Not in The Prophecies: Practical Attacks on Nostr

Distributed social networking services (SNSs) recently received significant attention as an alternative to traditional, centralized SNSs, which have inherent limitations on user privacy and freedom. We provide the first in-depth security analysis of Nostr, an open-source, distributed SNS protocol developed in 2019 with more than 1.1 million registered users. We investigate the specification of Nostr and the client implementations and present a number of practical attacks allowing forgeries on various objects, such as encrypted direct messages (DMs), by a malicious user or a malicious server. Even more, we show a confidentiality attack against encrypted DMs by a malicious user exploiting a flaw in the link preview mechanism and the CBC malleability. Our attacks are due to cryptographic flaws in the protocol specification and client implementation, some of which in combination elevate the forgery attack to a violation of confidentiality. We verify the practicality of our attacks via Proof-of-Concept implementations and discuss how to mitigate them.