Researchers have identified a hidden access mechanism, or "back door," in more than 20 router models made by ZBT Link. According to cybersecurity experts, the devices regularly contacted a server in China and could receive remote instructions, potentially allowing full system compromise without direct access. There is no public evidence the feature was used against customers. Experts disagree on whether it was intentional, a maintenance feature left in production or the result of a supply chain attack. ZBT Link says it was designed for after-sales maintenance and is working on a solution.