---
title: Secrets
description: Secure credential injection for sandboxes
icon: "key"
---

Secrets keep credentials on the host while giving sandboxed code a placeholder to use.

When you bind a secret to an environment variable, microsandbox puts a placeholder in the guest instead of the real value. By default that placeholder is `$MSB_<env_var>`, using the environment variable name exactly as provided, and you can provide a custom placeholder when needed. If the sandbox sends the placeholder to an allowed host, microsandbox swaps it for the real credential at the network boundary. Anywhere else, the placeholder remains meaningless.

That means the guest can call APIs without ever holding the credential itself.

Allowed hosts are checked against the sandbox's observed DNS and TLS identity. Keep allow lists narrow so placeholders can only turn into credentials at the destinations that actually need them.

## Add at create time

Bind secrets to environment variables when you create the sandbox, each scoped to the hosts allowed to receive it:

<CodeGroup>
```rust Rust
use microsandbox::Sandbox;

let sb = Sandbox::builder("worker")
    .image("python")
    .secret(|s| s
        .env("GITHUB_TOKEN")
        .value(std::env::var("GITHUB_TOKEN")?)
        .allow_host("api.github.com")
        .allow_host_pattern("*.githubusercontent.com")
    )
    .secret_env("SERVICE_API_KEY", service_api_key, "api.example.com")
    .create()
    .await?;
```

```typescript TypeScript
import { Sandbox } from "microsandbox";

await using sb = await Sandbox.builder("worker")
    .image("python")
    .secret((s) =>
        s.env("GITHUB_TOKEN")
            .value(process.env.GITHUB_TOKEN!)
            .allowHost("api.github.com")
            .allowHostPattern("*.githubusercontent.com"),
    )
    .secretEnv("SERVICE_API_KEY", process.env.SERVICE_API_KEY!, "api.example.com")
    .create();
```

```python Python
import os
from microsandbox import Sandbox, Secret

sb = await Sandbox.create(
    "worker",
    image="python",
    secrets=[
        Secret.env(
            "GITHUB_TOKEN",
            value=os.environ["GITHUB_TOKEN"],
            allow_hosts=["api.github.com"],
            allow_host_patterns=["*.githubusercontent.com"],
        ),
        Secret.env(
            "SERVICE_API_KEY",
            value=os.environ["SERVICE_API_KEY"],
            allow_hosts=["api.example.com"],
        ),
    ],
)
```

```go Go
sb, err := m.CreateSandbox(ctx, "worker",
    m.WithImage("python"),
    m.WithSecrets(
        m.Secret.Env("GITHUB_TOKEN", os.Getenv("GITHUB_TOKEN"),
            m.SecretEnvOptions{
                AllowHosts:        []string{"api.github.com"},
                AllowHostPatterns: []string{"*.githubusercontent.com"},
            },
        ),
        m.Secret.Env("SERVICE_API_KEY", os.Getenv("SERVICE_API_KEY"),
            m.SecretEnvOptions{AllowHosts: []string{"api.example.com"}},
        ),
    ),
)
```

```bash CLI
msb create python --name worker \
  --secret "GITHUB_TOKEN@api.github.com,*.githubusercontent.com" \
  --secret "SERVICE_API_KEY@api.example.com"
```

</CodeGroup>

In the CLI form, `ENV@HOST[,HOST...]` records a host-side source reference: the real value is read from the same-named host environment variable when the sandbox starts, and never lands in the durable config. The inline `ENV=VALUE@HOST` form is rejected on both `msb create` and `msb modify` (shell history and process listings would leak the value regardless), so providing a raw value is SDK-only.

<Warning>
  **Raw values are saved to disk.** When you pass a raw value through an SDK (`.value(..)`, `secret_env()`, or a value-based rotate), it is stored as-is in the sandbox config file and stays there until you rotate the secret to a reference. The sandbox behaves the same either way; the only difference is what ends up on disk. Prefer references whenever the value is available in a host environment variable.
</Warning>

## Change while running

<Tooltip tip="modify is not yet available on microsandbox cloud; recreate the sandbox to change its secrets."><span className="msb-badge-local">Local-only <Icon icon="circle-info" size={11} /></span></Tooltip>
Rotate or remove existing secrets without a restart. Adding a secret or changing its guest-visible placeholder requires a restart so the new environment reaches the guest. Later rotations keep that placeholder stable and only change the value injected at the network boundary.

<CodeGroup>
```rust Rust
let plan = sb.modify()
    .secret(|s| s
        .env("API_KEY")
        .source(SecretSource::Env { var: "API_KEY".into() })
        .allow_host("api.example.com"))
    .restart()
    .apply()
    .await?;
```

```typescript TypeScript
await sandbox.modify({
  secrets: {
    API_KEY: { env: "API_KEY", allowedHosts: ["api.example.com"] },
  },
  policy: "restart",
});

await sandbox.modify({ secretsRemove: ["SERVICE_API_KEY"] });
```

```python Python
from microsandbox import ModificationPolicy

await sb.modify(
    secrets={
        "API_KEY": {
            "env": "API_KEY",
            "allowed_hosts": ["api.example.com"],
        },
    },
    policy=ModificationPolicy.RESTART,
)

await sb.modify(secrets_rm=["SERVICE_API_KEY"])
```

```go Go
_, err := sb.Modify(ctx, m.ModifyOptions{
    Secrets: map[string]m.SecretModifySpec{
        "API_KEY": {
            Env:          "API_KEY",
            AllowedHosts: []string{"api.example.com"},
        },
    },
    Policy: m.ModificationPolicyRestart,
})

_, err = sb.Modify(ctx, m.ModifyOptions{
    SecretsRemove: []string{"SERVICE_API_KEY"},
})
```

```bash CLI
msb modify worker --secret GITHUB_TOKEN@api.github.com --restart  # add or rotate
msb modify worker --secret-rm SERVICE_API_KEY                       # remove
```
</CodeGroup>

Secret modification is available through every SDK and the CLI. See [Tuning](/sandboxes/tuning) for how changes are planned and applied.

For API details, see the SDK references: [Rust](/sdk/rust/secrets) | [TypeScript](/sdk/typescript/secrets) | [Python](/sdk/python/secrets) | [Go](/sdk/go/secrets).