Category Archives: Security

Computer Security Related topics

Another Strike against Domain Fronting

In 2014, Domain Fronting became the newest obfuscation technique for covert, difficult to censor communication. Even today, the Meek Pluggable transport serves ~400GB of Tor traffic each day, at a cost of ~$3000/month.

The basic technique is to make an HTTPS connection to the CDN directly, and then once the encryption has begun, make the HTTP request to the actual backing site instead. Since many CDNs use the same “front-end cache” servers for incoming requests to all of the different sites they host, there is a disconnect between the software handling SSL, and the routing web server proxying requests to where they need to go.

Even as the technique became widely adopted in 2014-2015, its demise was already predicted, with practitioners in the censorship circumvention community focused on how long it could be made to last until the next mechanism was found. This prediction rested on two points:

  1. The CDN companies will find themselves in a difficult position politically, since they are now in the position of supporting circumvention while also maintaining a relationship with the censoring countries.
  2. The technique has security and cost implications that make it not great for either the CDNs, or the practitioners.

We’ve seen both of these predictions mature.

Cloudflare, explicitly doesn’t support this mechanism of circumvention, and coincidentally has major Chinese partnerships and worked to deploy into China. Google also has limited the technique over periods as they have struggled with abuse (although mute in China, since the Google cloud doesn’t work there as a CDN.)

In terms of cost, the most notable incident is the “Great Cannon”, which targeted not only Github as widely reported, but also caused a significant amount of traffic to go to Amazon-hosted pages run by GreatFire, a dissident news organization, and costing them significant amounts of money. GreatFire had been providing a free browser that operated by proxying all traffic through domain-fronting. Due to a separate and less reported Chinese “DDOS” they ended up with a monthly bill for several tens of thousands of dollars and had to turn down the service.

The latest strike against domain fronting is a post a couple weeks back by Cobalt Strike that the technique is also gaining adoption for Malware C&C. This abuse case will further incentivize CDNs from allowing the practice to continue, since there will now be many legitimate western voices actively calling on them to stop. Enterprises attempting to track threats on their networks, and CDN customers wanting to not be blamed for attacks will both begin putting more pressure on the CDNs to remove the ability for different domains to be intermixed, and we should expect to see a continued drop in the willingness of providers to offer such a service.

Thoughts on IPv6 Measurement

About five years ago two projects, Zmap and Masscan, helped to shift the way that many researchers thought about the Internet. The tools both provide a relatively optimized code path for sending packets and collecting replies, and allow a researcher with moderate resources to attempt connections to every computer on the IPv4 Internet in about an hour.

These techniques are widely applied to monitor the Internet-scale security of services, with prominent examples of,, and For the security community, they have become a first-step for reconnaissance, allowing hackers to find origin IPs masked by CDNs, unadvertised points of presence, and vulnerable hosts within an organization.

While the core of the Internet and the services we actively choose to connect with remain staunchly IPv4, the networks that many end hosts are connected to are more rapidly adopting IPv6, responding to the exhaustion and density of the IPv4 address space.

This fall, a new round of research has focused on what is possible for the enumeration and exploration of the IPv6 address space. ‘You can -J reject but you can’t hide’ was presented at CCC, focusing on spidering DNS records to learn of active IPv6 addresses which are registered within the DNS system. Earlier in the fall, there were several sessions at IMC thinking about IPv6. Most notably, “Entropy/IP – uncovering entropy in IPv6″, which looks at how addresses are allocated in practice as seen by Akamai at the core of the network. In addition, IPv6 was the focus of a couple WIP sessions, expressing thoughts on discovering hosts through progressive ICMP probing, as well the continued exploration of what’s actually happening in the core as seen by Akamai.

Where does this growing understanding of wide-scale IPv6 usage take us?

  • Enumeration of candidate addresses is a new first step that will be needed for anything beyond a single prefix. Even then, scanning within a single organizational prefix can be considered an active brute-force attack, rather than the relatively ‘harmless’ reconnaissance of IPv4 scanning.
  • There are many potential sources to interact with for enumeration, including DNS records, observed network traffic, and default ::1 addresses. The Entropy/IP paper points out that has already been observed adding itself as a member of the NTP pool to harvest candidate IPv6 addresses for scanning.
  • Address generation for many hosts is not fully random, embedding a mac address, IPv4 address, or other non-random information. This can be used to discover a subset of hosts more efficiently, though still not at Internet scale. (for example, 2^32 attempts to look for hosts of a specific brand within a 2^64 network address space.) This would still sends several gigabytes of traffic to an individual network in the process of scanning. Non-random addresses tend to be more often associated with servers and routers than with end-clients.
  • Discovery of network topology is possible by enumerating where error responses to guessed addresses come back from. This doesn’t allow for discovery of individual machines either.

What do we do about it?

There will probably not be a for ipv6 in the same way there is for ipv4. Instead, much of the wide-scale scanning on the IPv6 network will be performed through reflection from hosts discovered through their participation in other active services, for instance bit torrent, NTP, or DNS.

Conversely, the number of vulnerable IPv6 hosts will keep growing, because they can exist for much longer before anyone will find them. This will likewise increase the value that can be obtained through scanning – both to hackers, and to academics looking at Internet dynamics. We can expect to see a marketplace for addresses observed passively by ISPs, the network core, and passive services.

