I've been asked a few times how I would do perimeter security testing. Personally, I'm not an offensive security guy, more a defensive one, meaning I'm more about security-related defensive methods rather than PEN testing of any kind. But still, even in a defensive position, having a "view" on how to do security testing is important. For me, I would use the following testing categorisation to look at IT architectures and see how they would react against certain attacks. I'm calling this one about perimeter testing as I am interested here in remote attacks (or differentiation), not local ones (which requires, in my opinion, a different way of looking at things).

  • Eggs and a basket
  • Overhead testing
  • Protocol insecurity or misuse
  • Application insecurity or misuse
  • Client insecurity
  • Correlation

Eggs and a basket

First of all, don't put all your eggs in the same basket. I would never trust myself enough to say things are secure. Always see if you can't benefit from other people's knowledge (or even other companies knowledge). If you are doing testing to choose a specific security-related technology, use analysis made by independent analysis firms or organizations to further steer your choice. But make sure that the organization is truely independent and doesn't give "reports" that are heavily in favor of whomever asked for them.

Overhead testing

Most technologies you use to counter certain threats will incur some overhead. This is true for application firewalls, network firewalls, isolation technologies, confidentiality technologies, access controls and more. You should set yourself a baseline of what you consider too much overhead and what not.

Overhead comes in many layers, so it is important to be able to perform load testing based on real loads, not fake lab-specific situations. Running one thousand clients with the same client certificate, same hosts, same reaction times against one SSL resource has an entirely different performance profile than running one thousand clients with different certificates, using different encryption libraries (other ciphers and such) and different speeds/reaction times (including things like SSL handshake timings). And that's just one example.

I always find it very important to be able to run load testing regularly. I would even go as far as recommend organizations to run load testing as a "business as usual" test, or at least allow your technology-inspired teams to easily request such loads against their new applications or technologies.

But enough of that. Let's talk about attack methods (or categorisation).

I tend to look first towards protocol insecurity, then application insecurity and finally client-level insecurity. Protocol insecurity is primarily about knowing how the protocol works (or should work) and finding ways to attack that. Some protocols are inherently insecure, and introducing proper protection against these is extremely important as the technology that implements the protocol might not be able to do that itself. Then I look at application-specific insecurity, which is more about knowing the application (vendor/product). And finally it is about client insecurity (such as browser-based attacks, ActiveX component attacks, and more).

In each of these cases, I consider the following attack methods:

  • Denial-of-Service - what could be done to disable the protocol or service behind it completely or partially
  • Out-of-order Execution - can the protocol or application be tricked into executing tasks when it isn't meant to, which most of the time leads to either information leakage or the next attack method
  • Privilege escalation - to get more rights/privileges (or switch from unauthenticated to authenticated access)
  • Remote command execution - executing whatever the attacker wants on the remote system
  • Application switching/routing - updating the behavior of the application to become a service that can be used to further expose/explore the remote servers' environment

Protocol insecurity or misuse

Many protocols are inherently insecure. Good security solutions will need to detect if a protocol is being used in a way that does not match the behavior expected. And this goes beyond the standard TCP/IP protocols and the application-level HTTP protocol. Consider SMTP and VoIP-related protocols as well as a nice example.

Denial of service attacks against TCP/IP are widely documented. Be it the well-known SYN flooding, a low-rate tcp-targeted DoS or messing with the TCP stack itself (like with the Microsoft Windows TCP/IP Stack Vulnerabilities), these attacks can be easily evaluated against your architecture.

With TCP/IP, I would generally also look at how the stacks present their information. Can an attacker use TCP sequence prediction attacks? Can he get information on when is the most feasible period to launch an attack (for instance from a reasonably stable TCP window size value reading)? And how about TCP session hijacking?

Or if we look at HTTP, can attacks such as Slowloris or an HTTP POST DOS attack bring down the service? And what if a user comes to a certain page after an obscure redirection, where the attacker hopes that the user authenticates against? Perhaps an attacker might hijack an HTTP session, or force a user to use a non-secure connection.

E-mail services too are particularly interesting to look at. Does it expose information (settings, or account identification)? Does it accept large time-outs (giving attackers time to just "play" with the service using netcat/telnet)?

And in case of VoIP, have you checked common voip-based attacks lately? VoIP is (imo) a complex set of protocols and whomever implements it has to follow strict rules. I would be very surprised if this can't be heavily influenced.

Application insecurity or misuse

Of course, protocols are implemented by applications, and applications have their own set of problems. And if you're running software that isn't properly configured or up to date, you'll definitely need to take a good read at my blog posting series on mitigating risks.

Consider Citrix for instance: a commonly found remote management toolsuite (well yeah, Citrix offers a lot more, I'm not going to delve into that right now). It has seen its share of vulnerabilities in the past, like DoS vulnerabilities, directory traversal or open proxy, command execution and more. And Citrix is far from an insecure platform.

Just like with all other applications, it is extremely important to have a good view / knowledge of each product you expose. Some applications can even mimic other protocols (like Nginx handling HTTP, IMAP, POP3, SMTP, WebDAV, ... which, if exploited by an attacker, can provide a new fall-out base to work from.

Client insecurity

Finally, the last thing to consider is most likely the one most difficult to manage: client insecurity. Especially with internet-facing services, it is very hard to protect yourself from client systems that are not properly protected. How to deal with user authentication if the user could have a keystroke logger running in the background? A browser is a commonly used application for service access, but what about things like a Citrix client (especially if local drive mapping is enabled)?


A good security system is integrated with the various security technologies in place. An attacked that did discovery or even tried out a few other attacks before should already alert most of your security components, possibly even invoking a temporary countermeasure against the users' location. It is not sufficient to block the IP address on the webserver when the attacker tried an HTTP-based attack only to have him try his luck on the next service that you expose...

Now for each "category" I tend to look at the attack from a "hit and run" aspect (exploitable with a single attack or burst), "build up" (most of the slower attacks tend to be like this) or "evaded" (trying to work around detection of the previous ones), and this for a single host, a relayed host or distributed. All these factors combined give me enough things to consider while evaluating an architecture (or security technology implementation) for remote attacks.


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