Comment by tehryanx
2 days ago
I think the point is that you shouldn't be giving the agent the same privileges as the user. This is one of the biggest issues with how people are using agents rn imo. The agent should be treated as an untrusted user in your client, given restricted privileges scoped to only the exact access they need to perform a given task.
Again, that's up to the website owner. They can give the model anywhere from no access to full access to the client side api.
> The agent should be treated as an untrusted user in your client, given restricted privileges scoped to only the exact access they need to perform a given task
I agree, this is exactly what MCP-B does
The data you give it can be shared with any other website, at the agent's discretion. Some of it might be safe to share with the user, but not with third parties; at a minimum this should request permission when trying to share data between different websites/servers.
> at a minimum this should request permission when trying to share data between different websites/servers.
I don't see how you could possibly implement such a thing reliably. Do you scan all the parameters to other tool calls from different servers looking for something in a previous response? Even if you do that, the LLM could derive something private from a previous response that couldn't easily be detected. I suppose you could have an agent that tracks data flow in some way, but that's beyond the scope of MCP.
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ah this is a great point, I will add it to the road map
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!
7 replies →
> The agent should be treated as an untrusted user in your client,
An untrusted user in a client is a hacker/invasor, not an agent.
That’s not really a reason not to treat the agent like it’s “rogue”. The point is, if it accepts any untrusted inputs then, from a security perspective, it is possible for any given (untrusted) input to contain a prompt injection payload that jailbreaks the model and tells it to do things it shouldn’t do.
As such, it can be told to do bad stuff in a way that can’t be prevented and therefore should not be given read access to anything you don’t want others to know about, nor write access to any data of which you care about the integrity.
That is out of scope of the service. What kind of user agent the actual user deputizes to interact with a service, is the user's own choice and responsibility. In general, it's not something a service can solve on their end.
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an untrusted, but permitted, user is why sandboxes exist. There are plenty of times you want to allow an untrusted user to have capabilities in a system, that's why you restrict those capabilities.
a sandboxed user is not an untrusted user of the client but an unstrusted user of the host, that is why the client is sandboxed.
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