DenoHow-ToBeginner · 3 min read

Why Deno Requires Permissions: Secure Runtime Explained

Deno requires explicit permissions to access files, network, environment variables, and other sensitive resources to protect users from malicious code. This permission system ensures scripts run with only the access they need, improving security by default.

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Syntax

Deno uses command-line flags to grant permissions when running scripts. Each flag controls access to a specific resource.

--allow-read: Allows reading files.
--allow-write: Allows writing files.
--allow-net: Allows network access.
--allow-env: Allows access to environment variables.
--allow-run: Allows running subprocesses.
--allow-hrtime: Allows high-resolution time measurement.

You can grant all permissions with --allow-all, but it is not recommended for security reasons.

bash

deno run --allow-read script.ts

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Example

This example shows a Deno script that reads a file. Without --allow-read, Deno will block the file access and show an error.

typescript

async function readFile() {
  try {
    const data = await Deno.readTextFile("example.txt");
    console.log("File content:", data);
  } catch (error) {
    console.error("Error reading file:", error.message);
  }
}

readFile();

Output

Error reading file: PermissionDenied: read access to "example.txt" is not allowed

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Common Pitfalls

Many beginners forget to grant the necessary permissions, causing their scripts to fail with permission errors. Another mistake is using --allow-all unnecessarily, which reduces security.

Always grant only the permissions your script needs.

bash

/* Wrong: No permissions granted */
deno run script.ts

/* Right: Grant only read permission */
deno run --allow-read script.ts

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Quick Reference

Permission Flag	Description
--allow-read	Allow reading files from disk
--allow-write	Allow writing files to disk
--allow-net	Allow network access
--allow-env	Allow access to environment variables
--allow-run	Allow running subprocesses
--allow-hrtime	Allow high-resolution time measurement
--allow-all	Allow all permissions (not recommended)

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Key Takeaways

Deno requires explicit permissions to protect users from unsafe code.

Grant only the permissions your script needs to keep security tight.

Use command-line flags like --allow-read and --allow-net to enable access.

Running without needed permissions causes runtime errors.

Avoid using --allow-all unless absolutely necessary.