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oauthex: oauth discovery checks (#1220)

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add extra checks for the default oauth discovery client:
* disallow redirects which downgrade to http
* disallow redirects to localhost or private ip
* disallow the initial call to a private ip
* proxy configuration opts out of dial time checking
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Yaroslav 2026-09-02 21:07:19 +02:00 • committed by GitHub
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docs/protocol.md
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@ -639,24 +639,24 @@ provided to
### Server-Side Request Forgery
The [mitigations](https://modelcontextprotocol.io/docs/tutorials/security/security_best_practices#mitigation-3) are as follows:
The OAuth discovery helpers apply these [mitigations](https://modelcontextprotocol.io/docs/tutorials/security/security_best_practices#mitigation-3) by default:
- _Enforce HTTPS_. The OAuth helpers provided by the SDK reject the `http://` URLs
except loopback addresses (`localhost`, `127.0.0.1`, `::1`).
- _Enforce HTTPS_. Reject `http://` URLs (except loopback: `localhost`, `127.0.0.1`, `::1`)
and redirects that downgrade `https` to a non-`https` scheme.
- _Block Private IP Ranges_. The OAuth helpers provided by the SDK allow passing
a custom `http.Client`. Developers are advised to customize the client it with
appropriate network protections, including IP range blocking. The SDK does not provide
this capability out of the box.
- _Block Private IP Ranges_. Reject targets that resolve to a non-public address
(private, link-local including the `169.254.169.254` metadata endpoint, CGNAT, multicast,
unspecified). The check runs on the initial URL and at dial time, guarding against DNS rebinding.
- _Validate Redirect Targets_. Similarly to previous point, customized `http.Client`
can be used to validate network hops. The SDK does not provide this capability out
of the box.
- _Validate Redirect Targets_. Reject redirects to non-public addresses and cap the redirect count.
- _Use Egress Proxies_. This is out of scope for the SDK and can be configured separately.
- _Use Egress Proxies_. Out of scope for the SDK; configure separately.
- _DNS Resolution Considerations_. The SDK has DNS rebinding protection on the server side which is enabled by default. For the client side, consider providing
a custom `http.Client` that would implement DNS pinning.
**Opting out.** Passing a custom `http.Client` can bypass these checks, making SSRF
protection your responsibility. Specifically: a custom `Transport.DialContext`/`DialTLSContext`,
a non-`*http.Transport`, or a configured `Proxy` disables the dial-time IP check; setting
`CheckRedirect` replaces all redirect validation. In these cases, implement your own IP
blocking, redirect validation, and DNS pinning.
### Session Hijacking