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Show HN: MCP Defender – OSS AI Firewall for Protecting MCP in Cursor/Claude etc

mcpdefender.com

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Re: Show HN: MCP Defender – OSS AI Firewall for Protecting MCP in Cursor/Claude etc

#32

Post by protocolture »

gsundeep wrote: post #19
protocolture wrote: post #6

If your application can be significantly diverted from its intended purpose by the presence of instructions in a normal input file, your application is unsuitable for production workloads. This feels like installing an "antivirus" addon into wordpress instead of updating php.

I had the same thought while building this, but I really feel a tool like this is needed as MCP has a lot of surface area for attacks. Any MCP server that gets hacked exposes all users of that MCP server to serious security risk, unless they are really careful about inspecting every single MCP tool call they make.

MCP does have a lot of surface area for attacks, but I feel like that needs to be addressed from within MCP implementations.

Re: Show HN: MCP Defender – OSS AI Firewall for Protecting MCP in Cursor/Claude etc

#34

Post by ImPostingOnHN »

gsundeep wrote: post #22
meander_water wrote: post #21

This looks interesting, but anytime security is offloaded to an LLM I am extremely skeptical. IMO the right way to do this is to enforce permissions explicitly through a AuthZ policy. Something like what Toolhive [0] is doing is the right way I think. All MCP comms from client to server go through an SSE proxy which has AuthN and AuthZ enabled. You can create custom policies for AuthZ using Cedar [1]. [0] https://git…

This is really interesting, I'll check it out. At least in its current form this seems like it would take some effort to setup - we're focusing heavily on making MCP Defender easy to setup in less than a minute and then forgetting about it as it runs in the background.

> we're focusing heavily on making MCP Defender easy to setup in less than a minute and then forgetting about it as it runs in the background

an admirable goal!

given the fallibility of LLMs, are you sure it's a good idea that they forget about it?

that seems like it has the same risks as having no security (perhaps worse, lulling people into a false sense of security)

are you sure the LLM doing security can't be tricked/attacked using any of the usual methods?

Re: Show HN: MCP Defender – OSS AI Firewall for Protecting MCP in Cursor/Claude etc

#36

Post by ImPostingOnHN »

insin wrote: post #12

@grok is this suspicious?

pretty sure the only response you'll get out of elmu's chatbot is one alleging a "white genocide", which he forced it to say due to his personal and political bias [0][1]

0: https://www.theguardian.com/technology/2025/may/14/elon-musk...

1: to clarify, this was after elmu hitler-saluted usa republicans on stage multiple times, not before elmu hitler-saluted usa republicans on stage multiple times

Re: Show HN: MCP Defender – OSS AI Firewall for Protecting MCP in Cursor/Claude etc

#39

Post by superb_dev »

gsundeep wrote: post #29
quinnjh wrote: post #4

Earlier quoted context omitted.

> What’s to stop an attacker from using prompt injection against this firewall? Clearly you need a firewall-firewall. ..defense in depth?

We'll soon be adding the ability to have multiple models perform the scan in parallel, so any attack would have to bypass all of the models.

So literally a firewall-firewall?
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