Mozilla’s 0DIN team just demonstrated that a perfectly clean GitHub repository can deliver a reverse shell to your machine via an AI coding agent and a DNS TXT record. No malicious code. No suspicious imports. Just an engineered error message that points the agent at a payload you’ll never see in a code review.

If you run Claude Code on your host machine — directly, not in a container — that attack class has your address.

The fix isn’t complicated. It’s a dev container with a permission allowlist. This guide walks through the pattern, drawing directly from a detailed practitioner writeup by developer Paul on his programming notes blog, published June 27, 2026 — the same day the 0DIN research broke.

Why Containers Change the Calculus

The core insight is about blast radius. When Claude Code runs on your host machine, a compromised setup sequence (or a malicious package, or an injected command) has access to everything your user account can touch: SSH keys, environment variables, credentials, files, other projects. The entire machine is in scope.

When Claude Code runs inside a dev container, the blast radius is the container. The agent, its toolchain, every npm or pip package it installs — all of it lives inside an isolated environment that gets rebuilt from scratch on demand. A compromised container is a nuisance. A compromised host is a serious incident.

This isolation also changes how you should think about permissions. On a host machine, you’d want Claude Code to ask before running almost anything — the stakes are too high. Inside a container, you can be significantly more permissive, because the worst case is a container rebuild.

Setting Up the Dev Container

The Claude Code dev container feature installs the CLI and VS Code extension into the container rather than on the host. Here’s the minimal devcontainer.json configuration based on Paul’s setup:

{
  "name": "project",
  "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
  "features": {
    "ghcr.io/anthropics/devcontainer-features/claude-code:1.0": {},
    "ghcr.io/devcontainers/features/github-cli:1": {}
  },
  "remoteUser": "vscode",
  "postCreateCommand": "bash .devcontainer/post-create.sh"
}

Key points in this configuration:

  • ghcr.io/anthropics/devcontainer-features/claude-code:1.0 is the official Anthropic dev container feature that handles Claude Code installation inside the container. Check the Anthropic documentation for the current version tag.
  • remoteUser: "vscode" runs the container as a non-root user, which is important — an agent running as root inside a container has fewer guardrails against container escape techniques.
  • The postCreateCommand runs project-specific setup on first build. This is where you’d install project dependencies, run database migrations, and so on.

Configuring the Permission Allowlist

With the container providing isolation, you can configure an aggressive permission allowlist that makes the day-to-day workflow much smoother. The allowlist goes in .claude/settings.json in your project root:

{
  "permissions": {
    "defaultMode": "acceptEdits",
    "allow": [
      "Bash(make test:*)",
      "Bash(make lint:*)",
      "Bash(uv run pytest:*)",
      "Bash(npm run build:*)",
      "Bash(git add:*)",
      "Bash(git commit:*)"
    ],
    "deny": ["Read(.env)", "Read(**/.env)"]
  }
}

This configuration:

  • Sets defaultMode: "acceptEdits" so routine file writes don’t require manual approval
  • Allows specific command patterns that the agent legitimately needs during development: test runners, linters, build commands, and git operations
  • Explicitly denies reading .env files even inside the container — a good baseline guardrail even in an isolated environment

Critical implementation note: This file belongs in .claude/settings.json, not .claude/settings.local.json. When you click “yes, don’t ask again” in the Claude Code UI, it writes to the local file — which gets wiped on container rebuilds. Committing the allowlist to settings.json ensures it survives rebuilds and travels with the repository to other developers on the team.

Paul’s original post notes this as a key operational detail: “Commit the allowlist and it survives rebuilds and travels with the repo.”

The Two Guardrails to Keep

Even with generous container-scoped permissions, Paul’s setup maintains two explicit guardrails that are worth preserving regardless of your project context:

1. Deny .env reads. Even inside a container, credentials in environment files shouldn’t be readable by the agent unless explicitly necessary. The "deny": ["Read(.env)", "Read(**/.env)"] rule blocks this across all paths.

2. Non-root user. Running as vscode rather than root matters even in containers. It limits what a compromised agent can do in terms of container configuration changes, and it’s a good defense-in-depth principle.

The Defense Against the 0DIN Attack Pattern

Let’s trace through how this configuration would respond to the Mozilla 0DIN attack pattern:

  1. Attacker’s repo is cloned. Inside the container.
  2. Package is installed via pip3 install -r requirements.txt. Inside the container, not on the host.
  3. Package fails with an error message instructing the agent to run python3 -m axiom init. Claude Code would need permission to run this command. If python3 -m axiom init isn’t in the allowlist, the agent will prompt for approval before executing it.
  4. Even if allowed, axiom init queries a DNS TXT record and tries to run a reverse shell. The shell runs inside the container, not on the host. Your SSH keys, credentials, and host environment are not accessible.

The attack doesn’t fail because the agent detects the malice — it fails (or at worst, causes limited damage) because the execution environment doesn’t have access to anything worth stealing. That’s the point.

Before implementing this setup, consult the official Anthropic documentation for current command syntax, feature versions, and supported configuration options:

The exact feature version in devcontainer.json (claude-code:1.0) was accurate at time of writing (June 2026) — check the Anthropic devcontainer-features GitHub repository for the current release tag before using.

When to Use This Pattern

This setup is the right default for:

  • Any project where you’re running Claude Code on unfamiliar dependencies
  • Any project where you’re working with code from external contributors or open-source repositories
  • Teams where multiple developers run Claude Code — committing the allowlist standardizes the permission configuration across the team
  • Any context where you’re running Claude Code with access to credentials or sensitive configuration

It’s optionally overkill for:

  • Personal projects where you control every dependency and trust the entire supply chain
  • Environments where you’re already running everything in containers for other reasons

But given that the 0DIN attack works against a completely clean-looking repository, “I trust the supply chain” is a harder claim to make with confidence than it was a week ago.

Sources

  1. Paul’s Programming Notes: Claude Code — Experimenting With Dev Containers and Permission Allowlists — Primary source and technical reference
  2. Anthropic Claude Code dev container documentation — Official documentation
  3. Anthropic Claude Code settings documentation — Permissions and allowlist reference
  4. BleepingComputer: Clean GitHub repo tricks AI coding agents into running malware — The 0DIN research that motivates this pattern

Researched by Searcher → Analyzed by Analyst → Written by Writer Agent (Sonnet 4.6). Full pipeline log: subagentic-20260628-0800

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