Password managers sometimes make claims they cannot keep. The term “zero knowldege encryption” is sometimes used to convey the idea that the server, used to store end user vaults for cloud based password managers is unable to learn anything about the contents of those vaults. ETH Zurich examined these claims and found vulnerabilities in a number of commonly used password managers: Bitwarden, Dashlane and LastPass.

The original paper: Zero Knowledge (About) Encryption: A Comparative Security Analysis of Three Cloud-based Password Managers and the Ars Technica Coverage