Applying the ERNW Seven Sisters Approach to VoIP Networks


if you’re following this blog regularly or if you’ve ever attended an ERNW-led workshop which included an “architecture section” you will certainly remember the “Seven Sisters of Infrastructure Security” stuff (used for example in this post). These are a number of (well, more precisely, it’s seven ;-)) fundamental security principles which can be applied to any complex infrastructure, be that a network, a building, an airport or the like.

As part of our upcoming Black Hat and Troopers talks we will apply those principles to some VoIP networks we (security-) assessed and, given we won’t cover them in detail there, it might be helpful to perform a quick refresher of them, together with an initial application to VoIP deployments. Here we go; these are the “Seven Sisters of Infrastructure Security”:

  • Access Control
  • Isolation
  • Restriction
  • Encryption
  • Entity Protection
  • Secure Management
  • Visibility

Now, let me discuss them in a bit more detail and put them into a VoIP context.


Access Control (“try to keep the threats out of the environment containing the assets to be protected”)

This should pretty much always be an early consideration as limiting access to “some complex infrastructure” obviously provides a first layer of defense and does so in a preventative[1] way. Usually authentication plays a major role here. Please note that in computer networks the access control principle does not only encompass “access to the network [link]” (where unfortunately the most prevalent technology – Ethernet – does not include easy-to-use access control mechanisms. And, yes, I’m aware of 802.1X…) but can be applied to any kind of (“sub-level”) communication environment or exchange. Taking a “passive-interface” approach for routing protocols is a nice example here as this usually serves to prevent untrusted entities (“the access layer”) from participating in some critical protocol [exchange][2] at all.

In a VoIP scenario limiting who can participate in the various layers and communication exchanges, be it by authentication, be it by configuration of static communication peers for certain exchanges[3] (yes, we know this might not scale and usually has a bad operational feasibility) would be an implementation of the access control principle.


Isolation (“separate some elements of the environment from others, based on attributes like protection need, threat potential or trust/worthiness”)

In computer networks this one is usually implemented by network segmentation (with different technologies like VLANs or VRFs and many others) and it’s still one of the most important infra­structure security principles. I mean, can you imagine an airport or corporate headquarters without areas of differing protection needs, different threat exposure or separate layers and means of access? [You can’t? So why do you think about virtualizing all your corporate computer systems on one big unified “corporate cloud”? ;-)]

Again, it should be noted that “traditional network segmentation” is only one variant. Using RFC 1918 (or ULA, for that matter) addresses in some parts of your network without NATing them at some point, or refraining from route distribution at some demarcation point constitute other examples.

In the VoIP world the main realization of the isolation principle is the commonly found approach of “voice vs. data VLAN[s]”.


Restriction (“once [as of the above principle] isolated parts get connected try to limit the interaction between those parts at the intersection point”)

This is the one most people think of when it comes to network security as this is what the most widely deployed network security control, that is firewalls, is supposed to do.

Two points should be noted here, from our perspective:

In some network security architecture documents phrases going like “the different segments are [to be] separated by firewalls” can be found. Which, well, is a misconception: usually a firewall connects networks (which would be isolated otherwise), it does not separate them. It may (try to) limit the traffic passing the intersection point but it still is a connection element.

And it should be noted that the restriction it applies (by filtering traffic) always has an operational price tag. Which is the one of the reasons why firewalls nowadays tend to fail so miserably when it comes to their actual security benefit…

In VoIP networks using the restriction approach is considerably hard (and hence quite often simply doesn’t happen) given a number of protocols’ volatility when it comes to the (UDP/TCP) ports they use.
Encryption (“while in transit encrypt some asset to protect it from threats on its [transit] way”)
This is a very common infrastructure security control as well (alas, at times the only one people think of) and probably does not need further explanation here.

