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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsMCP servers must use URL-mode elicitation—not form-mode elicitation—to collect secrets such as passwords, API keys, access tokens, and payment credentials. URL mode keeps those inputs out of the MCP client and model context, but it does not prove who opened the browser link. A forwarded link can let another person authorize an account, so the server must verify that the browser user is the same person who initiated the elicitation.
How MCP URL elicitation handles secrets
The MCP elicitation specification defines two modes. Form mode collects structured information through the client. URL mode sends the user to an out-of-band interaction, such as a third-party OAuth or payment flow. Servers must not request sensitive information through form mode; they must use URL mode for it.
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The boundary matters: the user enters the third-party secret or completes authorization in a browser flow, rather than providing the credential through the MCP client or model context. The MCP client’s bearer token does not change as a result. The server manages credentials it receives through the third-party flow.
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Can a forwarded MCP authorization link be used to authorize the wrong account?
Yes. The specification describes this cross-user scenario: Alice starts an elicitation, then Bob opens its URL and completes the third-party authorization. If the server attaches the resulting credentials to Alice’s pending request without checking who completed the flow, tokens may be bound to Alice’s identity, potentially enabling account takeover.
The direct defense is identity continuity. The specification states: “To prevent this attack, the server MUST ensure that the user who started the elicitation request (the end-user who is accessing the server via the MCP client) is the same user who completes the authorization flow.”
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How an MCP server should verify who completed authorization
The server must verify the browser user’s identity server-side and compare it with the identity associated with the initiating MCP authorization. It must not rely on an identity asserted only by the client. If the server stores pending elicitation state, that state must be protected and securely associated with individual users.
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For a web-accessible server, the specification offers one non-normative example: route the user through a connection endpoint on the server’s own domain, verify an authenticated browser session there, and compare its authoritative identity with the identity tied to MCP authorization before redirecting to the third-party authorization server. This illustrates the check; it is not a universal implementation recipe. Other architectures need a robust mechanism suited to their authentication model.
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What MCP clients must do before opening an elicitation URL
Server-side identity binding does not replace safe link handling by the client. For URL-mode elicitation, the specification requires clients to:
- Show the full URL so the user can inspect the destination.
- Obtain explicit user consent before opening it.
- Never automatically pre-fetch the URL or its metadata.
- Open it in a secure manner that prevents the client or LLM from inspecting page content or user inputs.
The specification identifies iOS SFSafariViewController as a suitable example and WkWebView as unsuitable. Clients should also highlight the domain and warn about ambiguous or suspicious URIs, including Punycode. These client safeguards help users make informed navigation decisions, but they cannot establish that the person completing authorization is the same person who initiated it.
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How URL elicitation relates to MCP authorization security
MCP authorization is a separate flow from the third-party authorization that URL elicitation may start. The MCP authorization security considerations say clients must include the OAuth resource parameter in authorization and token requests, and servers must validate that tokens were issued for them. They also require exact validation of registered redirect URIs and recommend checking OAuth state values.
These controls remain important for MCP authorization, but they do not by themselves prevent the described URL-elicitation attack. That scenario requires the server to bind the browser flow’s completing user to the initiating MCP user.
Quick Recap
Implementation checks
- Secret collection: Use URL mode for passwords, API keys, access tokens, payment credentials, and similar sensitive data; do not request them through form mode.
- User continuity: Verify the browser user server-side and compare that identity with the initiator before associating third-party credentials with a pending request.
- Pending state: Protect stored elicitation state and associate it securely with the correct user.
- Client navigation: Display the full destination, require consent, avoid pre-fetching, and isolate the browser interaction from client and model inspection.
- MCP authorization: Separately validate token audience, exact registered redirect URIs, and OAuth state as applicable.
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