Incident Analysis: Anatomy of a Network Outage Caused by Perimeter Filtering

Incident Analysis: Anatomy of a Network Outage Caused by Perimeter Filtering

There is a fine line between a robust cybersecurity posture and excessive heuristic filtering. This threshold is frequently crossed in perimeter firewalls when uncontextualized "web reputation" heuristics and blind IP blocking collide with distributed CDN architectures.

This document details the post-mortem analysis of a real infrastructure incident in which aggressive perimeter filtering inadvertently disconnected critical internal corporate services.

The Genesis of the Disaster: "This IP has an ugly face"

It all starts with a phone call or a ticket early in the morning. "The internal Nextcloud is no longer working." "Strange, the servers are up, RAM is at 20%, the disks are breathing. Does it work from the outside?" "Yes, it opens from mobile. From inside the office it times out."

The analyst on duty, armed with curl -vL and saintly patience, runs a couple of checks and uncovers the mystery. The domain resolves to an IP address belonging to a well-known and gigantic global CDN (let's call it, at random, StormCloud). And that specific IP, from inside the corporate network, responds only with a desolate silence. Packets go out, but they don't come back.

Why? Simple: the perimeter firewall's "threat intelligence" system detected that on that IP, three days earlier, there was a Korean website distributing malware. And so, with the speed and acumen of a bull in a china shop, it blacklisted the IP.

Too bad that IP belongs to a CDN that handles about 20% of global web traffic.

The CDN Paradox Explained to the Stubborn

At this point, the theological debate opens with the perimeter administrators. "We blocked the IP because it has a bad reputation", they say triumphantly from the heights of their policies.

Now, try explaining to these people that CDNs work with shared and dynamic IPs. It's like blocking the entire highway because a stolen car passed by ten minutes ago.

• If you block a CDN IP, you are not blocking the hacker. You are also blocking your own services hosted behind that CDN.
• You are blocking the banking portals your employees need to use.
• You are blocking technical documentation pages.
• And above all, tomorrow the CDN will assign another IP from its immense pool to that service, and the block will become useless for security but will remain very useful for breaking things.

The standard response to this objection is usually: "Well, if we lift the block, then the hacker will break in. Security comes first".

Security. That is, the art of making systems so unusable that even legitimate users cannot work on them. The wet dream of every lazy sysadmin: a server turned off and unplugged from the network. Now that's a 100% security level.

Do-It-Yourself Delisting and Political Provocation

When the discussion hits these levels of surrealism, frustration can drive one to extreme responses.

Some suggest that you, the service manager, should go to the CDN to request the "delisting" of the IP. Sure, you call the support of a multi-billion dollar giant to tell them: "Listen, sorry, my local firewall blocked one of your random IPs, could you change it?". They laugh in your face in fifteen different languages.

And this is where the desperate move kicks in: the political provocation.

If the network team tells you that the only way to be secure is to lock down everything to the point of a blackout, you answer them with a smile: "You know what? You're right. Let's move all our security services out of your datacenter. We'll buy a nice twenty-euro VPS from a German provider and manage everything ourselves from there".

The reaction is immediate. The mere idea of the security team deciding to flee to the outside because it doesn't trust the internal infrastructure is a devastating political torpedo. It is the collapse of the credibility of the entire IT department.

In a few minutes, magically, the firewall rules are revised, the IP blocks demoted to intelligent monitoring, and the service starts working again.

We didn't win with logic, nor with technology. We won by using management's paranoia against the sysadmins' laziness.

[May the Talpone be with you]

Discussion & Support

Discussion and support channel. To discuss configurations or other topics, join us on fluxer.dreadful.work.