Saturday, February 11, 2012

Captive Portal Security part 1


by Peter Willis


This brief paper will focus on the security policies of networks which require payment or web-based authentication before authorization. The topics of discussion will range from common set-up of captive portals, the methods available to circumvent authorization and ways to prevent attacks. A future paper will go over the captive portals themselves, their design and attack considerations.




What is a captive portal? A captive portal (or CP) is a generic term for any network which presents an interface, usually via a website, to authorize a user to access a network or the internet. Typically these will be in the fashion of a website where one accepts an agreement not to tamper with the network. It can also cover college campus log-in screens, corporate guest account access, for-pay wifi hotspots, etc.

In this paper I will mostly be discussing the "wifi hotspot" form of CP, where either after payment, user authorization or clicking on an "Accept" button will get you access to the internet. Many attacks to circumvent these networks rely on remote dedicated servers and thus won't be suitable to bypass all forms of CPs (as some networks are intended to grant access to internal-only networks).


There are many different attacks one can perform to circumvent a CP, ranging from the simplistic to complex. Each has its benefits and weaknesses, relating to performance, reliability and possibility of detection by IDS. I will briefly outline the methods here with more examples/detail given during the paper.

1. Tunnel over DNS

This method involves tunneling network and transport-level packets encoded over DNS records. The reason this attack works is due to the design of the DNS system. A domain or sub-domain can delegate where the answers for their DNS records come from using an NS record. The NS record points to your 3rd-party server. The useful part for bypassing CPs comes in here because most caching nameservers (like those on a CP) must forward on requests to your server if they want to be answered properly, which means we can use the CP as a sort of DNS proxy. Set up a custom nameserver that can pack and unpack packets and a client to transmit them and you have yourself a two-way connection with a server on the internet. Make an SSH connection through it and you have a secure tunnel.

One tool used to exploit this property of DNS is iodine[1]. This tool has enhanced security and methods to auto-detect the type of network it passes through and get the best connection possible. Unfortunately it doesn't work as well in the real world as it does on paper.

The original implementation, OzymanDNS by Daniel Kaminsky, is (once modified) actually the most reliably working solution, even if it works as a presentation layer instead of tunneling raw IP packets. This actually gives a couple benefits in that it can be run by an unpriviledged user (on the client side) and is modular/easy to hack on (written in perl). I have modified a version of this code[2] to optimize it and reduce CPU use, but it's not pretty: an average of about 7 kilobits per second is what you'll see. But if you really need brief access it works just about anywhere.

2. Tunnel over ICMP

Surprisingly difficult to exploit in the real world, this protocol offers an alternative tunnel through a firewall for those that don't block it. Networks use ICMP for all kinds of things, most notably passing errors and routing information when necessary as well as the ubiquitous 'ping' messages. If you can't ping there may still be alternative methods to tunnel data through this protocol, but in general the ping is the simplest.

The attack works like this: send an ICMP message (which being layer 3 is not always restricted the way IP sub-protocols are) with an IP or other packet encoded in its payload similar to tunneling over DNS. The benfit of this method is (when it works) it can provide a highly reliable connection without excessive lag (around 24 kilobits per second bandwidth on average). The tool icmptx[3] works well.

A bigger problem looms with IPv6 though, as there is a newer ICMP (icmpv6) which imposes restrictions on the number of icmp packets per second - around 6 per second. This seriously hampers the performance of any application trying to tunnel data through the protocol, but luckily nobody uses IPv6 yet. In general most CPs seem to block ICMP packets.

3. Firewall pinholes

Seldom probed by anyone but the most desperate CP hackers, most firewalls do allow one or more tcp or udp ports outbound to remote hosts (usually on the internet). These will allow you to tunnel an arbitrary protocol as long as it works over the transport protocol of the pinhole.

My favorite of these is tunneling OpenVPN tunnels over udp port 53 - the DNS port. Some CPs allow port 53 outbound to any host on the internet without checking to see if it's passing DNS requests, which is a big mistake. Unfortunately some CPs intercept and rewrite any traffic going over port 53 to provide their own custom DNS responses which breaks this method.

Sometimes strange high-numbered ports are open to the remote host. The simple way to tell this is to run a tcpdump or custom application on the remote host and scan every port from the CP's network to identify the one that's open. With an automated script this can be accomplished easily and once a hole is found the tunnel will provide full bandwidth to the attacker. For attacking CPs that allow access to internal networks you may be able to craft specific packets which when successfully passed through present a different response, thus enumerating open ports.

4. Transparent proxies

Often times the best holes come from an incorrectly set-up network proxy. Often the ACLs on HTTP proxies are set up only enough to block basic requests and don't handle the whole spectrum of possible HTTP requests. Sometimes you can use the proxy as simply as configuring the CP's gateway IP in your browser's proxy settings. However, sometimes a 3rd party server will help us with the extra mile of getting through their ACLs.

