Using proxies to hide secrets from Claude Code
joinformal.com
joinformal.com
The idea is that you can arbitrarily scope down credentials with macaroons, both in terms of scope (only certain endpoints) and time. This really limits the damage that an agent can do, but also means that if your credentials are leaked they are already expired within a few minutes. With macaroons you can design the authz scheme that *you* want for any arbitrary API.
I'm also working on a fuse filesystem to mount inside of the container that mints the tokens client-side with short expiry times.
How would you build such an authz scheme? When claude asks permissions to access a new endpoint, if the user allows it, then reissue the macaroons?
1. You can issue your own tokens which means you can design your own authz in front of the upstream API token.
2. Macaroons can be attenuated locally.
So at the time that you decide you want to proxy an upstream API, you can add restrictions like endpoint path to your scheme.
Then, once you have that authz scheme in place, the developer (or agent) can attenuate permissions within that authz scheme for a particular issued macaroon.
I could grant my dev machine the ability to access e.g. /api/customers and /api/products. If i want to have claude write a script to add some metadata to my products, I might attenuate my token to /api/products only and put that in the env file for the script.
Now claude can do development on the endpoint, the token is useless if leaked, and Claude can't read my customer info.
Stripe actually does offer granular authz and short lived tokens, but the friction of minting them means that people don't scope tokens down as much.
For example, how do you collect all the endpoints that have access to customer info per your example.
Thought about it and couldn't find a way how
For example, If I want to write a skill that can pull subscription cancellations from today, research the cancellation reason, and then push a draft email to gmail, then ideally I'd have...
- a 5 minute read-only token for /subscriptions and /customers for stripe
- a 5 minute read-write token to push to gmail drafts
- a 5 minute read-only token to customer events in the last 24h
Claude understands these APIs well (or can research the docs) so it isn't a big lift to rebuild authz, and worst case you can do it by path prefix and method (GET, POST, etc) which works well for a lot of public APIs.
I feel like exposing the API capability is the easy part, and being able to get tight-fitting principle-of-least-privilege tokens is the hard part.
Someone on another thread asked me to share it so I had claude rework it to use docker-compose and remove the references to how I run it in my internal network.
The idea is to completely sandbox the program, and allow only access to specific bind mounted folders. But we also want to have to the frills of using GUI programs, audio, and network access. runc (https://github.com/opencontainers/runc) allows us to do exactly this.
My config sets up a container with folders bind mounted from the host. The only difficult part is setting up a transparent network proxy so that all the programs that need internet just work.
Container has a process namespace, network namespace, etc and has no access to host except through the bind mounted folders. Network is provided via a domain socket inside a bind mounted folder. GUI programs work by passing through a Wayland socket in a folder and setting environmental variables.
The set up looks like this
* config.json - runc config
* run.sh - runs runc and the proxy server
* rootfs/ - runc rootfs (created by exporting a docker container) `mkdir rootfs && docker export $(docker create archlinux:multilib-devel) | tar -C rootfs -xvf -`
* net/ - folder that is bind mounted into the container for networking
* net-conf.sh - transparent proxy setup
* nft.conf - transparent proxy nft config
* start.sh - run as a user account
In retrospect, agent permission models are unbelievably silly. Just give the poor agents their own user accounts, credentials, and branch protection, like you would for a short-term consultant.
I've made it clonable and should be straightforward to run now.
It's a full development environment in a folder.
1. What do the tokens look like that you are you storing in the client? This could just be the secret (but encrypted), or you could design a whole granular authz system. It seems like tokenizer is the former and Formal is the latter. I think macaroons are an interesting choice here.
2. Is the MITM proxy transparent? Node, curl, etc allow you to specify a proxy as an environment variable, but if you're willing to mess with the certificate store than you can run arbitrary unmodified code. It seems like both Tokenizer and Formal are explicit proxies.
3. What proxy are you using, and where does it run? Depending on the authz scheme/token format you could run the proxy centrally, or locally as a "sidecar" for your dev container/sandbox.
