x402 Facilitator
The service in the x402 protocol that verifies a client's signed payment against the server's payment terms and settles it on chain, often paying the gas on the client's behalf.
In x402, the resource server does not usually talk to the blockchain itself. It forwards the client's payment to a facilitator, which exposes two operations: verify (is the signature valid, is the amount, recipient, token and validity window right, has the authorization been used) and settle (submit the transfer on chain, for example an EIP-3009 transferWithAuthorization).
Two security properties follow. First, verify alone does not consume the payment: until settlement, the same authorization still passes verification, so a server that serves after verify and settles later is exposed to a replay attack. Second, a facilitator that sponsors gas pays for every settlement it submits, including ones designed to revert, so it should simulate before settling, cap gas per request and rate limit per payer.
A 2026 study of 15 facilitators, together behind almost every x402 payment, found that every one broke at least one of eight security rules; the issues were disclosed and fixed.
Related Terms
x402
An open payment protocol that uses the HTTP 402 Payment Required status code so clients, including AI agents, can pay for web resources with signed stablecoin authorizations.
EIP-3009
An Ethereum token standard, implemented by USDC, whose transferWithAuthorization function moves tokens on the strength of a signed message from the holder, with a validity window and a single-use nonce.
Replay Attack
An attack where a valid transaction or message is maliciously resubmitted to execute the same action multiple times.
Gas
A unit measuring computational effort required to execute operations on Ethereum, paid by users to incentivize validators.
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