  The introduction of the WebRTC API to modern browsers has brought about a new
threat to user privacy. This API causes a range of client IP addresses to
become available to a visited website via JavaScript even if a VPN is in use.
This a potentially serious problem for users utilizing VPN services for
anonymity. In order to better understand the magnitude of this issue, we tested
widely used browsers and VPN services to discover which client IP addresses can
be revealed and in what circumstances. In most cases, at least one of the
client addresses is leaked. The number and type of leaked IP addresses are
affected by the choices of browser and VPN service, meaning that
privacy-sensitive users should choose their browser and their VPN provider with
care. We conclude by proposing countermeasures which can be used to help
mitigate this issue.
