On 14 January 2016, ExpressVPN was criticized by former Google information security engineer Marc Bevand for using weak encryption.[21] Bevand had discovered that only a 1024-bit RSA key was used to encrypt the service's connections after using it to test the strength of the Great Firewall of China.[21] Bevand described ExpressVPN as "one of the top three commercial VPN providers in China" and asserted that the Chinese government would be able to factor the RSA keys to potentially spy on users.

https://en.wikipedia.org/wiki/ExpressVPN#cite_note-bevand-criticism-21

On 14 January 2016, ExpressVPN was criticized by former Google information security engineer Marc Bevand for using weak encryption.[21] Bevand had discovered that only a 1024-bit RSA key was used to encrypt the service's connections after using it to test the strength of the Great Firewall of China.[21]

https://en.wikipedia.org/wiki/ExpressVPN#cite_note-bevand-criticism-21