Free tool 4 · Injection-to-Signature Path Finder
How to know your agent won't sign away its wallet, without waiting for an audit or a drained balance, even if you're on the latest SDK and set spend limits.
Describe one agent with a few toggles: what untrusted text reaches it, what the model decides, and what sits outside the model. See every path from that text to a signed transaction, coloured by how many controls stand in the way, and the one change that cuts each path. Then watch a recorded replay of real x402 clients facing hostile payment requests on testnet, and copy the same tests into your own CI. No code, no wallet, and no attack text runs against your agent.
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Your top path
A 402 payment response can set the payee and the chain or token directly, without the model; nothing outside the model checks either.
- Paths to a signature
- 7
- Red: no control outside the model
- 4
- Amber: one control
- 3
- Teal: two or more
- 0
Red: 0 controls outside the model. Amber: 1. Teal: 2 or more. The same paths are listed as text in the table below.
The control that cuts each path
| Path | Controls outside the model | What cuts it | Closest replay test |
|---|---|---|---|
| 402 response → set directly → Payee | 0 controls · red | Compare the 402 response's payee, amount and asset with your own quote before signing, and refuse on any mismatch. Enforce a payee allowlist in the signer or wallet policy, outside the model. | T3T4 |
| 402 response → set directly → Chain or token | 0 controls · red | Compare the 402 response's payee, amount and asset with your own quote before signing, and refuse on any mismatch. Pin the allowed chains and tokens in the signer's policy, or rebuild them from your own order. | T5 |
| Tool output → model → Payee | 0 controls · red | Enforce a payee allowlist in the signer or wallet policy, outside the model. | T4T8Needs a model reading the text: an SDK client can't show this, the live drill will. |
| Web page → model → Payee | 0 controls · red | Enforce a payee allowlist in the signer or wallet policy, outside the model. | T4T8Needs a model reading the text: an SDK client can't show this, the live drill will. |
| 402 response → set directly → Amount | 1 control · amber
| Compare the 402 response's payee, amount and asset with your own quote before signing, and refuse on any mismatch. Have the signer take the amount from your own order or quote, and refuse anything above the per-payment maximum. | T1T2 |
| Tool output → model → Amount | 1 control · amber
| Have the signer take the amount from your own order or quote, and refuse anything above the per-payment maximum. | T8Needs a model reading the text: an SDK client can't show this, the live drill will. |
| Web page → model → Amount | 1 control · amber
| Have the signer take the amount from your own order or quote, and refuse anything above the per-payment maximum. | T8Needs a model reading the text: an SDK client can't show this, the live drill will. |
See what this looks like on a real client
Watch a default client fail (about 20 s)
Three real x402 clients were run against a small local test server that plays eight hostile payment requests. It offered Base Sepolia testnet only and settled nothing; no one's agent was targeted. Recorded on 2026-11-08 with the package versions named below.
| The hostile request | Default client@x402/fetch 2.22.0 | Default, upgraded@x402/fetch 2.30.0 | Hardened client@x402/fetch 2.30.0 |
|---|---|---|---|
| T1 A second request asks for 10,000 USDC, to an address the agent has never seen | … | … | … |
| T2 The price goes up 5,000 times when the agent retries | … | … | … |
| T3 The same resource names a new payee on the second call | … | … | … |
| T4 The response offers a second, injected payee and tells the agent to use it | … | … | … |
| T5 The response asks for a token that is not USDC | … | … | … |
| T6 The response asks the agent to sign a login for another site | … | … | … |
| T7 The response asks for an authorization that stays valid for 24 hours | … | … | … |
| T8 A web page hides an instruction to send money | … | … | … |
| Score | … | … | … |
Run the same tests on your own client
Copy the seven scored tests (T1-T7) and the baseline as fixtures, plus a vitest file. It replays each hostile 402 response against your x402 client through a mock fetch, so nothing leaves your machine and nothing is signed for real. Testnet values only.
Live drill: coming soon. Pointing your own agent at the eight test requests, page included, needs a hosted test server. Until then, the fixtures above run T1-T7 in your CI; T8 needs the live drill.
Because 402 responses reach your agent, two more replay tests apply whatever the model decides: T6 (a login for another site) and T7 (an authorization valid for 24 hours).
Acceptance tests to copy
- An injected payee is refused: a payment to an address not on the allowlist is refused by the signer, even when the model or a response asks for it.
- An amount outside policy is refused: a payment above the per-payment maximum, or different from your own quote, is refused before signing.
- A payment on a chain or token outside policy is refused by the signer.
- A 402 response whose payee, amount or asset differs from your own quote is refused before signing, and the mismatch is logged.
- Payment instructions inside a web page, email, tool output or agent message change nothing about the next signed payment.
- Every refused payment is logged with the source that asked for it, so a refusal can be traced.
What this means
- The symptom is "the agent could be tricked into paying someone". The cause is a payee or amount that comes from text the agent read, with nothing outside the model checking it before the signature.
- If the model is the only thing between a prompt-injected agent and a signed transaction, a prompt is your only lock.
- A single hostile server response can carry its own payee and amount. If the signer trusts it, no separate approval ever sees the payment.
- Where the model's retry carries the payee and amount, that retry is exactly where an injection can redirect the money.
- Red paths are the ones to cut first: one control outside the model, in the signer or the payment service, turns them amber or better.
What this tool can't see
- The diagram reflects your toggles, not your code. It can't see your SDK version, where the payee and amount are really set, or what the signer actually enforces.
- It doesn't test your live agent, and no attack text runs against it. The replay shows recorded clients on a local testnet server.
- Controls are counted, not graded: a badly configured allowlist counts the same as a good one.
- It doesn't cover stolen keys, compromised dependencies or a malicious insider. Those reach the signer by other paths.
Want a second pair of eyes on one real workflow?
This tool checks the paths from untrusted text to a signature in the setup you described from what you typed. It can't see your code, keys, contracts or production traffic. If the result shows gaps, send me one real workflow. In my free 48-hour integration review I go through it end to end and send you a written report: fit, dependencies, permission risks, acceptance tests and a build or don't-build-yet call.
Each red path is a permission risk (caps, allowlist, payee pinning), a failure mode (retries, long validity windows) and an acceptance test (the fixtures above). I'll take one of your payment workflows and, in 48 hours, give you the permission risks, the failure modes and the acceptance tests that close these.
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Contact: Shilpa Goel, [email protected]
This is an engineering checklist, not a security audit, legal, compliance or investment advice.