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Detect and destroy web proxy servers

John Strand, Contributor

In an effort to control browser use within an organization, enterprises often use Web filters as part of their security gateway technology strategy. Some employees, however, eager to bypass the devices and continue their Web use without restriction, respond by using anonymous proxies, tools that can trick Web filters into incorrectly allowing access to an inappropriate site. In this tip, after reviewing how Web anonymizers can lead to compromised systems, we'll look at ways companies can shore up their content-filtering processes.

Why anonymous Web proxies are bad
First, I want to establish the dangers of allowing users to use anonymous proxy software to surf the Web. Let's be clear: If employees are using them on your network, you can guarantee that at least some portion of your user population is doing something that they should not be doing, perhaps going to porn sites, gambling sites or other non-approved places like personal blogs, MySpace.com or YouTube. Keep in mind that these websites often host malware, including Trojan backdoors in the form of ads and/or "special programs."

Also, remember that many enterprise Web content-filtering devices are not merely preventing attack and infection from malicious sites and domains of ill-repute. The products also protect an enterprise from content on legitimate sites that are unknowingly hosting malware via third-party ads by trying to block malware that may be dispersed via the adds. The Internet and the browsers that access it are your single greatest vulnerability. Without the protection of a Web content-filtering device, your only layer of defense against malicious code from the Web is antivirus, a defense which can be bypassed when a hacker has the right tools.

So how do users bypass Web-filtering devices? Quite easily; there are many freely available tools to do so. Employees, for example, can make connections and request objects through "go-between" anonymous proxy software like Privoxy, instead of having to connect directly to an enterprise server and be subject to content filtering.

To boot, keeping up with emerging Web proxy tools and techniques is no easy task. The blacklist approach that Sophos takes to unauthorized proxies is to continually search through forums, IRC channels and discussion boards where users set up and share proxies. The antivirus vendor then adds the proxies to their blacklist. While the Sophos approach is interesting, it is flawed in that it does not help when a user sets up a proxy at home and tunnels out of an enterprise environment via SSH or SSL as we discussed above. In this case, the proxy of choice may not have reached the critical mass to attract the blacklist vendor's attention.

Sophos’ Paul Ducklin responds to John Strand
TechTarget ANZ yesterday received this correspondence from Sophos’ Head of Technology for Asia Pacific, Paul Ducklin:
“The author doesn't seem to have read the page he links to on our site.
As you will see on this page, if we (or someone else we trust) has already classified a site as a known proxy, then that site is going to be on our blocklist. You won't even be able to make a GET request to that site through our security product because we know it's dodgy and we block it up front. So it is true that we include a reputation blocklist approach in our proxy detection and prevention.
But the Sophos Web Appliance also analyses HTTP requests *and* responses to and from sites which are not on our reputation blocklist, actively inspecting traffic for signs that it is being routed through an anonymising proxy.
This provides real-time proxy detection, so that we are able proactively to detect and block proxy sites which are only covertly shared amongst users (and which thus don't get onto conventional blocklists), and even to block proxies set up by individual users for their own policy-busting use.
In short, the Sophos Web Appliance does not depend on a proxy having reached a ‘critical mass’, does not rely on a ‘blacklist approach’, and *is* able to close the door on proxies not identified through our reputation blocklist service.
With this in mind, we are very disappointed to read the words, ‘Whilst the Sophos approach is interesting, it is flawed’, since it is precisely because of the problem of relying entirely on blocklisting that we offer our two-pronged approach.
It would be much fairer, and more accurate, to mentioned our dual-layer defence, for example by writing instead that ‘a pure blocklist approach is flawed...[T]he proxy of choice may not have reached critical mass...
Incidentally, Sophos avoids this limitation by providing a real-time proxy detection feature in addition to a blocklist, which allows the proactive detection of hitherto unknown proxies."

How to defend against anonymous proxy software
The first action an organization should take is to review its Web surfing policies. Acceptable use policies should state that user activity to and from the Internet will be monitored, and that no methods of bypassing corporate Web filters will be tolerated. Users should have no reasonable expectation of privacy. This may be a tough sell due to political issues, but it will go a long way toward securing and monitoring an environment.

To gain visibility into the network, which is an important aspect of anti-proxy defenses, any outgoing traffic that is not associated with a business driver should be blocked. Exceptions may be necessary for some external sites. For example, your users may need to access other sites for research or data entry. Try to identify which sites are essential in order for the business to function and make restrictions accordingly.

While we as a community of security professionals have become quite good at only allowing certain protocols into our network (ingress filtering), many of our networks are still lacking when it comes to the traffic we allow out of our environment (egress filtering). It's critical to apply the same mindset to both inbound and outbound traffic. If enterprises redirect outgoing traffic destined to common Web ports like port 80 or 443, but neglect to monitor or restrict traffic to any other port, an internal user can circumvent outbound Web filtering by using the browser to manually input the server address and the port (i.e. wwx.homeproxy.com:8888). Attackers may also be able to create an SSH tunnel out of the enterprise environment to an external proxy. Ideally, you should have a default drop rule that any traffic not explicitly allowed is implicitly denied.

After getting a handle on the traffic leaving the environment, monitor the sites that internal users are accessing. Because there will inevitably be proxy sites that Web content-filtering software misses, make sure to review Web content-filtering logs on a weekly basis, at least.

Following all of the above steps helps in a number of ways. These best practices will cut back on the amount of unauthorized Web surfing, help with an audit and limit the effectiveness of client-side attacks against an enterprise environment. Many malware infections delivered via a Web browser utilize the Web as a command and control mechanism. Others utilize non-standard ports and protocols like port 6667 or ICMP as a tunneling mechanism. With outbound ports shut down, aside from those being redirected through a Web content filtering device, a major command and control vector for external attackers can be taken down. Considering this, any traffic that is about to leave the network -- and that has not been explicitly allowed and monitored -- should be treated as a possible compromise and should be investigated. 

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