Show HN: An MCP Gateway to block the lethal trifecta | Hacker News Reader

Show HN: An MCP Gateway to block the lethal trifecta

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51 points76SlashDolphin22 comments

Hi there, me and some friends were inspired by Simon Willison's recent post on the "lethal trifecta" (https://simonwillison.net/2025/Jun/16/the-lethal-trifecta/ ) and started building a gateway to defend against it.

The idea: instead of connecting an LLM directly to multiple MCP servers, you point them all through a Gateway.

The Gateway:

- Connects to each MCP server and inspects their tools + requirements

- Classifies tools along the "trifecta" axes (private data access, untrusted content, external comms)

- When all three conditions are about to align in a single session, the Gateway blocks the last step and tells the LLM to show a warning instead.

That way, before anything dangerous can happen, the user is nudged to review the situation in a web dashboard.

We'd love for the HN community to try it out: https://github.com/Edison-Watch/open-edison

Any feedback very welcome - we'll be around in the thread to answer questions.

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I think the "lethal trifecta" framing is useful and glad that attempts are being made at this! But there are two big, hard-to-solve problems here:

1. The "lethal trifecta" is also the "productive trifecta" - people want to be able to use LLMs to operate in this space since that's where much of the value is; using private / proprietary data to interact with (do I/O with) the real world.

2. I worry that there will soon be (if not already) a fourth leg to the stool - latent malicious training within the LLMs themselves. I know the AI labs are working on this, but trying to ferret out Manchurian Candidates embedded within LLMs may very well be the greatest security challenge of the next few decades.

Those are really good points and we do have some plans for them, mainly on the first topic. What we're envisioning in terms of UX for our gateway is that when you set it up it's very defensive but whenever it detects a trifecta, you can mark it as a false positive. Over time the gateway will be trained to be exactly as permissive as the user wishes with only the rare false positive. You can already do that with the gateway today (you get a web notification when the gateway detects a trifecta and if you click into it, you get taken to a menu to approve/deny it if it occurs in the future). Granted, this can make the gateway overly-permissive but we do have plans on how to improve the granularity of these rules.

Regarding the second point, that is a very interesting topic that we haven't thought about. It would seem that our approach would work for this usecase too, though. Currently, we're defending against the LLM being gullible but gullible and actively malicious are not properties that are too different. It's definitely a topic on our radar now, thanks for bringing it up!

"without risk", "solves", and "Guaranteed" are big words – you might want to temper them.

Agreed. If someone could help answer the question of "how" I'd appreciate it. I'm currently skeptical but not sure I'm knowledgeable enough to prove myself right or wrong.

But, it just seems to me that some of the 'vulnerabilities' are baked in from the beginning, e.g. control and data being in the same channel AFAIK isn't solvable. How is it possible to address that at all? Sure we can do input validation, sanitization, restrict access, etc. ,etc., and a host of other things but at the end of the day isn't it still non-zero chance that something is exploited and we're just playing whack-a-mole? Not to mention I doubt everyone will define things like "private data" and "untrusted" the same. uBlock tells me when a link is on one of it's lists but I still click go ahead anyways.

At least in its current state we just use an LLM to categorise each individual tool. We don't look at the data itself, although we have some ideas of how to improve things, as currently it is very "over-defensive". For example, if you have the filesystem MCP and a web search MCP, open-edison will block if you perform a filesystem read, a web search, and then a filesystem write. Still, if you rarely perform writes open-edison would still be useful for tracking things. The UX is such that after an initial block you can make an exception for the same flow the next time it occurs.

Well, I guess 80-90% protective is better than nothing. Better might be a lock that requires positive confirmation by the user.

It is possible to configure it like that - when a trifecta is detected, it is possible for the gateway to wait for confirmation before allowing the last MCP call to proceed. The issue with that MCP clients are still in early stages and some of them don't like waiting for a long time until they get a response and act in weird or inconvenient ways if something times out (some of them sensibly disable the entire server if a single tool times out, which in our case disables the entire gateway and therefore all MCP tools). As it is, it's much better to default to returning a block message, and emit a web notification from the gateway dashboard to get the user to approve the usecase, then rerun their previous prompt.

