engineering techniques to trick users into installing and running malware by themselves. Unfortunately, the Web is rich with deceptive content that lures users into downloading malware.

One common class of attacks includes images that resemble popular video players, along with a false warning that the computer is missing essential codecs for displaying the video, or that a newer version of the video player plugin is required to view it. Instead, the provided link is for downloading a trojan that, once installed, gives the adversary full control over the user’s machine.

A more recent trick involves fake security scans. A specially crafted Web site displays fake virus scanning dialogs, along with animated progress bars and a list of infections presumably found on the computer. All the warnings are false and are meant to scare the user into believing their machine is infected. The Web site then offers a download as solution, which could be another trojan, or ask the user for a registration fee to perform an unnecessary clean-up of their machine.

We have observed a steady increase in fake anti-virus attacks: From July to October 2008, we measured an average of 60 different domains serving fake security products, infecting an average of 1,500 Web sites. In November and December 2008, the number of domains increased to 475, infecting over 85,000 URLs. At that time the Federal Trade Commission reported that more than one million consumers were tricked into buying these products, and a U.S. district court issued a halt and an asset freeze on some of the companies behind these fake products. This does not ap-

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pear to have been sufficient to stop the scheme. In January 2009, we observed over 450 different domains serving fake security products, and the number of infected URLs had increased to 148,000.

Malware activities on the user’s machine. Whether a user was compromised by a social engineering attack or a successful exploit and drive-by download, once the adversaries have control over a user’s machine, they usually attempt to turn their work into profit.

In prior work, ¹⁰ we analyzed the behavior of Web malware installed by drive-by do wnloads. In many cases, malware was equipped with key-loggers to spy on the user’s activity. Often, a back

door was installed, allowing the adversary to access the machine directly at a later point in time. More sophisticated malware turned the machine into a bot listening to remote commands and executing various tasks on demand. For example, common uses of botnets include sending spam email or harvesting passwords or credit cards. Botnets afford the adversary a degree of anonymity since spam email appears to be sent from a set of continuously changing IP addresses making it harder to blacklist them.

To help improve the safety of the Internet, we have developed an extensive infrastructure for identifying URLs that trigger drive-by downloads. Our analysis starts by inspecting pages in Google’s large Web repository. While exhaustive inspection of each page is prohibitively expensive as the repository contains billions of pages, we have developed a lightweight system to identify candidate pages more likely to be malicious. The candidate pages are then subjected to more detailed analysis in a virtual machine allowing us to determine if visiting a page results in malicious changes to the machine itself. The lightweight analysis uses a machine-learning framework that can detect 90% of all malicious pages with a false positive rate of only 10–3. At this false positive rate, the filter reduces the workload of the virtual machines from billions of pages to only millions. The URLs that are determined to be malicious are further processed into host-suffix path-prefix patterns. Since 2006, our system has been used to protect Google’s search. Our data is also published via Google’s Safe Browsing API to browsers such a Firefox, Chrome, and Safari. These browsers employ our data to prevent users from visiting harmful pages.

challenges

Despite our efforts to make the Web safer for users, there are still a number of fundamental challenges requiring future work, including:

Securing Web Services. Establishing a presence on the Web, ranging from simple HTML pages to advanced Web applications, has become an easy process that enables even people with little technical knowledge to set up a Web service. However, maintaining such

a service and keeping it secure is still difficult. Many Web application frameworks require programmers to follow strict security practices, such as sanitizing and escaping user input. Unfortunately, as this burden is put onto the programmer, many Web applications suffer from vulnerabilities that can be remotely exploited. ^{12, 14} For example, SQL injection attacks are enabled by a programmer neglecting to escape external input.

Popular Web applications such as bulletin boards or blogs release security updates frequently, but many administrators neglect to update their installations. Even the Web server software itself, such as Apache or IIS, is often out-of-date. In previous work, ¹⁰ we found over 38% of Apache installations and 40% of PHP installations in compromised sites to be insecure and out-of-date.

To avoid the compromising of Web applications, it is important to develop mechanisms to keep Web servers and Web applications automatically patched. Some Web applications already notify Web masters about security updates, but the process of actually installing security patches is often still manual and complicated.

It is difficult to be completely safe against drive-by downloads. All that is required for an adversary to gain control over your system is a single vulnerability. Any piece of software that is exposed to Web content and not up-to-date can become the weakest link.

Many browser plugins and add-ons, such as toolbars, do not provide automatic updates. Furthermore, system updates often require a restart after installation discouraging users from applying the security patches on time.

Even if a system was fully patched, the window of vulnerability for some software is often very large. According to Krebs, major browsers were unsafe for as long as 284 days in 2006, and for at least 98 days criminals actively used vulnerabilities for which no patches were available to steal personal and financial data from users. ^{5, 6} Although progress on providing fault isolation in browsers that may prevent vulnerabilities from being exploited has been made, ^{1, 4} a completely secure browser still needs to be developed.

Detecting Social Engineering At-

46 communicAtionS of the Acm | APriL 2009 | voL. 52 | no. 4