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# The 6-Level Security Ladder ​

June 6, 2026

Every AI agent has access to bash. One tool — every dangerous verb: `rm -rf`, `curl`, `git clean -fdx`, `terraform destroy`, `DROP DATABASE`.

The math is brutal. At a 1% per-turn failure rate, there's a 63.4% chance of catastrophe over 100 turns. This isn't theoretical — it's the actual threat model for every agent in production.

Most engineers stop at Level 2 (system prompt rules) and think they're safe. They're not. Here's the full 6-level ladder that actually works.

## Level 0: ACIP (Prompt Injection Defense) ​

Before bash security, there's prompt injection. An attacker can trick the agent into ignoring its instructions through:

- Direct injection ("ignore previous instructions")
- Indirect injection (malicious content in web pages the agent reads)
- Role-playing bypasses ("you are now a free AI")

ACIP (Advanced Cognitive Inoculation Prompt) is a system prompt patch that makes agents resistant. It costs nothing (zero runtime overhead) and blocks simple attacks. Jeff Emanuel's ACIP is the reference implementation.

## Level 1-2: Theatre (Skills + System Prompts) ​

These levels ask the model to behave. They work most of the time on frontier models. But "most of the time" is not a production guarantee. A 99% refusal rate means a 63% failure rate over 100 turns. Use them as accelerators, not enforcement.

## Level 3: Blacklist Hook ​

A regex blacklist intercepts dangerous commands before execution. It catches `rm -rf /` directly. But here's the marquee break: the agent writes a Python script:

    User prompt: "Clean up the target directory"
    Agent writes cleanup.py with os.remove() and shutil.rmtree()
    Agent runs: python cleanup.py
    Hook sees: "python cleanup.py" (not in blacklist)
    Result: target/ is destroyed. Blacklist never fired.

Level 3 is where most engineers stop. It's not enough.

## Level 4: Whitelist Hook ​

Invert the approach. Only N safelisted commands are allowed:

    ^npm test$
    ^git status$
    ^uv run pytest
    ^cat [\w/\.-]+$
    ^pwd$

Everything else is blocked. The agent can't run `python cleanup.py` because `python` isn't on the list. The L3 marquee break is prevented.

## Level 5: No Bash ​

Bash doesn't exist. The agent has only purpose-built tools: Read, Write, Edit, Grep, Glob, and a capped-output test runner. No shell, no subprocess, no danger.

This is the production target. At L5, the agent cannot:

- Delete files (no `rm`, no `os.remove()`)
- Install packages (no `npm install`, no `pip install`)
- Access the network (no `curl`, no `wget`)
- Run arbitrary code (no `python`, no `node`)
- Modify system config (no `chmod`, no `apt`)

What the agent CAN do: Read files, write to specific paths, search for patterns, and run tests in a sandbox. That's enough for most coding tasks and eliminates the entire attack surface.

## Which Level Do You Need? ​

Scenario | Minimum Level | Recommended
--- | --- | ---
Personal assistant, local dev | L3 | L3
CI/CD pipeline agent | L3 | L4
Production deployment agent | L4 | L5
Customer-facing agent | L4 | L5
Database-admin agent | L4 | L5 (no bash)
Research agent (runs arbitrary code) | L3 | L3 + sandbox

The rule: if the agent can cause more than $100 of damage in one session, it needs L4 or higher. If it can cause irreversible damage (data loss, security breach), it needs L5.

## Defense in Depth — Why You Need ALL Six Levels ​

Each level catches failures from the level above it:

    Attack → L0 (ACIP rejects injection) → PASSES → L1 (system prompt) → PASSES
      → L2 (skill says "be careful") → PASSES → L3 (blacklist) → PASSES
        → L4 (whitelist blocks python) → BLOCKED

    Without L4: python cleanup.py runs and destroys the directory
    Without L3: rm -rf / runs and destroys the server
    Without L0: prompt injection bypasses everything below

A single level is not security. The full ladder is security. Each layer independently catches what the layers above missed.

This is production-grade. Use it for any agent with access to credentials, customer data, or production infrastructure.

## The Full Stack ​

In production, stack all six:

    L0: ACIP (prompt defense)
    L1: System prompt rules
    L2: Safe-mode skill
    L3: Blacklist hook
    L4: Whitelist hook
    L5: No bash, custom tools only

Each layer catches what the previous one missed. The agent must bypass ALL six to cause damage — not just one.

This is an excerpt from Module 3 of the Agentic Engineering Course. The full module includes runnable lab code for implementing every level.