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There is Always a Back Door

Russ White

A long time ago, I worked in a secure facility. I won't disclose the facility; I'm certain it no longer exists, and the people who designed the system I'm about to describe are probably long retired. Soon after being transferred into this organization, someone noted I needed to be trained on how to change the cipher door locks. We gathered up a ladder, placed the ladder just outside the door to the secure facility, popped open one of the tiles on the drop ceiling, and opened a small metal box with a standard, low-security key. Inside this box was a jumper board that set the combination for the secure door.

First lesson of security: there is (almost) always a back door.

I was reminded of this while reading a paper recently published about a backdoor attack on certificate authorities. There are, according to the paper, around 130 commercial Certificate Authorities (CAs). Each of these CAs issue widely trusted certificates used for everything from TLS to secure web browsing sessions to RPKI certificates used to validate route origination information. When you encounter these certificates, you assume at least two things: the private key in the public/private key pair has not been compromised, and the person who claims to own the key is really the person you are talking to. The first of these two can come under attack through data breaches. The second is the topic of the paper in question.

How do CAs validate the person asking for a certificate actually is whom they claim to be? Do they work for the organization they are obtaining a certificate for? Are they the "right person" within that organization to ask for a certificate? Shy of having a personal relationship with the person who initiates the certificate request, how can the CA validate who this person is and if they are authorized to make this request?

They could research the person — check their social media profiles, verify their employment history, etc. They can also send them something that, in theory, only that person can receive, such as a physical letter, or an email sent to their work email address. To be more creative, the CA can ask the requestor to create a small file on their corporate web site with information supplied by the CA. In theory, these electronic forms of authentication should be solid. After all, if you have administrative access to a corporate web site, you are probably working in information technology at that company. If you have a work email address at a company, you probably work for that company.

These electronic forms of authentication, however, can turn out to be much like the small metal box which holds the jumper board that sets the combination just outside the secure door. They can be more security theater than real security.

In fact, the authors of this paper found that some 70% of the CAs could be tricked into issuing a certificate for just about any organization — by hijacking a route. Suppose the CA asks the requestor to place a small file containing some supplied information on the corporate web site. The attacker creates a web server, inserts the file, hijacks the route to the corporate web site, so it points at the fake web site, waits for the authentication to finish, and then removes the hijacked route.

The solution recommended in this paper is for the CAs to use multiple overlapping factors when authenticating a certificate requestor — which is always a good security practice. Another solution recommended by the authors is to monitor your BGP tables from multiple "views" on the Internet to discover when someone has hijacked your routes, and take active measures to either remove the hijack, or at least to detect the attack.

These are all good measures — ones your organization should already be taking.

However, the larger point should be this: putting a firewall in front of your network is not enough. Trusting that others will "do their job correctly," and hence that you can trust the claims of certificates or CAs, is not enough. The Internet is a low trust environment. You need to think about the possible back doors and think about how to close them (or at least know when they have been opened).

Having personal relationships with people you do business with is a good start. Being creative in what you monitor and how, is another. Firewalls are not enough. Two-factor authentication is not enough. Security is systemic and needs to be thought about holistically.

There are always back doors.

By Russ White, Infrastructure Architect at Juniper Networks
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