Security firm V12 said on 22 October 2026 that it found 25 vulnerabilities in the x402 codebase, the open standard that lets software such as AI agents pay for an API call inside an ordinary HTTP exchange (a server answers 402 Payment Required, the client signs a payment and retries). V12 described three it rated critical. First, the documented default client setup enforced no per-payment limit or total budget, so, according to V12, a hostile API could turn a routine request into a request for the wallet's full balance of a compatible token, and with an automatic signer and no application-supplied policy the client could sign it.

Second, the TypeScript Solana facilitators did not check the instruction type, so a token approval, which moves no tokens, was accepted as a payment in V12's local test; V12 called it "a complete payment bypass" and says the test did not submit a live transaction. Third, the Sign-In-With-X client signed login messages without checking that the website named in them was the one it had contacted, which V12 says could let a malicious server log in to another site as the agent's wallet.

V12 says it reported the 3 critical and 13 high-severity findings to the x402 team on 12 August 2026 and that the 3 critical flaws were fixed by 20 August. The x402 Foundation's TypeScript changelogs for 2.23.0 list the matching changes: client spendControls that by default allow only recognised pegged assets with a $1 USD cap, a required origin check before signing Sign-In-With-X messages, and a validation guard for TransferChecked in the Solana package.

According to The Hacker News, neither V12 nor the x402 Foundation has publicly reported attacks using the flaws, the foundation has published no security advisory for them, and npm audit and PyPI listed no advisories for the last releases before the fixes as of 23 October.