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Exploiting Browser Flaws

Thus far, we've focused on attacking Web applications involving bad guys undermining the logic that lives on Web servers for nefarious purposes. However, a significant and scary trend involves attackers coopting e-commerce sites and using them as a delivery mechanism for malicious code to vulnerable Web browsers.

Numerous browser vulnerabilities are discovered on a regular basis, especially (but not exclusively) in Internet Explorer. There are several types of browser holes, including buffer overflows, flaws that let an attacker escape the security restrictions on scripts or other active Web content (such as the Java runtime environment), exploits that let malicious code bypass cryptographic signature checks, and problems that let malicious code execute in a different security zone than it should. All of these problems could be triggered if the victim surfs to the wrong Web site with a vulnerable browser.

Microsoft, as well as other vendors, has historically not rated such browser flaws as critical, because they say that the victim user must be tricked into surfing to the attacker's Web site. If users surf only to trusted sites, they should be unaffected by such problems, or so the thinking goes.

However, this assumption is false, as we saw in several major attacks, with many more likely in the future. In these attacks, the bad guys first undermined trusted midsized e-commerce sites. The attackers installed code on these Web sites that would exploit browser vulnerabilities when an unsuspecting, but trusting, user surfed to these e-commerce sites. Later, when users surfed to the e-commerce sites, their browsers were exploited, and malicious code was inserted on their machines.

In June 2004, this attack was pulled off using the Download.Ject flaw in Internet Explorer that let a Javascript run arbitrary code on a vulnerable browser that surfed to a site hosting Download.Ject exploitation software. Attackers took over a dozen e-commerce sites using various buffer overflow attacks, and installed browser-exploiting code there. When a user surfed to one of the infected Web sites, the Download.Ject flaw in the user's browser was triggered, causing the victims to download a keystroke logger program called berbew from a Russian Web site. This keystroke logger grabbed financial information from the browser, including account numbers and passwords for e-commerce sites and banks, as illustrated in Figure 7.40. Here is the flow of these increasingly common types of attacks:

  1. The attacker takes over some e-commerce or other trusted site on the Internet. The attacker installs code on this site that can exploit browser vulnerabilities.
  2. An innocent victim surfs to the infected Web site.
  3. The infected Web site responds with a Web page that exploits the browser.
  4. Based on the exploitation of Step 3, the browser connects to the attacker's site and grabs some malicious code from it, such as a keystroke logger, a bot, or a worm.
  5. The evil code on the victim's machine now runs, doing nasty stuff to the user, such as stealing his or her keystrokes.
Figure 7.40

Figure 7.40 Compromising an e-commerce site and using it to deliver keystroke loggers to victims with vulnerable browsers.

In November 2004, we saw a similar attack, this time exploiting an at-that-time-unpatched buffer overflow in Internet Explorer called the IFRAME flaw. This time, the attackers took over some advertising sites that posted banner ads on a variety of other news and e-commerce Web sites. If you viewed any of these ads at any of these sites with a vulnerable browser, you'd get a worm called Bofra installed on your machine. Bofra would steal sensitive information and try to take over other nearby systems.

As users increasingly deploy personal firewalls to block the automated propagation of malicious code to their machines, such browser-based attacks will likely grow in prominence. By riding through a user's normal Web surfing and exploiting browser holes, the attacker's actions bypass the personal firewall on a machine. The vast majority of personal firewalls are configured to allow one or more Web browsers to access the Internet, thus poking a significant hole in the protection offered by the firewall if the browser itself is vulnerable.

Defending Against Browser Exploits

These browser-based exploits are an increasing threat, but how do you defend against such attacks?

First, keep your browsers patched. If there's a new hole reported in a browser, make sure to patch it immediately. Unfortunately, both the June and November 2004 attacks exploited holes for which there was no patch yet released. Still, it's a good idea to keep your systems patched.

Next, utilize an up-to-date antivirus tool on all systems, especially those machines that browse the Internet. Happily, the code used in most of these attacks so far was detectable with antivirus tools by the time the attack was widespread, which prevented many users from being compromised.

Furthermore, you might want to consider using a browser other than Internet Explorer. I don't want to start a product war here. However, Internet Explorer is a major target for these types of attacks, given its market dominance. Other browsers have holes, too, but they are less likely to be targeted by attackers, simply because fewer people use them. The attackers are looking for lots of easy prey, and Internet Explorer users sure are a large population. However, please do not underestimate the amount of work needed to transition to another browser. For personal users, learning a new browser might take some time. In enterprise environments, a different browser might break some of your critical applications. Recoding those applications could take significant resources, thus making a transition to another browser financially impossible.

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