The simplest attack in this case is simply looking for an open proxy on some host on the network. Nmap[4] is bundled with scripts which will quickly detect open proxies. Sometimes one needs to abuse the HTTP specifications to find a hole in a particular implementation of an HTTP proxy. Some ACLs can be bypassed by merely changing CRLF to LF in your request or using a different HTTP method. Some authentication/authorization software even have rules in place to allow you to bypass authorization by adding a "?.jpg", "?.css", "?.xml" or other extention to your request. Sometimes header injection can be used to provide a quick jump through a backend proxy.

However, this may only give you HTTP or HTTPS access through the CP. To pass arbitrary data through the proxy it must allow you to use the CONNECT method to reach a remote host. Usually this is allowed because the SSL port (tcp port 443) needs to transmit encrypted packets which are only passed through a proxy with the CONNECT method. If a CONNECT is supported we can take advantage of a 3rd-party server on the internet to tunnel arbitrary packets over HTTP.

An HTTP proxy is set up on the internet which passes connections on to an internal SSH server on port 22. A client then uses proxytunnel[5] to connect to the CP's gateway and from there issues a CONNECT to the 3rd-party proxy, usually on port 443 (but port 80 is a good alternative to have set up). The final request is made to CONNECT to that machine's ssh port. If all goes well you should be able to use this new connection with your SSH client and forward arbitrary data over it. All the CP sees is HTTP or HTTPS traffic being forwarded from one proxy to another. The end result is a high-bandwidth tunnel through the proxy.

A tool[6] included with this paper will allow you to quickly probe ports found on a CP gateway for common problems in HTTP implementations and look for a way out.

5. MAC spoofing

The last method is the only one which is present in all wireless and some wired CPs by design. Once a host is authorized by the CP, its mac and IP address are allowed unrestricted access. All one needs to do is sniff traffic on the network, find a host that is authorized, and spoof its IP and mac address. Spoofing a mac is dependent on your network card and driver but most modern network devices today support it. The downside of course is you have to observe someone already authenticated, but in places such as a crowded airport lobby this may be less difficult than it seems.

6. Miscellaneous

Of course there are many more methods to circumvent CP access with varying degrees of success. Other methods include use of fuzzers, creative source routing/probing, the abuse of cisco protocols for proxy clustering, and the abuse of "convenience" features of some authentication servers.


I can't tell you how to perform these attacks without at least trying to tell you how to fix your own broken networks, so here's the white-hat portion of the paper.

First of all, block all unauthenticated traffic (of any layer) destined for the internet. There's no reason for any client to be able to pass any packets without being authed, so just put that rule at the top of your firewall. That'll stop firewall pinhole attacks and IP over ICMP.

To stop IP over DNS, simply have all DNS requests from unauthed sources return the same record every time... The IP of your CP gateway. Not only will this work fine with your transparent HTTP proxy (your proxy should be sending a redirect to your gateway's website anyway for authentication purposes), you can also run many service-specific proxies that will handle requests and inform the client to auth first via http. SSL won't work, but screw them for trying to be secure.

To stop HTTP transparent proxy abuse, pick a very secure and well-established proxy server that's stable and has all security patches and bug fixes up to date. Make sure you craft your ACLs to explicitly stop ALL requests coming from an unauthed host. It should only return the redirect to your authentication website. If for example the server differentiates between requests which are terminated by LF or CRLF, you should probaly write a lot more rules than less and cover all the possibilities.

For other methods of abuse, make sure your CP solution has no unneeded services or ports available on the network. These can be easy targets for attack (who here has checked if snmp is enabled on their gateway? ssh?) and the less attack vectors the less likely an attacker will get through. Audit all your systems to ensure they don't accidentally allow more access than is necessary.


Almost all CPs today can be bypassed in one way or another. Clearly there's a certain amount of risk vs reward where a company just doesn't care if the top 1% of users can gain access without authorization. That's good for us hackers, but can be bad for the CP provider.

Did you know that since its inception, Starbucks and McDonald's for-pay wifi had open squid proxies on every default gateway? All you needed to do was run an nmap scan to find it, set it in your browser's proxy list and surf for free. (Oh, and the payment network McDonald's uses to process credit cards uses the same network) Did you know there's a major mobile voice and data service provider which has not one but THREE holes in its service, allowing anyone to use their service for free anywhere in the USA with 3G coverage?

Even if Bradley Manning was not able to get a CDROM of US embassy cables out of a secured facility, he might have been able to find a way though network firewalls using these same techniques and tunnel the data out. It's not that far-fetched: as corporations increasingly lock the internet away from their employees, industrious hackers find new more challenging methods to circumvent their restrictions and get the access they want. With this in mind, those implementing the controls should be wary of the many methods of attack involved and the potential for abuse.

This paper has not covered the possibility of attacking the web applications which authenticate users trying to gain access to the network. I am not a web application pen tester so i'll let someone else review those holes.


1. iodine -

2. dnstunnel -

3. icmptx -

4. nmap -

5. proxytunnel -

6. -


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