In 2013, a hacker (or hackers) apparently obtained access to the Trump Organization’s domain registration account and created at least 250 website subdomains that cybersecurity experts refer to as “shadow” subdomains. Each one of these shadow Trump subdomains pointed to a Russian IP address, meaning that they were hosted at these Russian addresses. (Every website domain is associated with one or more IP addresses. These addresses allow the internet to find the server that hosts the website. Authentic Trump Organization domains point to IP addresses that are hosted in the United States or countries where the company operates.) The creation of these shadow subdomains within the Trump Organization network was visible in the publicly available records of the company’s domains.

[…]

The subdomains and their associated Russian IP addresses have repeatedly been linked to possible malware campaigns, having been flagged in well-known research databases as potentially associated with malware. The vast majority of the shadow subdomains remained active until this week, indicating that the Trump Organization had taken no steps to disable them. This suggests that the company for the past four years was unaware of the breach. Had the infiltration been caught by the Trump Organization, the firm should have immediately decommissioned the shadow subdomains, according to cybersecurity experts contacted by Mother Jones.