Build and HMAC-SHA256 sign a MoonPay sell widget URL (sell.moonpay.com). Returns a ready-to-redirect URL with apiKey + signature appended. Requires MOONPAY_PUBLISHABLE_KEY and MOONPAY_SECRET_KEY in the environment.
AI agents invoke sign_sell_url to trigger actions in Mcp Afip. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.
Although nominally operating within a tax authority (AFIP) context, this tool executes cryptographic signing logic and constructs authenticated URLs that facilitate redirection to external financial services. This constitutes an Execute-category action because it triggers external operations (payment widget initialization via MoonPay) whose effects depend on the URL parameters and signature generated.
From the tool's definition The tool 'sign_sell_url' builds and HMAC-SHA256 signs a MoonPay sell widget URL, returning a ready-to-redirect URL.
Documented attack patterns abuse exactly the kind of access sign_sell_url gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Mcp Afip, and nothing reaches the server without passing your rules. This is the rule we recommend for sign_sell_url:
{
"version": "1",
"default": "deny",
"tools": {
"sign_sell_url": {
"limits": [
{
"counter": "sign_sell_url_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}sign_sell_url stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
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Build and HMAC-SHA256 sign a MoonPay sell widget URL (sell.moonpay.com). Returns a ready-to-redirect URL with apiKey + signature appended. Requires MOONPAY_PUBLISHABLE_KEY and MOONPAY_SECRET_KEY in the environment. It is categorised as a Execute tool in the Mcp Afip MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Mcp Afip MCP server in PolicyLayer and add a rule for sign_sell_url: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Mcp Afip. Nothing to install.
sign_sell_url is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the sign_sell_url rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for sign_sell_url. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
sign_sell_url is provided by the Mcp Afip MCP server (codespar/mcp-dev-latam). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Mcp Afip, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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