The company said in an advisory that it was revealing details of the flaw even though Apple had not yet released a fix for it because it was important to bring it to the notice of both the ProtonVPN community and other VPN providers and their users.
The normal procedure is to notify a vendor of a bug and then wait for 90 days before revealing details.
Explaining the flaw, ProtonVPN said the typical behaviour when one connected to a VPN was that the operating system would close all existing Internet connections and then set them up again through the VPN tunnel.
While most connections did not last very long and would be re-established through the VPN tunnel, some lasted for a long time and could stay open outside the VPN tunnel for minutes to hours.
The advisory said this vulnerability also existed in iOS version 13.4.
The flaw could result in user data being exposed if the affected connections were not encrypted. But a more common problem was IP leaks, where an attacker was able to view a user's IP address and the IP address of the server to which a connection was being made.
Apart from this, the server to which the user made a connection would be able to view the IP of the user rather than that of the VPN.
The advisory said that no VPN service could provide a fix as iOS did not allow a VPN to take down existing network connections.
One workaround that was listed was to turn on airplane mode after connecting to a ProtonVPN server, which would result in all connections being killed and temporarily disconnect the VPN.
After this, if airplane mode was turned off, ProtonVPN would reconnect and other connections within the VPN tunnel would also reconnect.
Apple suggested using Always-on VPN to mitigate the issue but this does not fix the issue for third-party applications.