At risk are photographs, home videos and music collections as well as scans of documents such as passports, tax forms and other sources of personal data. In some cases, back-up files are being made available that, if downloaded and restored, could let attackers take over a victim's online life.
Security firms suggest that attackers have already found out about this easy-to-access source of saleable data and are starting to actively seek it out and share it.
Those at risk are people who use home data storage devices known as Network Attached Storage (NAS). Correctly configured, these devices act as a common data store accessible by any other device connecting to that home network.
However, many people have set them up incorrectly and have accidentally made this data accessible not just to their home network but to the internet at large. Visiting this data is as easy as visiting any other webpage.
To find out how many people are accidentally sharing their data online, the BBC turned to the Shodan search engine. While Google, Bing and others seek out data on the net, Shodan looks for devices.
In the past, security researchers have used Shodan to expose insecure and poorly protected computers controlling industrial plants, power plants, heating and ventilation systems and CCTV streams.
A search via Shodan turned up tens of thousands of NAS systems in UK homes.
Working out which ones of these are sharing personal data is difficult because British computer misuse laws do not allow the BBC to visit them to see which are happy to share data with anyone.
Montash is a multi-award winning, global technology recruitment firm. Specialising in permanent and contract positions across mid-senior appointments which cover a wide range of industry sectors and IT functions, including:
ERP, BI & Data, Information Security, IT Architecture & Strategy, Energy Technologies, Demand IT and Business Engagement, Digital and E-commerce, Infrastructure and Service Delivery, Project and Programme Delivery.
With offices based in London, Montash has completed assignments in over 30 countries and has appointed technical professionals from board level to senior and mid-management in permanent and contract roles.