#WannaCry and The Rise and Fall of the Firewall

The now infamous WannaCry Ransomware outbreak was the most widespread malware outbreak since the early 2000s. There was a very long gap between the early 2000s “worm” outbreaks (think Sasser, Blaster, etc) and this latest 2017 WannaCry outbreak. The usage of the phrase “worm” was itself widespread, especially as it was included in CISSP exam syllabuses, but then it died out. Now its seeing a resurgence, that started last weekend – but why? Why is the worm turning for the worm (I know – it’s bad – but it had to go in here somewhere)?

As far as WannaCry goes, there has been some interesting developments over the past few days – contrary to popular belief, it did not affect Windows XP, the most commonly affected was Windows 7, and according to some experts, the leading suspect in the case is the Lazarus group with ties to North Korea.

But this post is not about WannaCry. I’m going to say it: I used WannaCry to get attention (and with this statement i’m already more honest than the numerous others who jumped on the WannaCry bandwagon, including our beloved $VENDOR). But I have been meaning to cover the rise and fall of the firewall for some time now, and this instance of a widespread and damaging worm, that spreads by exploiting poor firewall configurations, brought this forward by a few months.

A worm is malware that “uses a computer network to spread itself, relying on security failures on the target computer”. If we think of malware delivery and propagation as two different things – lots of malware since 2004 used email (think Phishing) as a delivery mechanism but spread using an exploit once inside a private network. Worms use network propagation to both deliver and spread. And that is the key difference. WannaCry is without doubt a Worm. There is no evidence to suggest WannaCry was delivered on the back of successful Phishing attacks – as illustrated by the lack of WannaCry home user victims (who sit behind the protection of NAT’ing home routers). Most of the early WannaCry posts were covering Phishing, mostly because of the lack of belief that Server Message Block ports would never be exposed to the public Internet.

The Infosec sector is really only 20 years old in terms of the widespread adoption of security controls in larger organisations. So we have only just started to have a usable, relatable history in infosec. Firewalls are still, in 2017, the security control that delivers most value for investment, and they’ve been around since day one. But in the past 20 years I have seen firewall configurations go thru a spectacular rise in the early 2000s, to a spectacular fall a decade later.

Late 90s Firewall

If we’re talking late 90s, even with some regional APAC banks, you would see huge swaths of open ports in port scan results. Indeed, a firewall to many late 90s organisations was as in the image to the left.

However – you can ask a firewall what it is, even a “Next Gen” firewall, and it will answer “I’m a firewall, i make decisions on accepting or rejecting packets based on source and destination addresses and services”. Next Gen firewall vendors tout the ability of firewalls to do layer 7 DPI stuff such as IDS, WAF, etc, but from what I am hearing, many organisations don’t use these features for one reason or another. Firewalls are quite a simple control to understand and organisations got the whole firewall thing nailed quite early on in the game.

When we got to 2002 or so, you would scan a perimeter subnet and only see VPN and HTTP ports. Mind you, egress controls were still quite poor back then, and continue to be lacking to the present day, as is also the case with internal firewalls other than a DMZ (if there are any). 2002 was also the year when application security testing (OWASP type vulnerability testing) took off, and I doubt it would ever have evolved into a specialised area if organisations had not improved their firewalls. Ultimately organisations could improve their firewalls but they still had to expose web services to the planet. As Marcus Ranum said, when discussing the “ultimate firewall”, “You’ll notice there is a large hole sort of in the centre [of the ultimate firewall]. That represents TCP Port 80. Most firewalls have a big hole right about there, and so does mine.”

During testing engagements for the next decade, it was the case that perimeter firewalls would be well configured in the majority of cases. But then we entered an “interesting” period. It started for me around 2012. I was conducting a vulnerability scan of a major private infrastructure facility in the UK…and “what the…”! RDP and SMB vulnerabilities! So the target organisation served a vital function in national infrastructure and they expose databases, SMB, and terminal services ports to the planet. In case there’s any doubt – that’s bad. And since 2012, firewall configs have fallen by the wayside.

WannaCry is delivered and spreads using a SMB vulnerability, as did Blaster and Sasser all those years ago. If we look at Shodan results for Internet exposure of SMB we find 1.5 million cases. That’s a lot.

So how did we get here? Well there are no answers born out of questionnaires and research but i have my suspicions:

  • All the talk of “Next Generation” firewalls and layer 7 has led to organisations taking their eye off the ball when it comes to layer 3 and 4.
  • All the talk of magic $VENDOR snake oil silver bullets in general has led organisations away from the basics. Think APT-Buster ™.
  • All the talk of outsourcing has led some organisations, as Dr Anton Chuvakin said, to outsource thinking.
  • Talk of “distortion” of the perimeter (as in “in this age of mobile workforces, where is our perimeter now?”). Well the perimeter is still the perimeter – the clue is in the name. The difference is now there are several perimeters. But WannaCry has reminded us that the old perimeter is still…yes – a perimeter.
  • There are even some who advocated losing the firewall as a control, but one of the rare success stories for infosec was the subsequent flaming of such opinions. BTW when was that post published? Yes – it was 2012.

So general guidelines:

  • The Internet is an ugly place with lots of BOTs and humans with bad intentions, along with those who don’t intend to be bad but just are (I bet there are lots of private org firewall logs which show connection attempts of WannaCry from other organisations).
  • Block incoming for all ports other than those needed as a strict business requirement. Default-deny is the easiest way to achieve this.
  • Workstations and mobile devices can happily block all incoming connections in most cases.
  • Egress is important – also discussed very eloquently by Dave Piscitello. Its not all about ingress.
  • Other pitfalls with firewalls involve poor usage of NAT and those pesky network dudes who like to bypass inner DMZ firewalls with dual homing.
  • Watch out for connections from any internal subnet from which human-used devices derive to critical infrastructure such as databases. Those can be blocked in most cases.
  • Don’t focus on WannaCry. Don’t focus on Ransomware. Don’t focus on malware. Focus on Vulnerability Management.

So then perimeter firewall configurations, it seems, go through the same cycles that economies and seasonal temperature variations go through. When will the winter pass for firewall configurations?

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