Comment by tehryanx
2 days ago
I'm not following.
Say I have your browser extension running, and it's interfacing with an MCP-B enabled banking application using my session to access my data in that app.
I also have it connected to MCP-B enabled rogue web app that I mistakenly trust.
My browser has an entire architecture built around preventing data from crossing between those two origins, but what's stopping a malicious instruction from the rogue app asking the extension agent to include some data that it pulled into the context window from the banking app?
Further, when I use MCP in my IDE I have to deliberately provide that MCP server with a token or credentials to access a protected resource. With MCP-B, isn't it just automatically provided with whatever credentials are already stored in cookies/etc for a given MCP-B enabled app? If I load an MCP-B enabled app, does the agent automatically have access or do I have to configure it somewhere?
> If a website wants to expose a "delete all user data" tool, that's on them. It's no different than putting a big red delete button on the page.
It is different though, because the directive to push that button can come from somewhere other than the user, unless you've somehow solved prompt injection.
The point I'm driving toward is that I think you're violating the most common assumption of the web's long-standing security model, that data is protected from leaking cross origin by the browser. There's no SOP or CORS for your agent extension, and that's something that web apps have been built to expect. You're basically building an SOP bypass extension.
Ah I see. Yes this is a concern, but this issue is actually not unique to MCP-B and is just a generally issue with agentic workflows that rely on a dynamic toolset from 3p vendors. (which any MCP server local or remote has the ability to be)
> With MCP-B, isn't it just automatically provided with whatever credentials are already stored in cookies/etc for a given MCP-B enabled app?
Not exactly, MCP-B just allows your extension agent to call functions that the website owner explicitly exposes. The client itself is not given an credentials like traditional MCP.
> If I load an MCP-B enabled app, does the agent automatically have access or do I have to configure it somewhere?
Theres more in the blog post but how much access the agent has and how much human approval is needed to grant this access is completely up to the website creator.
FWIW your points are valid and MCP-B should enforce some guardrails when any domain shift happens via elicitation: https://modelcontextprotocol.io/specification/draft/client/e...
I'll add it to the road map. Thanks for bringing it up!
I do think the threat model here is a bit unique though.
If I'm running two MCP servers on my machine, I'm the one that installed them, I'm the one that assigned what permissions they have in my environment, and I'm the one that explicitly decided what level of access to give them within whatever resource they're accessing. That gives me reasonably strong control over, or at least full knowledge of, what data can be shared between them.
With MCP, I can use oauth to make very deliberate decisions about the scope of access I want to give the agent.
With MCP-B, it's the web application owner that installed the interface and what access it has to my data, and the agent running in my client gets access to whatever that third party deemed appropriate.
With MCP-B the agent has the same access I do by default, with the only restrictions being up to the app owner rather than it being up to me.
MCP auth is not perfect by any stretch, but the key thing it gives the user is the capacity to restrict what the agent has access to with some granularity. That's super important because the agent can't be trusted when it's consuming inputs the user didn't explicitly define. MCP-B doesn't have this, if you have the agent in your browser it has access to whatever resources you have so long as they were exposed by a tool call, which isn't somethign the user has any say in.
I see your point. The MCP-B zero config nature from a user perspective is simultaneous it's biggest strength and weakness. You can think of it kind of like putting your Social Security number into a website. You are putting a bunch of trust that they are going to protect it properly.
With MCP-B you are putting trust in both the model and the website owner. It opens up the user to risk for sure, but it's up to them to determine if the upside is worth it.
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> with the only restrictions being up to the app owner rather than it being up to me.
I don't see any reason sites using MCP-B couldn't have settings to restrict access to certain data based on user configuration.
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