Ukrainian cyber-security expert Dyma Budorin hasn’t slept much since he relocated his Kyiv-based company a fortnight before the Russian invasion. He saw the writing on the wall and hastily moved with most of his staff to Barcelona, where his wife anxiously checks on her parents in Mariupol. Bleary but resolute, he details plans hatched in a Spanish village to unleash mayhem upon Russian targets.

Aided by an underground band of volunteers – part of an ‘IT army’ galvanised by a direct appeal from the Ukrainian government – his cybersecurity company Hacken has managed to adapt a tool originally designed to stress-test company systems and protect against fraud. Volunteers rewrote code in record time to allow disBalancer to work across all platforms, beyond Windows.

“They did in three days what it would have taken six months to achieve,” says Budorin. “DisBalancer has become a powerful tool for Ukraine and it’s in operation now in Russia.”

It has been adopted...