some of the open challenges associated with this rising threat.
As Web browsers have become more capable and the Web richer in features, it is difficult for the average user to understand what happens when visiting a Web page. In most applications visiting a Web page causes the browser to pull content from a number of different providers, for example, to show third-party ads, interactive maps, or display online videos. The shear number of possibilities to design Web pages and make them attractive to users is staggering. Overall, these features increase the complexity of the components that constitute a modern Web browser. Unfortunately, each browser component may introduce new vulnerabilities an adversary can leverage to gain control over a user’s computer. Over the past few years we have seen an increasing number of browser vulnerabilities, 5, 8 some of which have not had official fixes for weeks.
For an adversary to exploit a vulnerability, it requires the user visit a Web page that contains malicious content. One way to attract user traffic is to send spam email messages that advertise links to malicious Web pages. However, this delivery mechanism has some drawbacks. For the exploit to be delivered, the user must open the spam email and then click on the embedded link. The ubiquitous Web infrastructure provides a better solution to this bottleneck. While it is easy to exploit a Web browser, it is even easier to exploit Web servers. The relative simplicity of setting up and deploying Web servers has resulted in a large number of Web applications with remotely exploitable vulnerabilities. Unfortunately, these vulnerabilities are rarely patched, and therefore, remote exploitation of Web servers is increasing. To exploit users, adversaries just need to compromise a Web server and inject malicious content, for example, via an IFRAME pointing to an exploit server. Any visitor to such a compromised Web server becomes a target of exploitation. If the visitor’s system is vulnerable, the exploit causes the browser to download and execute arbitrary payloads. We call this process “drive-by download.” Depending on the popularity of the com-
promised Web site, an adversary may get access to a large user population. Last year, Web sites with millions of visitors were compromised that way.
Taking Over Web Servers. Turning Web servers into infection vectors is unfortunately fairly straightforward. Over the last couple years, we have observed a number of different attacks against Web servers and Web applications, ranging from simple password guessing to more advanced attacks that can infect thousands of servers at once. In general, these attacks aim at altering Web site content to redirect visitors to servers controlled by the adversary. Here, we expand on some examples of recent dominant server attacks.
SQL Injection Attacks. SQL injection is an exploitation technique commonly used against Web servers that run vulnerable database applications. The vulnerability happens when user input is not properly sanitized (for example, by filtering escape characters and string literals) therefore causing well crafted user input to be interpreted as code and executed on the server. SQL injection has been commonly used to perpetrate unauthorized operations on a vulnerable database server such as harvesting users’ information and manipulating the contents of the database. In Web applications running a SQL database to manage users’ authentication, adversaries use SQL injection to bypass login and gain unauthorized access to user accounts or, even worse, to gain administrative access to the Web application. Other variants of these attacks allow the adversary to directly alter the contents of the server’s database and inject the adversary’s own content.
Last year, a major SQL injection attack was launched by the Asprox botnet. In this attack several thousand
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bots were equipped with an SQL injection kit that starts by sending specially crafted queries to Google searching for servers that run ASP.net, and then launches SQL injection attacks against the Web sites returned from those queries. In these attacks the bot sends an encoded SQL query containing the exploit payload (similar to the format shown here) to the target Web server.
http://www.victim-site.com/asp_ap- plication.asp?arg=<encoded sql query>
The vulnerable server decodes and executes the query payload which, in the
44 communicAtionS of the Acm | APriL 2009 | voL. 52 | no. 4
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