It’s worth also watching the watchers here: which providers are “selling me out” so to speak? It would be worth building the honey-pots to observe which services and servers leak client information and lead top probing and the potential for compromise of end hosts.

First-party Google Analytics

Third party analytics services are suffering from the growing prevalence of ad blocking, tracking protection, and the trend of minimizing connections and requests. However, from a site owner perspective, receiving usage information remains important for measuring site growth.

My expectation is that we are already on the curve where ads and tracking software will be more tightly integrated into websites and make it significantly more difficult for clients to disambiguate
“good” and “bad” scripts, which are mostly done today from the URL.

Google already provides the tools needed to relay analytics communication through a third party server, and it took under an hour to put together a proof of concept that removes the final third-party requests that are required when viewing this page. In essence, my server proxies all the requests that would normally go to Google, and adds on a couple extra parameters to track who the real client is.

The modified loading script for google analytics, and the corresponding nginx configuration to make my server a relay are here.

Watch your PAC

In the last week at Blackhat / Defcon two groups looked deeply at one of the lesser known implementations of network policy called Proxy Autoconfig. (In particular, badWPAD by Maxim and Crippling HTTPS with unholy PAC by Safebreach.)

Proxy AutoConfig (PAC) is a mechanism used by many organizations to configure an advanced policy for connecting to the Internet. A PAC file is written in JavaScript to provide a dynamic determination of how different connections should be made, and which proxy they should use. In particular, international companies with satellite offices often find the PAC system useful in routing some traffic through a corporate proxy for compliance or geographical reasons while other traffic is routed directly to the Internet.

These two talks both focus on what a malicious individual could do to attack the standard, and each find an interesting line of attack. The first attack is that the PAC file is allowed to make DNS requests in determining how to proxy connections, and in many browsers sees the full URL being accessed rather than only the domain. This means that even when the user is communicating with a remote server over HTTPS, the local network can learn the full URL that is being visited. The second attack has to do with where computers look for PAC files on their local network – for a file called `wpad.dat`.

While there is certainly the potential for an attacker to target a victim through these technologies, they are more accessible and arguably more valuable to a ISP or state level actor interested in passive surveillance. This explicit policy for connectivity is not inherently more invasive than policies employed by many ISPs already, and could likely be deployed on many networks without consumer push-back as a performance enhancement for better caching. It is also appropriate for targeted surveillance, since vulnerability can be determined passively.

The viability of surveillance through WPAD and PACs is a bit of a mixed bag. Most ISPs use DHCP already and set a “search domain”, which will result in a recognizable request for proxy information from vulnerable clients. While organizations often require all clients to enable discovery, this is not true of many consumer machines. Unfortunately, some versions of windows have proxy discovery enabled by default.

The NMAP tool used for network exploration, and pitched towards use as a tool facilitating network attackers, already has support for WPAD. In contrast, the network status and monitoring tools, like Netalyzr and OONI do not yet monitor local proxy status and won’t provide indication of malicious behavior.


I started running a public sp3 server today. It’s a small side-project I’ve hacked together over the last couple weeks to make it easier for people to play with packet spoofing. The server works similarly to a public proxy, but with the trade-off that while it won’t send high-volumes of traffic, it will allow you to send arbitrary IPv4 packets from any source you want.

There are a few fun applications that need this capability that I’ve been thinking of: helping with NAT holepunching of TCP connections; characterizing firewall routing policies; and for cover traffic in circumvention protocols. I think there are others as well, so I wanted to start running a server to see what people come up with.

The code is on github.

The state of Internet Censorship

I’ll be presenting next week at 32C3 on the state of Internet access, transparency, and measurement. Lots of the work is done each year on measuring and learning about the state of access, but this phenomenon with growing relevance to many countries is poorly publicized. Much of this is a fear that being too public about what can be measured will make the network operators move to even more opaque techniques, since in many instances these systems are seen to thrive in structures without accountability.

Needless to say, it has been a busy year in the space, with increased funding for the measurement community and a multitude of new policy in response to ISIS and other perceived threatening uses of Internet Speech.

I’m excited to be heading back to Germany for the holidays, and hope to provide a reasonable survey of what’s out there and make the network measurement field a bit more accessible!

IETF and the HRPC working group

The Internet Engineering Task Force, the multi-stakeholder organization which shepherds the standards process for many of the technologies used on-line, is continuing to evolve that process. Protocol standards are already expected to include discussions on their security and privacy implications, in order to force an explicit conversation on those issues and hopefully encourage the development of secure systems. Beyond these, a new working group, the Human Rights Protocol Considerations group, was chartered last week. The group exists as part of the process of having another conversation around new protocols as they exist: what are the implications for freedom of expression and freedom of assembly that are wrapped up in our protocol design.

It seems like a question worth considering, especially as the IETF’s major contribution will be increasingly international. Many protocols emerging today are build by individual companies and are proprietary. We can hope however that it is at the boundaries of these walled ecosystems we create that standard protocols will need to be agreed upon. These boundaries will parallel our cultural discontinuities, and represent important places to have these conversations.

The group is drafting a methodology document as part of the background for proposing the update to the standards process. It’s an interesting way of thinking about protocols – how do they control or support individual expression? – that I hadn’t thought of before in those terms.