Advertising networks are more than just information-disclosure risks. They also serve as a malware attack vector. Advertising services pay web site owners for publishing advertisements on their web sites. A very common technique is the banner ad we've all seen at the top of web pages. Such ads usually take the form of animated GIF files, but they now include many image and video formats. Individuals and organizations that want to advertise using such a service create a media file and pay an advertiser a fee, and the advertiser serves the image to thousands of visitors of sites that belong to its advertising network. The risks here are twofold. Attackers have created misleading advertisements as a means to draw traffic to a malware serving or other malicious web site.47 The users' trust of the advertisement company and the hosting web site increases their trust of the advertisements, leaving web surfers more vulnerable to such an attack. Virus writers have used the Google Adwords service to serve text ads that appeared to link to legitimate destination sites, but silently infected vulnerable web surfers by routing users through an intermediate, malicious site. Attackers also have used a vulnerability in Internet Explorer to compromise visitors as they passed through the intermediate site, before ultimately arriving at the legitimate site.48
In part 1 of an interview with Greg Conti, the author of Googling Security examine the privacy implications of popular online tools and services.
Note: Google has taken an active role in countering malicious advertisements and websites. For example, Google warns users of potentially malicious sites by clearly labeling suspect sites in their search result listings. As another example, Google's I'm Feeling Lucky search button no longer automatically redirects a user to a suspicious site; instead, the user is presented with a list of search results instead.53
Beyond malicious ad serving, advertisers employ another concerning strategy that I call malicious interfaces. In the idealistic world of interface design theory, interface designers always operate in the best interests of their users. Designers carefully study user tasks and painstakingly craft interfaces and applications to help users accomplish them. However, in the world of online advertising, the exact opposite is true. Designers frequently violate design best practices to coerce or mislead users into viewing advertising. Examples abound on the web: fake hyperlinks that pop up advertisements, giant advertisements that cover the text of articles, distracting advertising videos that begin playing the moment a page is viewed, banner ads with fake buttons that appear to be a part of the interface, advertisements embedded in video clips … the list goes on. Malicious interface designers are creative; new "innovations" are coming out regularly. The only constraining factor appears to be the tolerance of the user. The invasiveness of advertisements is getting worse. You may have heard the term "banner blindness." Users quickly learned that banner ads are of little value and ignore the advertisements, to the point that they barely perceive banner ads anymore. This defense mechanism has forced advertisers to become more aggressive in capturing user attention. 54, 55 Although, I don't claim that Google employs malicious interface design, the trend is concerning; it seems that malicious interface designers are carefully seeking the sweet spot between making advertising profit and annoying the user so much that they abandon a given site altogether.
Warning: In many cases the link displayed by an embedded advertisement is not the actual link. Nor will hovering over the link display the destination URL in the user's browser status bar. The actual link goes first to the ad server so the click can be logged. The user's browser is then redirected to the page chosen by the advertiser.
As carriers of key components of network infrastructure, ISPs and web hosting services are flexing their muscle to place advertisements in front of users. For example, domain registrar and web hosting provider Network Solutions hijacked customers' unused subdomains to resort to ad-laden "parking" pages. 56 As another example, bloggers Lauren Weinstein and Sarah Lai Stirland reported Canadian ISP Rogers modification of web pages en route.
48. Brian Krebs, "Virus Writers Taint Google Ad Links," Security Fix Blog, WashingtonPost.com, 25 April 2007. http://blog.washingtonpost.com/securityfix/2007/04/virus_writers_taint_google_ad.html, last accessed 4 May 2008.
49. Brian Krebs, "Hacked Ad Seen on MySpace Served Spyware to a Million," Security Fix Blog,WashingtonPost.com, 19 July 2006.
50. Betsy Schiffman, "Hackers Use Banner Ads on Major Sites to Hijack Your PC," Wired.com, 15 November 2007. http://www.wired.com/techbiz/media/news/2007/ 11/doubleclick, last accessed 4 May 2008.
51. Nonmalicious Flash applets hosted on web pages have also been shown to be vulnerable to attack. See Dan Goodin's "Serious Flash Vulns Menace at Least 10,000 Websites," www.theregister.co.uk/2007/12/21/flash_vulnerability_menace/
52. Eric Auchard, "Adobe, Yahoo! test running ads inside PDF documents." Reuters, 28 November 2007. www.reuters.com/article/marketsNews/idUKN2754715120071129?rpc=44&sp=true, last accessed 4 May 2008.
53. Robert Freeman, "I'm Feeling Lucky," Frequency X Blog, IBM Internet Security Systems, 29 April 2008. http://blogs.iss.net/archive/FeelingLucky.html, last accessed 17 May 2008.
54. Jakob Nielsen, "Banner Blindness: Old and New Findings,"Alertbox blog, 20 August 2007. www.useit.com/alertbox/banner-blindness.html, last accessed 7 May 2008.
55. An interesting exception is NCSoft's game City of Heroes, which makes viewing ingame advertisements optional; see http://yro.slashdot.org/article.pl?sid=08/04/06/ 0554230.
56. Cade Metz,"Network Solutions Hijacks Customer Subdomains for Ad Fest," The Register, 11 April 2008. www.theregister.co.uk/2008/04/11/network_solutions_sub_ domain_parking/, last accessed 8 May 2008.
Reproduced from the book Googling Security & the Privacy Implications of Cloud Computing Copyright , Addison Wesley Professional. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other users.