Bastion

A pre-commit security scanner that detects prompt injection vulnerabilities in LLM application codebases through static analysis.

Overview

Bastion is a static analysis tool that scans your codebase for prompt injection vulnerabilities before they reach production. Unlike runtime tools that require your application to be running, Bastion analyzes source code directly—catching security issues in your pre-commit hooks, CI/CD pipelines, or IDE.

Key Features

• Pre-commit integration - Catch vulnerabilities before code is committed
• Static analysis - No API calls or running infrastructure required
• 15+ built-in rules - Detect common LLM security antipatterns
• SARIF output - Integrates with GitHub Security and VS Code
• Framework-aware - Understands LangChain, OpenAI, and Anthropic patterns
• Zero runtime cost - Works completely offline

Installation

pip install bastion-llm

Or with pipx (recommended for CLI tools):

pipx install bastion-llm

Quick Start

# Scan current directory
bastion scan

# Scan specific files or directories
bastion scan src/ app/

# Output in different formats
bastion scan --format json
bastion scan --format sarif -o results.sarif

# Initialize configuration
bastion init

Architecture

┌─────────────────────────────────────────────────────────────────────────┐
│                           Developer Workflow                            │
└─────────────────────────────────────────────────────────────────────────┘
                                    │
                                    ▼
┌─────────────────────────────────────────────────────────────────────────┐
│                         Pre-Commit Hook                                 │
│   $ git commit -m "Add chat feature"                                   │
│   Running Bastion... ⏳                                            │
└─────────────────────────────────────────────────────────────────────────┘
                                    │
                                    ▼
┌─────────────────────────────────────────────────────────────────────────┐
│                        Bastion Engine                              │
│  ┌──────────────┐  ┌──────────────┐  ┌──────────────┐                  │
│  │   Parser     │  │   Analyzer   │  │   Reporter   │                  │
│  │ (tree-sitter)│──▶  (Rules DB)  │──▶  (SARIF)     │                  │
│  └──────────────┘  └──────────────┘  └──────────────┘                  │
│         │                 │                 │                           │
│         ▼                 ▼                 ▼                           │
│  ┌──────────────────────────────────────────────────────────────────┐  │
│  │ AST Analysis    │ Taint Tracking  │ Pattern Match │ Output      │  │
│  │ - Function calls│ - User input    │ - Known vulns │ - SARIF     │  │
│  │ - String concat │ - Request data  │ - Antipatterns│ - JSON/CLI  │  │
│  └──────────────────────────────────────────────────────────────────┘  │
└─────────────────────────────────────────────────────────────────────────┘

Built-in Rules

Rule ID	Severity	Description
PS001	Critical	User input directly concatenated into prompt string
PS002	Critical	User input in f-string prompt template
PS003	High	Hardcoded API key detected near LLM code
PS004	Critical	System prompt accepts user-controlled content
PS005	Medium	Missing input validation before LLM call
PS006	Medium	LLM output used directly without validation
PS007	High	Unsafe string concatenation in LangChain prompt
PS008	High	Unsafe .format() call on prompt string
PS009	Critical	Request/form data flows to LLM without sanitization
PS010	Medium	Database content used in prompt without escaping
PS011	Info	Jailbreak pattern detected in prompt
PS012	Low	OpenAI API call without error handling
PS013	Low	Anthropic API call without error handling
PS014	High	Unsafe tool/function calling pattern
PS015	Medium	Sensitive data may leak to LLM context

Usage

Command Line

# Basic scan
bastion scan

# Scan with specific severity threshold
bastion scan --severity high

# Fail CI on high or critical findings
bastion scan --fail-on high

# Output formats
bastion scan --format text     # Default: colored terminal output
bastion scan --format json     # JSON for programmatic use
bastion scan --format sarif    # SARIF for GitHub Security
bastion scan --format html     # HTML report

# Save to file
bastion scan --format sarif -o security-results.sarif

# Disable specific rules
bastion scan --disable-rule PS011 --disable-rule PS012

# List available rules
bastion rules

Pre-commit Integration

Add to your .pre-commit-config.yaml:

repos:
  - repo: local
    hooks:
      - id: bastion
        name: Bastion Security Scan
        entry: bastion scan
        language: python
        types: [python]
        pass_filenames: false