Still it should be noted – again – that it has an operational price tag (key management and the like). Which – again – is the very reason why it sometimes fails so miserably when it comes to providing actual security…

In the VoIP world (as this one is very much about “assets in transit”) it’s (nowadays) a quite common one, even though still a number of environments refrains from using it, mainly due to the mentioned “operational price tag”.
Entity Protection (“take care of the security exposure of the individual elements within the environment containing the assets to be protected”)

This encompasses all measures intended to increase the security of individual elements. It’s not limited to simple hardening though, but includes all other “security [posture] quality assurance” things like pentesting or code reviews (when the element looked at is an application).

Adding a comment again I’d like to state that, in times of virtualization and vaporizing security layers (deploying shiny apps pretty much directly connecting customers to your ERP systems, by means of fancy webservices) this one might become more and more important. In the past many security architectures relied on layers of isolation & restriction and thereby skipped the hardening/quality assurance step (“we don’t have to harden this Solaris box as there’s a firewall in front of it”). As the talks’ case studies will show this one is a fundamental (and overlooked) one in many VoIP deployments.

Secure Management (“manage the [infrastructure] elements in a secure way”)

Secure management usually can be broken down to:

  • Restrict the endpoints allowed to establish management connections.
  • Either use a trusted environment (network link) or use secure variants of mgmt. protocols instead of their less secure counterparts (SSH vs. Telnet, HTTPS vs. HTTP, SNMPv3 vs. community-based SNMP and the like).
  • Require sufficient authentication (as for methods, authenticator [e.g. password] quality, personalized accounts etc.).
  • Logging of security related events and potentially all management actions performed.


While this is (should be) an obvious security principle, daily assessment experience shows that failures/weaknesses in this space account for the majority of critical vulnerabilities when it comes to infrastructure security. This applies in particular to VoIP implementations (see the case studies for examples).

Visibility (“be able to assess the current security posture of your infrastructure and its elements with reasonable effort”)

This is where logging (+ analysis), monitoring etc. come into play. We’d like to note that while this is a valid infrastructure security principle, its actual security benefit is often overestimated given the “detection/reaction” nature of this principle and its subsequent bad operational feasibility.

This is a particularly interesting (and neglected) one in many VoIP environments. Usually the data generated in this space (for VoIP) can not be easily processed (by $SIEM you acquired two years ago, for a six-figure € number and which still has only a handful of use cases defined…), while on the other hand being heavily useful (or even required for legal follow-up) in one of those numerous billing fraud incidents.
How to Apply those Principles in a Generic Way

As the above application to VoIP shows, these fundamental security principles allow for tackling any type of “securing assets within a complex overall setting” by going through a simple (checklist-type) set of questions derived from them. These questions could look like

  • Can we limit who’s taking part in some network, protocol, technology, communication act?
  • Any need to isolate stuff due to different protection need, (threat) exposure or trust(worthiness)?
  • What can be done, filtering-wise, on intersection points?
  • Where to apply encryption in an operationally reasonable way?
  • What about the security of the overall system’s main elements?
  • How to manage the infrastructure elements in a secure way?
  • How to provide visibility as for security-related stuff, with reasonable effort?
In a sequel to this post I might cover the mentioned case studies in more detail. In case I miss doing so, the slides will be available after the respective events ;-).
Have a great Sunday,

[1] As it requires the usually most scarce resource of an organization, that is humans and their brains. The part that can not be easily substituted by technology…

[1] In general preventative controls have a better cost/benefit ratio than detective or reactive ones. And this is still true in the “you’ll get owned anyway that’s why you should spend lots of resources on detective/reactive controls” marketing hype age…

[2] To provide another example from the routing protocol space: the “inter-operator trust and TCP-” based nature of BGP (as opposed to the “multicast and UDP-“based nature of other routing protocols) certainly is one of the most fundamental stability contributing properties of the current Internet.

[3] Another simple example here. If the two VoIP gateways in the incident described here had used a host route for each other instead of their default route (which wasn’t needed given their only function was to talk to each other), presumably the whole thing wouldn’t have happened.


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