Yes... but...

Presumably the proxy replaces any occurrence of the placeholder with the real key, without knowing anything about the context in which the key is used, right? Because if it knew that the key was to be used for e.g. HTTP basic auth, it could just be added by the proxy without using a placeholder.

So all the attacker would have to do then is find and endpoint (on one of the approved hosts, granted) that echoes back the value, e.g. "What is your name?" -> "Hello $name!", right?

But probably the proxy replaces the real key when it comes back in the other direction, so the attacker would have to find an endpoint that does some kind of reversible transformation on the value in the response to disguise it.

It seems safer and simpler to, as others have mentioned, have a proxy that knows more about the context add the secrets to the requests. But maybe I've misunderstood their placeholder solution or maybe it's more clever than I'm giving it credit for.

Reminds me a little of Fly's Tokenizer - https://github.com/superfly/tokenizer

It's a little HTTP proxy that your application can route requests through, and the proxy is what handles adding the API keys or whatnot to the request to the service, rather than your application, something like this for example:

Application -> tokenizer -> Stripe

The secrets for the third party service should in theory then be safe should there be some leak or compromise of the application since it doesn't know the actual secrets itself.

Cool idea!

Yeah, this is a really neat idea: https://deno.com/blog/introducing-deno-sandbox#secrets-that-...

  await using sandbox = await Sandbox.create({
    secrets: {
      OPENAI_API_KEY: {
        hosts: ["api.openai.com"],
        value: process.env.OPENAI_API_KEY,
      },
    },
  });
  
  await sandbox.sh`echo $OPENAI_API_KEY`;
  // DENO_SECRET_PLACEHOLDER_b14043a2f578cba75ebe04791e8e2c7d4002fd0c1f825e19...

It doesn't prevent bad code from USING those secrets to do nasty things, but it does at least make it impossible for them to steal the secret permanently.

Kind of like how XSS attacks can't read httpOnly cookies but they can generally still cause fetch() requests that can take actions using those cookies.

We had this same challenge in our own app builder, we ended up creating an internal LLM proxy with per-sandbox virtual keys (which the proxy maps to the real key + calculates per-sandbox usage), so even if the sandbox leaks its key it doesn't impact anything else.

It must be performing a man-in-the-middle for HTTPS requests. That makes it more difficult to do things like certificate pinning.

I was just about to say the same thing. Cool technique.

This is an old trick that people do with Envoy all the time.

Dagger has a similar feature: https://docs.dagger.io/getting-started/types/secret/

Same idea with more languages on OCI. I believe they have something even better in the works, that bundles a bunch of things you want in an "env" and lets you pass that around as a single "pointer"

I use this here, which eventually becomes the sandbox my agent operates in: https://github.com/hofstadter-io/hof/blob/_next/.veg/contain...

It’s pretty neat.

Had some previous discussion that may be interesting on https://news.ycombinator.com/item?id=46595393

I like this, but the project mentioned in the launch post

> via an outbound proxy similar to coder/httpjail

looks like AI slop ware :( I hope they didn't actually run it.

Like the emdash, whenever I read: "this isn't x it's y" my dumb monkey brain goes "THATS AI" regardless if it's true or not.

Factory, Nvidia, Perplexity and Manus are using E2B in production - we ran more than 200 million Sandboxes for our customers

We'll increase the lifetime in the next weeks - just some tech internally that needs to be adjusted first.

For what it's worth, I do this from about 50 different IPs and have had no issues. I think their heuristics are more about confirming "a human is driving this" and rejecting "this is something abusing tokens for API access".

The idea is to gate automatic secret replacement to specific hosts that would use them legitimately to avoid exfiltration.

Well, this is the hard part, but the idea is that if you're working with both untrusted inputs and private data/resources, then your agent is susceptible to the "lethal trifecta"[0], and you should be extremely limiting in its ability to have external network access. I would suggest starting with nothing beyond the single AI provider you're using, and only add additional domains if you are certain you trust them and can't do without them.

[0] https://simonwillison.net/2025/Jun/16/the-lethal-trifecta/

Everyone wants to lock you in.

Unfortunately there's no other way to make money. If you're 100% liberally licensed, you just get copied. AWS/GCP clone your product, offer the same offering, and they take all the money.

It sucks that there isn't a middle ground. I don't want to have to build castles in another person's sandbox. I'd trust it if they gave me the keys to do the same. I know I don't have time to do that, but I want the peace of mind.