# Five Attacks on x402 Agentic Payment Protocol

arXiv: https://arxiv.org/abs/2605.11781 · HTML: https://mlsanity.com/paper/2605.11781/

## Key facts

- Cited by 4 papers in the map (data through 14 Oct 2026).
- First posted on arXiv 12 May 2026.

## Built on by (most cited first, 4 of 4)

- [Can Trustless Agents Be Trusted? An Empirical Study of the ERC-8004 Decentralized AI Agent Ecosystem](https://mlsanity.com/paper/2606.26028/) (2026)
- [A Formal Analysis of Agent Payment Protocols](https://mlsanity.com/paper/2609.00060/) (2026)
- [Your Agent Says Yes: Interpreting Adversarial Market Behavior Beyond Individual Transactions](https://mlsanity.com/paper/2609.07675/) (2026)
- [SoK: Decentralized Agent Economic Infrastructure](https://mlsanity.com/paper/2610.01756/) (2026)

## Abstract (arXiv)

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 in both design and implementation. We present five concrete attacks that reveal weaknesses in authorization, binding, replay protection, and web-layer handling, showing that x402 is vulnerable across multiple stages of the payment workflow. We validate these attacks through a reproducible testbed on local chains, Base Sepolia, and live endpoints and further audit three open-source SDKs and endpoints. Our results show that all five attacks are practical and can cause either unpaid service or paid-but-denied outcomes. We also propose practical mitigations.