Test for network leaks, discover a product flaw and get vendor to fix

Mikko Kenttälä
Published in
4 min readDec 14, 2020


Plot twist: this time it is not about us doing vulnerability research and reporting. This is a story about our customer in action, told to us by their CISO with a promise to share it anonymously.

When there is a failure in network isolation — a leak — it gets blamed on bad design, faulty configuration or human error. It may feel like that the blame is on you. Sometimes that misses the mark. This time was different. The root cause was a network product behaving badly. For once they, the vendors, get the spotlight. But let’s not jump ahead, let’s hear what an anonymous CISO has to say.


“We have a critical mission and a mission critical environment. It is extremely important to us that our network isolation works as expected, always and in all cases.” — CISO

This was not empty talk. They put their money where their mouth is and joined a pilot campaign aimed to test mission critical, especially operational technology (OT), networks against isolation failures.

“We were confident that we had done everything right and there would be no leaks to be found.” — CISO

And they had indeed done everything right. Alas, sometimes things are out of your control and in your vendors’ hands. But you are not alone, the campaign that they engaged in was based on a product designed to help you by relentlessly running tests designed to find leaks, no matter what is the root cause.

The find and the fix

“Much to our surprise a leak vulnerability was found. It was in such rare circumstances and had such a short time window to exploit that we would never have found it by manual testing.” — CISO

A bit of background, we have done our share of security audits in the past 15 years and our methodology has had some quite unorthodox bits. One approach that I remember fondly is the torture phase in network equipment testing, if possible we like to power them on and off, sometimes few times in row, while observing for a behavioural change. More than once, something unexpected has happened when flipping the power switch on and off.

You can guess where this is going? We have made our Beacon product based on everything we have learned and researched. This time in the left corner were the tests codified in the SensorFu Beacon and in the right corner was a communication system used to secure supervisory, control and data acquisition systems (SCADA) and other OT environments. Who won?

“We worked with SensorFu to drill down to the exact circumstances and to isolate the problem in the affected product. This enabled us to describe the problem to the vendor of that product in detail.” — CISO

When the affected device was rebooting there was a brief moment when it didn’t work as intended, and the Beacon caught it. That device was used as a VPN tunneling solution to connect a remote part of infrastructure to the core network. When the root cause was determined and flaw reproduced our customer swiftly reported it to the vendor. Vendor promptly worked on the fix and mitigations, and published the details for the vulnerability. This vulnerability was assigned CVE-2020–24684.

You can think that this is like having Dr. Jekyll and Mr. Hyde in your network, all fine and dandy in the “uptime” and all cranky and outright dangerous at the “reboot time”.


“We think this was a significant find. We must be able to protect our networks and block any exfiltration potential.” — CISO

For full disclosure, we have a bias, we are the proud makers of the SensorFu Beacon. It is a product that detects new network leak paths from isolated networks and network segments. These leak paths can be a result of human error, vulnerability or malice, and they may violate your security policies or contractual obligations. ​Beacons continuously seek out new network leak paths by the power of active network-fu.

“We’ve been convinced by the value that SensorFu Beacon delivers to us and we will continue to boldly take it to new places where no Beacon has gone before.” — CISO

Thank you CISO, you folks did the heavy lifting. Thank you vendor, now with the vulnerability advisory the other users of the product are better off.

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