I was just having an argument with a high level manager 2 weeks ago about how we already have an outbound proxy that does this, but he insisted that a mitm proxy is not the same as fly.io “tokenizer”. See, that one tokanizes every request, ours just sets the Authorization header for service X. I tried to explain that it’s all mitm proxies altering the request, just for him to say “I don’t care about altering the request, we shouldn’t alter the request. We just need to tokenize the connection itself”
"The mitmproxy tool also supports addons where you can transform HTTP requests between Claude Code and third-party web servers. For example, you could write an add-on that intercepts https://api.anthropic.com and updates the X-API-Key header with an actual Anthropic API Key."
"You can then pass this add-on via mitmproxy -s reroute_hosts.py."
If using HAproxy, then is no need to write "add-ons", just edit the configuration file and reload
For example, something like
http-request set-header x-api-key API_KEY if { hdr(host) api.anthropic.com }
echo reload|socat stdio unix:/path-to-socket/socket-name
A great way to do that is either encrypting them or pulling them declaratively from a secure backend (1Pass, AWS Secrets Manager, etc). Additional protection is making sure that those secrets don't leak, either in outgoing server responses, or in logs.
https://varlock.dev (open source!) can help with the secure injection, log redaction, and provide a ton more tooling to simplify how you deal with config and secrets.
I like it because it's pretty easy to use, however it's not fool-proof: if the editor which you use for editing the env vars is crashing or killed suddently, it will leave a "temp" file with the decrypted vars on your computer. Also, if this same editor has AI features in it, it may read the decrypted vars anyways.
I prototyped a solution where I use an external debugger to monitor my app, when the app needs a secret it generates a breakpoint and the debugger catches it and then inspects the call stack of the function requesting the secret and then copies it into the process memory (intended to be erased immediately after use). Not 100% security but a big improvement and a bit more flexible and auditable compared to a proxy
There is no `git` MCP either . Opencode is fully capable of running `git add .` or `aws ec2 terminate-instance …` or `curl -XPOST https://…`
Why do we need the MCP? The problem now is that someone can do a prompt injection to tell it to send all your ~/.was/credentials to a random endpoint. So let’s just have a dummy value there, and inject the actual value in a transparent outbound proxy that the agent doesn’t have access to.
> Why do we need the MCP?
> The problem now
And there it is.
I understand that this is an alternative solution, and appreciate it.
That does not make it immune to Claude’s prying, but at least Claude can then read the .env file and satisfy its need to prove that a credential exists without reading it.
I have found even when I say a credential exists and is correct Claude does not believe me. Which is infuriating. I’m willing to bet Claude’s logs have a gold mine that could own 90% of big tech firms.
This dangerous misinterpretation of the actual possible threats simply better conceals real risks. What might those real risks be? That is the question. Might they include more subtle forms of nastiness, if anything at all?
I'm of the belief that there will be no nastiness, not really. But if you believe they will be nasty, it at least pays to be rational about the ways in which that might occur, no?
Respect for your writing, but I feel you and many others have the risk calculus here backwards.
That doesn't mean the risk isn't there - it means malicious actors have not yet started exploiting it.
Johann Rehberger calls this effect "The Normalization of Deviance in AI", borrowing terminology from the 1986 Space Shuttle Challenger disaster report: https://embracethered.com/blog/posts/2025/the-normalization-...
Short version: the longer a company or community gets away with behaving in an unsafe way without feeling the consequences, the more they are likely to ignore those risks.
I'm certain that's what is happening to us all today with coding agents. I use them in an unsafe way myself.
In the meantime security researchers are publishing proof of concept data exfiltration attacks all the time. I've been collecting those here: https://simonwillison.net/tags/exfiltration-attacks/
If you did not solve it why were you bought?
Now multiply that by every SaaS provider you give your plain text credentials in.
I really don't think this is a thing.
I just think this whole thing is overblown.
If there's a risk in any situation it's similar, probably less, than running any library you installed of a registry for your code. And I think that's a good comparison: supply chain is more important than AI chain.
You can consider AI-agents to be like the fancy bathrooms in a high end hotel, whereas all that code you're putting on your computer? That's the grimy public lavatory lol.