x402 SDK Default Client Exposed AI Agent Wallets to Full-Balance Drains via Unsigned 402 Responses

V12's code review and local tests identified the missing spend limit plus two other critical issues: the TypeScript facilitator accepting Solana token approvals as transfers without checking the instruction byte, and a login message signature that could be replayed on a different domain. These affected only agents using automatic signing with no app-level payment policy. The x402 Foundation, seeded by Coinbase and including Visa and Mastercard on its steering committee, shipped fixes without security advisories or CVE entries, leaving npm audit and PyPI silent on the pre-fix releases.

The pattern matches earlier payment-protocol failures where default behavior favored convenience over explicit authorization, such as early ERC-20 approval exploits and Solana transaction-injection cases documented in 2024-2025 incident reports. Automatic agent payments amplify the blast radius because a single 402 response can now trigger on-chain movement without human review, and the absence of public advisories reduces upgrade pressure on downstream developers.

Post-fix defaults now enforce $1 caps and recognized stablecoins only, yet the foundation's changelog still lists changes without security context. Developers relying on registry scans received no alerts as of October 23.

Next steps include independent audits of the remaining 22 V12 findings and monitoring for on-chain drains tied to x402 traffic after the August cutoff.