Tom Olzak

Fighting Unwanted Browsing: Web filtering is not always effective

In Access Controls, Business Continuity, Content Filtering, Data Leak Prevention, Insider risk, malware on September 23, 2009 at 12:22

Many organizations use Web filtering to block employee access to “unsuitable” sites.  Blocking usually takes the form of products like WebSense and services such as OpenDNS (from free, through SMB and Enterprise).  However, savvy employees will find a way around these controls. 

Definitions of what constitutes an unsuitable site vary from business to business, but there is a general set of objectives which typically underlies them all.

  • Prevent viewing of pornography, hate sites, or any other material which may be interpreted as creating a hostile work environment
  • Prevent activities which may put the organization at risk, such as visiting sites
    • which present a known high risk of infecting the network with malware
    • which provide an easy way for employees to wile away the workday focused on social networking, shopping, sports, or other non-business related media

Whether an organization uses Web filtering to achieve one or all of these objectives, users will find a way around restrictions.  One of the best ways is to encrypt outgoing sessions with a client-based or hosted proxy.  Yes, most if not all Web filters allow you to block access to these sites.  And yes, restricting employee rights to install applications can help.  However, there are services which circumvent both controls.

Web filters rely on their ability to see destination information and compare it to a database of blocked sites, usually organized by category.  If a user connects to an external proxy service (not in the blocked sites list) via SSL/HTTPS, no traffic from the end-user device to the Internet is visible to the Web filter.  The result?  The user can browse to any and all sites on the Web.

Take, for example, Megaproxy.  Figure 1 is the message I receive on my test machine if I try to go directly to the Megaproxy site.  Why?  Because the site is considered a proxy site.  All proxy sites must be blocked—as they are on this network–or Web filtering is the proverbial exercise in futility.  But Megaproxy provides an easy way around this.

Figure 1: Megaproxy blocked

Figure 1

The Megaproxy service periodically changes the URL used to get to the proxy sign-on prompt shown in Figure 2.  So Web filtering vendors have to play catch-up to block the current URL.  This is only possible when using the for-fee service, which a user can simply set up from home.  The fee is so low that any user with a strong desire to break out of IS constraints imposed on browsing will quickly get out the credit card.  I’ve been testing the same URL for about three weeks now with no problem.

Figure 2: Megaproxy login

Figure 2

Once logged on, the service asks for the URL for the page I want to visit, as shown in Figure 3.  The Web filter system I’m testing blocks remote access services, such as GoToMyPC.  So, I entered 

Figure 3: Enter URL

Figure 3

Figure 4 shows the result; I easily access with full functionality.  I could just as easily access  Note that I have to enter all addresses for sites I want to visit into the address bar provided by Megaproxy.  If I use the standard browser address bar, I will leave Megaproxy, and my traffic will once again be visible to the filtering solution.

Figure 4:

Figure 4

Megaproxy is not malware.  Nor is it intended to make your life as a security professional miserable.  It is designed to provide safe browsing from hotels, airports, and other hot spots.  The changing URL allows use of secure browsing even if the hotspot tries to prevent it by blocking proxy access.

The bottom line? An organization cannot rely on Web filtering alone to prevent unsuitable Web behavior.  Rather, other controls—preventive and detective, administrative and technical—must support filtering.  For example, some organizations simply block all SSL traffic not explicitly approved for business purposes.  If your organization is using Web filtering, take a look at the gaps unique to your organization and plug them.

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