---
id: "CVE-2026-113603"
kind: "vulnerability"
aliases: ["CVE-2026-113603"]
title: "x402 SIWX extension lets paid clients unlock unpaid routes"
severity: "medium"
cvss: 6.9
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["go", "node", "package", "python", "self-hosted", "x402"]
not_affected_if: ["no-siwx-extension", "single-price-route"]
published_at: "2026-10-20T16:17:10+00:00"
first_seen_at: "2026-10-23T21:57:20+00:00"
updated_at: "2026-10-25T03:40:29+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-113603", "first_seen_at": "2026-10-23T21:57:20+00:00"}]
---

# x402 SIWX extension lets paid clients unlock unpaid routes

The x402 SIWX server extension in the npm, PyPI and Go packages does not check that a payment matches the route it unlocks. A paying client can pay for a cheap route, point the payment's resource URL at an expensive route, and then reuse a sign-in header to access the expensive route without paying.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed version of `@x402/extensions` in package.json or the lockfile, and of `x402` in the Python environment (`pip show x402`), and the Go module version in go.mod.
- Look for SIWX (SIGN-IN-WITH-X) route protection in your x402 server configuration.

## Fix

- Upgrade `@x402/extensions` past `2.28.0` (fixed version unknown from the sources).
- Upgrade the PyPI `x402` package past `2.25.0` and the Go module past `2.28.0` (fixed versions unknown from the sources).

## Timeline

- 2026-10-23T21:57:20+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-113603

## From the sources

The x402 SIWX server extension in @x402/extensions 2.3.1 through 2.28.0, x402 PyPI 2.11.0 through 2.25.0, and Go module 2.16.0 through 2.28.0 contains an authorization bypass vulnerability that allows paying clients to unlock unpaid routes by supplying a mismatched PaymentPayload.resource URL. Attackers can pay for the cheapest SIWX-protected route while setting resource.url to an expensive route, then repeatedly access it using only a SIGN-IN-WITH-X header without further payment.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_