Security Property-Based Testing

When to Use

• Testing that security invariants hold across many generated inputs
• Fuzz-testing parsers, validators, and input handlers
• Verifying authentication/authorization boundaries
• Testing cryptographic implementations for correctness
• Finding edge cases in input validation logic

When NOT to Use

• Testing specific known attack payloads (use unit tests)
• Load testing or performance testing
• UI testing or visual regression testing

Core Concept

Property-based testing generates thousands of random inputs and verifies that security properties always hold:

For all inputs X: security_invariant(process(X)) == True

Frameworks by Language

Language	Framework	Install
Python	Hypothesis	pip install hypothesis
JavaScript/TS	fast-check	npm install fast-check
Rust	proptest	cargo add proptest --dev
Go	rapid	go get pgregory.net/rapid
Java	jqwik	Maven/Gradle dependency

Python (Hypothesis) Examples

Input Validation

from hypothesis import given, strategies as st

@given(st.text())
def test_sanitizer_removes_script_tags(input_str):
    """XSS property: output never contains <script>"""
    result = sanitize_html(input_str)
    assert "<script" not in result.lower()

@given(st.text())
def test_path_traversal_blocked(user_path):
    """Path traversal property: resolved path stays within base dir"""
    try:
        resolved = resolve_user_path(user_path)
        assert resolved.startswith(BASE_DIR)
    except ValidationError:
        pass  # Rejection is safe

Auth Boundaries

@given(st.sampled_from(["admin", "user", "guest"]),
       st.sampled_from(["/api/admin", "/api/user", "/api/public"]))
def test_auth_boundaries(role, endpoint):
    """Authorization property: guests cannot access admin endpoints"""
    response = make_request(role, endpoint)
    if endpoint.startswith("/api/admin") and role == "guest":
        assert response.status_code == 403

Serialization Roundtrip

@given(st.dictionaries(st.text(), st.text()))
def test_serialize_roundtrip_safe(data):
    """Serialization property: roundtrip preserves data without code execution"""
    serialized = safe_serialize(data)
    deserialized = safe_deserialize(serialized)
    assert deserialized == data

JavaScript (fast-check) Examples

const fc = require('fast-check');

test('SQL parameterization prevents injection', () => {
  fc.assert(fc.property(fc.string(), (userInput) => {
    const query = buildParameterizedQuery(userInput);
    // Property: query never contains unescaped user input directly
    expect(query.params).toContain(userInput);
    expect(query.sql).not.toContain(userInput);
  }));
});

Security Properties to Test

Property	Description
No injection	Output never contains unescaped control characters
Path containment	File paths stay within allowed directory
Auth enforcement	Protected resources reject unauthorized access
Idempotent validation	Validating twice gives same result
No information leak	Error messages don't vary by secret values
Constant-time comparison	Comparison time doesn't depend on input
Roundtrip safety	Serialize/deserialize preserves data exactly

Best Practices

1. Start with the most security-critical invariants
2. Use custom strategies that generate attack-like inputs
3. Combine with coverage tracking to find untested paths
4. Save failing examples as regression unit tests
5. Run with high example counts in CI (at least 1000)