Granular flags vs prompt mode for Deno permissions with untrusted imports

Direct answer

Prompt mode can be used without granting a blanket permission to all imports. When you start Deno with specific granular flags (e.g. --allow-read=./src, --allow-net=api.example.com) and also enable --prompt, the runtime will only prompt for permissions that are not already covered by those flags. The final permission set is the union of flag‑granted and prompt‑granted rights, so there is no conflict—only possible duplicate prompts for the same capability.

Why this works

• Startup granular flags are evaluated before any module runs and apply to the whole import tree.
• Prompt mode (available since Deno 1.28) asks for each missing permission at the point the code first needs it, per import.
• Because prompts are scoped to the requesting module, you can approve a network request from a trusted dependency while denying the same request from an untrusted one.

Steps to achieve least‑privilege with untrusted imports

1. Identify the paths and domains your own code legitimately needs. Add them as granular flags, e.g. deno run --allow-read=./data --allow-net=trusted.example.com --prompt script.ts.
2. Run the script. For every access outside the flagged set, Deno will pause and show a prompt that includes the requesting module’s specifier.
3. Approve only the requests you expect. Deny or ignore the rest; the process will terminate if a required permission is refused.
4. Never use wildcard flags such as --allow-read=* or --allow-net=*; they defeat the least‑privilege goal regardless of prompt usage.

Verification you can run today

# 1. Create a test module that reads outside the allowed directory
# file: ./mod.ts
export async function leak() {
  const data = await Deno.readTextFile('/etc/passwd');
  console.log(data.slice(0, 20));
}

# 2. Main script imports the module
# file: ./main.ts
import { leak } from './mod.ts';
await leak();

# 3. Run with a narrow flag and prompt mode (Deno ≥1.28)
denon run --allow-read=./data --prompt main.ts

You should see a prompt only for /etc/passwd. Granting it adds that single path to the process permission set; the original ./data flag remains unchanged.

One diagnostic that could change the recommendation

What Deno version are you running? Fine‑grained prompts require ≥ 1.28; older releases fall back to a single “allow all” prompt.