server.

Figure 1: A typical Interaction with of drive-by download victim with a landing URL .

Figure 1 illustrates the main phases in a typical interaction that takes place when a user visits a website with injected malicious content. Upon visiting this website, the browser downloads the initial exploit script (e.g., via an IFRAME). The exploit script (in most cases, javascript) targets a vulnerability in the browser or one of its plugins. Interested readers are referred to Provos et al. [20] for a number of vulnerabilities that are commonly used to gain control of the infected system. Successful exploitation of one of these vulnerabilities results in the automatic execution of the exploit code, thereby triggering a drive-by download. Drive-by downloads start when the exploit instructs the browser to connect to a malware distribution site to retrieve malware executable(s). The downloaded executable is then automatically installed and started on the infected systemSome compromised web servers also trigger dialog windows asking users to manually download and run malware. However, this analysis considers only malware installs that require no user interaction..

Finally, attackers use a number of techniques to evade detection and complicate forensic analysis. For example, the use of randomly seeded obfuscated javascript in their exploit code is not uncommon. Moreover, to complicate network based detection attackers use a number or redirection steps before the browser eventually contacts the malware distribution site.

Niels Provos 2008-05-13