Paper 2025/1278 Original: eprint.iacr.org/2025/1278 ↗

On the Security Risks of Covert Password Manager Providers

Abstract

Password managers have become widely adopted and are commonly recommended to improve user security. However, current cloud-based solutions rely on the implicit assumption that password manager providers are fully trusted. This assumption is never questioned, as password managers are typically operated by their own designers, who are therefore judge and jury. Users are thus exposed to significant risks: a malicious provider could perform covert actions to access or alter credentials without being detected. Most password managers rely solely on the strength of a user-chosen master password. This design enables a covert adversary to mount large-scale offline attacks to crack weak master passwords. Even more concerning, some popular password managers do not encrypt credentials on users' devices, transmitting them in plaintext prior to server-side encryption, as is the case with Google default configuration. On the other hand, key-protected password managers such as KeePassXC offer stronger protection but lack native support for multi-device synchronization, limiting their adoption. Motivated by these observations, we define a comprehensive set of security properties that any cloud-based password manager should satisfy. We demonstrate that none of the widely deployed mainstream solutions fulfill these fundamental requirements. Nevertheless, we argue that it is feasible to design a password manager that is resilient to covert adversaries while supporting secure cross-device synchronization. To support our claims, we propose a provably secure design that meets all identified properties.