Thanks for the follow up. I can see the value in trying to look at the chained read - search - write or similar patterns to alert the user. Awareness of tool activity is definitely helpful.

Fair criticism! We wrote the Readme earlier on when we were still ironing out the requirements. I'll fix it up shortly.

I'm trying to wrap my head around this:

1. How are you defending against the case of one MCP poisoning your firewall LLM into incorrectly classifying other MCP tools?

2. How would you make sure the LLM shows the warning, as they are non-deterministic?

3. How clear do you expect MCP specs in order for your classification step to be trustworthy? To the best of my knowledge there is no spec that outlines how to "label" a tool for the 3 axes, so you've got another non-deterministic step here. Is "writing to disk" an external comm? It is if that directory is exposed to the web. How would you know?

Really good questions, let's look at them one by one:

1. We are assuming that the user has done their due diligence verifying the authenticity of the MCP server, in the same way they need to verify them when adding an MCP server to Claude code or VSCode. The gateway protects against an attacker exploiting already installed standard MCP servers, not against malicious servers.

2. That's a very good question - while it is indeed non-deterministic, we have not seen a single case of it not showing the message. Sometimes the message gets mangled but it seems like most current LLMs take the MCP output quite seriously since that is their source of truth about the real world. Also, while the message could in theory not be shown, the offending tool call will still be blocked so the worst case is that the user is simply confused.

3. Currently we follow the trifecta very literally, as in every tool is classified into a subset of {reads private data, writes on behalf of user, reads publicly modifiable data}. We have an LLM classify each tool at MCP server load time and we cache these results based on whatever data the MCP server sends us. If there are any issues with the classification, you can go into the gateway dashboard and modify it however you like. We are planning on making a improvements to the classification down the line but we think it is currently solid enough and we would like to get it into users' hands to get some UX feedback before we add extra functionality.

Wouldn't the LLM running in the gateway also be susceptible to the same jailbreaks?

That's a good question! We do use an LLM to categorise the MCP tools but that is at "add" or "configure" time, not at the time they are called. As such we don't actively run an LLM while the gateway is up, all the rules are already set and requests are blocked based on the hard-set rules. Plus, at this point we don't actually look at the data that is passed around, so even if we change the rules for the trifecta, there's no way for any LLM to be poisoned by a malicious actor feeding bad data.

couldnt the configuring LLM be poisoned by tool descriptions to grant the lethal trifecta to the run time LLM?

It is possible thay a malicious MCP could poison the LLM's ability to classify it's tools but then your threat model includes adding malicious MCPs which would be a problem for any MCP client. We are considering adding a repository of vetted MCPs (or possibly use one of the existing ones) but, as it is, we rely on the user to make sure that their MCPs are legitimate.

Malicious servers are a separate threat I think. If the server is lying about what the tools do, an LLM can't catch that without seeing server source code, thus defeating the purpose of MCP.

How do you determine if the tools access private data? Is it based solely on their tool description (which can be faked) or by trying them in a sandboxed environment or by analyzing the code?

It is based on what the MCP server reports to us. As with most current LLM clients we assume that the user has checked the MCP servers they're using for authenticity.

Seen a hack using whatsapp mcp recently - this seems promising

Curious to learn more- have a pointer to it?

So is any combination of MCP servers basically going to require human in the loop approval for everything?

Sounds like it defeats the point.

That is a fair concern, and while that would happen often in some cases, there are others which rarely export data or rarely read public data where you can manually approve each usecase. Still, we are very interested in seeing how people use MCPs so we can improve the UX, which is why we're publishing this release. If users report that they get too many false positives we can always increase the granularity of the trifecta categories (say, exports data can be exports data publicly or privately. Or reads public data can have different tiers, etc.)

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