BRUGES, Belgium — Hackers who cranked up fridge temperatures at supermarkets across Europe may have gained access through public internet-connected systems protected by default passwords or outdated software, a cybersecurity expert said. The second cyberattack in three weeks hit fridge systems at supermarket chains including Carrefour and Delhaize in Belgium, and ICA and Coop in Sweden, forcing retailers to discard spoiled food and raising concerns about food security. Three blood banks in France were also hit. Authorities have not publicly disclosed how the attackers gained access, and no perpetrator has been identified. Laurens Singier, an ethical hacker and researcher at Cyber 3 Lab, part of Howest University of Applied Sciences in Bruges, Belgium, said internet-connected systems which can be accessible remotely often have weak protections. "In this case, the weak point was mainly putting those devices on the public internet. They shouldn't even be there," he said, speaking from a lab full of servers and systems similar to those that could control supermarket store fridges. A common