Standard USB ports and cables have several electrical channels, some of which are used to transfer power, the others data. When you plug in you risk:
- Moving viruses between the computer and your device (in either direction)
- Your contacts and photos being copied to the computer. This can happen nefariously or, more commonly, because the computer helpfully performs a backup
A USB condom simply blocks the current on the data channels, meaning only the power channels make the connection between the device charging and the one being charged.
Simpler options are available, such as buying USB cables that only have the power parts installed, but they don’t offer anywhere near as good a headline 🙂
- ‘USB condom’ prevents juicejacking attacks (wired.co.uk)