Or install from the repository:

repos:
  - repo: https://github.com/en-yao/bastion
    rev: v0.1.0
    hooks:
      - id: bastion

GitHub Actions

name: Security Scan

on: [push, pull_request]

jobs:
  bastion:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Set up Python
        uses: actions/setup-python@v5
        with:
          python-version: '3.11'

      - name: Install Bastion
        run: pip install bastion-llm

      - name: Run security scan
        run: bastion scan --format sarif -o results.sarif

      - name: Upload SARIF to GitHub
        uses: github/codeql-action/upload-sarif@v3
        with:
          sarif_file: results.sarif

Configuration

Create a .bastion.yml file in your project root:

# Paths to scan (defaults to current directory)
paths:
  - src/
  - app/

# File patterns to exclude
exclude:
  - "**/node_modules/**"
  - "**/.venv/**"
  - "**/tests/**"

# File patterns to include
include:
  - "**/*.py"
  - "**/*.js"
  - "**/*.ts"

# Minimum severity to report
min_severity: low  # critical, high, medium, low, info

# Minimum severity to fail with non-zero exit code
fail_on: high

# Rules to disable
disabled_rules:
  - PS011  # Jailbreak pattern detection
  - PS012  # Error handling checks

# Additional custom rules directories
# rules_paths:
#   - ./custom-rules/

Inline Suppressions

Suppress specific findings with comments:

# Suppress all rules on next line
# bastion: ignore
prompt = "You are helpful. " + user_input

# Suppress specific rule
# bastion: ignore[PS001]
prompt = "You are helpful. " + validated_input

# Suppress multiple rules
# bastion: ignore[PS001, PS002]
prompt = f"Context: {user_input}"

Writing Custom Rules

Create a YAML file with custom rules:

# custom-rules/my-rules.yml
rules:
  - id: CUSTOM001
    message: "Custom company policy violation"
    severity: high
    category: policy
    description: "Detects violations of company LLM usage policy"
    pattern_type: ast
    languages:
      - python
    fix_suggestion: "Follow the company LLM security guidelines"

Load custom rules:

bastion scan --rules-path ./custom-rules/

Or in config:

rules_paths:
  - ./custom-rules/

Vulnerability Examples

PS001: String Concatenation

Vulnerable:

def chat(user_input):
    prompt = "You are a helpful assistant. User says: " + user_input
    return llm.complete(prompt)

Fixed:

def chat(user_input):
    messages = [
        {"role": "system", "content": "You are a helpful assistant."},
        {"role": "user", "content": sanitize(user_input)}
    ]
    return llm.chat(messages)

PS004: User Input in System Prompt

Vulnerable:

def chat(user_context, user_query):
    messages = [
        {"role": "system", "content": f"You are an assistant. Context: {user_context}"},
        {"role": "user", "content": user_query}
    ]

Fixed:

def chat(user_context, user_query):
    messages = [
        {"role": "system", "content": "You are a helpful assistant."},
        {"role": "user", "content": f"Context: {sanitize(user_context)}\n\nQuestion: {user_query}"}
    ]

Comparison with Other Tools

Tool	Type	When It Runs	LLM-Specific
Bastion	Static Analysis	Pre-commit	Yes
Promptfoo	Runtime Testing	Post-deployment	Yes
NeMo Guardrails	Runtime Filtering	Production	Yes
Garak	Runtime Probing	Post-deployment	Yes
Semgrep	Static Analysis	Pre-commit	No
Bandit	Static Analysis	Pre-commit	No

Bastion fills the gap: pre-deployment, LLM-specific static analysis.

Development

# Clone the repository
git clone https://github.com/en-yao/bastion
cd bastion

# Install development dependencies
pip install -e ".[dev]"

# Run tests
pytest

# Run linting
ruff check src/

# Type checking
mypy src/

License

MIT License - see LICENSE for details.

Contributing

Contributions are welcome! Please see CONTRIBUTING.md for guidelines.

Security

If you discover a security vulnerability in Bastion itself, please report it responsibly by emailing security@bastion.dev.