x402 security audit

Grade any x402 seller's payment-security posture from the outside - a read-only black-box check mapped to the 'Five Attacks on x402' failure modes. Send GET /api/x402-audit with the required field url and pay $0.01 per call over x402 or MPP (there is no free tier). It returns a JSON object with url, reachable, status, paymentRequired, x402Version and 4 more.

Probes the URL's 402 challenge (never pays) and scores TLS transport, gated-response cache hygiene (Attack III / cache leakage), error/info-leak hygiene, and payment-terms well-formedness, then returns a letter grade with per-check findings and an honest note on what only insider/active testing can confirm. ?url=https://api.example.com/paid&method=GET

Category: Payments & x402 · Tags: x402 audit security posture cache tls grade five-attacks

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Parameters

Example request

curl -i "https://agent402.tools/api/x402-audit?url=https%3A%2F%2Fagent402.tools%2Fapi%2Fhash&method=POST"

Without payment this returns HTTP 402 Payment Required with the exact price for x402-audit; any x402 v2 or MPP client pays it and retries.

Example response

{
  "url": "https://api.example.com/paid",
  "reachable": true,
  "status": 402,
  "paymentRequired": true,
  "x402Version": 2,
  "score": 92,
  "grade": "A",
  "checks": [
    {
      "id": "transport-tls",
      "title": "Payment challenge served over TLS",
      "attack": "credential interception",
      "severity": "high",
      "status": "pass",
      "detail": "https"
    },
    {
      "id": "cache-hygiene",
      "title": "Gated response is not shared-cacheable",
      "attack": "III - cache leakage",
      "severity": "high",
      "status": "pass",
      "detail": "Cache-Control: no-store, private"
    }
  ],
  "summary": "A (92/100) - 6 passed, 1 warning, 0 failed. Note: replay/idempotency (II) and router Sybil (IV) can't be graded from outside."
}

From an MCP client

catalog.call {
  "slug": "x402-audit",
  "params": {
    "url": "https://agent402.tools/api/hash",
    "method": "POST"
  }
}

The hosted connector at https://agent402.tools/mcp needs a payment for x402-audit; the stdio package pays it from a wallet or from AGENT402_CREDITS_KEY. Local install: npx -y agent402-mcp.

Errors and behavior

• url is required. An input the tool rejects returns an HTTP 4xx whose body carries error, tool, expected, required and example, so the caller can correct it.
• A paid call that ends in any status of 400 or above is not charged: settlement is cancelled when the tool fails.
• Wallet-only: this tool reaches the network or stored state, so it has no proof-of-work tier. A prepaid card-credits key (Authorization: Bearer a402_...) also pays it.
• A POST with a JSON body to /api/x402-audit is served as this GET, with the body as the input.
• An Idempotency-Key header makes a retried paid call replay the first 200 instead of charging again.

Paid call (JavaScript agent)

import { wrapFetchWithPayment } from "@x402/fetch";
import { x402Client } from "@x402/core/client";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const client = new x402Client();
client.setSpendControls?.(false); // keep your own spending ceiling in code
registerExactEvmScheme(client, { signer: privateKeyToAccount(KEY) });
const payFetch = wrapFetchWithPayment(fetch, client);

const res = await payFetch("https://agent402.tools/api/x402-audit?url=https%3A%2F%2Fagent402.tools%2Fapi%2Fhash&method=POST");

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