Web filter evasion part 4: Public HTTP Proxies

View entire series: Web Filter Evasion

A lot of times, you kind of want access to your favorite content, even if it's just to read it, while at work or school. Some places with more draconian Internet access policies block pretty much everything "cool" and paint their restrictions with a very wide brush. In this series, we'll uncover a few ways around these restrictions.

With each article, I will delve into techniques that are progressively more advanced and difficult to implement. Keep in mind that this is a thought exercise in evasion. Implementing this stuff could get you expelled from school, fired from your job, or banned from public-access computers in a library or coffee shop. With a little bit of wisdom, you can often get around the restrictions set in place without getting yourself into hot water.

Today, we will cover using "wide open" proxies to bounce our connections through. Since proxies are transient and often configured poorly, they pass the ire of most web filters. The rub? You usually have to find one that operates on a well-known port such as 80 (http) or 443 (https/ssl). Finding a working proxy can be a pain, and using it involves tweaking your browser settings and/or installing special software. Group policies could leave you in the cold on this one. The increased difficulty of this particular evasion technique brings this trick in at #4 in our series.

Public Proxies
Public proxies are (often SOCKS v5) proxy servers that have either been intentionally or accidentally configured to allow anyone on the Internet to access proxied content without proper access controls. I could go on and on about the various security implications of these proxies but that's not what this series is about. Simply put, HTTP proxies differ from Web Proxies in that the browser directly handles communication with the proxy behind the scenes. Regardless of what protocols the proxies are capable of handling, we're only concerned about Web traffic (http/https) for the time being.

Browser Configuration:
Yesterday, I linked to various proxy configuration instructions for different browsers. Today, you can use the same instructions to add the proxies you discover on your own. If you find that there is already a proxy server configured, you might be out of luck for this trick (and maybe for the fifth and final one, as well). If your employer is already forcing your web traffic through their own proxy, there's a good chance that they're blocking all outgoing traffic from your workstation to the Internet, making HTTP Proxies AND Tunneling difficult, if not impossible.

Security Risks:
The same risks apply here as apply to web proxies. Your data is going through a third party and you're subject to data manipulation, sniffing, or other bad things. Essentially, you're trusting a potentially unknown and untrusted third party with your traffic. By now, it should be obvious that these more aggressive evasion techniques can't be easily justified. You have been warned.

Finding public HTTP Proxies:
Public proxies play a huge role in the so-called underground. I'm talking mostly about illegal or frowned-upon activities such as phishing, spamming, and illegal file sharing. As such, there are groups that go out of their way to sniff out and enumerate poorly-configured proxy servers that are easily hijacked for whatever purpose others desire. I don't advocate using services without permission. In a moment, I'll discuss how to set up your own public HTTP Proxy.

One source that provides a list of proxies that can be sorted on several criteria: http://www.xroxy.com/proxy-port.htm

Assuming your employer or school has you locked down to using only port 80 or 443, use the above tool to find proxies that operate on those ports. Then, update your proxy settings to match one of the servers in the list. If it's too slow or fails to work, try again. Soon enough, you should be able to access all of the content your heart desires.

Squid at home (or on another server outside of work/school)
You can also configure Squid to act as your own proxy. Making it a public proxy is a bad idea, so lock it down to your remote computer's IP Address. First, find your public-facing IP address from the computer you normally use, and write it down. The easiest way is to visit WhatIsMyIP.com. Then, using the Squid configuration guide from yesterday, add the following lines:

At the top of the block of "acl" lines (replace with your actual ip address in place of
acl myremote src

Above the other http_access lines:
http_access allow myremote all

Finally, you will need Squid to listen on a port you can use. For this example, we will use Port 80. Keep in mind this will interfere with any web server running on this host. Below the existing http_port line(s), add:
http_port 80

Restart Squid, then point your browser's proxy to the IP address of your squid server, port 80.

The IP Address restriction of "myremote" will keep others from hijacking your proxy. Add as many rules as you need to for other remote IP addresses.

View entire series: Web Filter Evasion

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