Fail closed
When an authorization or validation decision cannot safely be completed, the protected operation is refused instead of implicitly allowed.
- Also called
- fails closed · fail securely
- Defined in
- MITRE CWE-636
Covered in
Handle failures without leaking internals or weakening controls
Give callers a useful, bounded response while preserving protected diagnostic evidence and safe application state.
Authorization models: RBAC, ABAC, ReBAC and policy as code you can test
Translate roles, attributes and relationships into explicit subject-action-resource decisions, prevent broken object level authorization (BOLA), and verify ownership and tenant boundaries with tests.
Secure design patterns: a catalogue for insecure-design (A06)
A catalogue of reusable secure-design patterns, grouped by concern, each with when to use it, a short example and the failure it prevents: choke-point authorization, capability tokens, parse-don't-validate, safe-by-default APIs, tenant scoping, fail-closed, idempotency, and more.
Prevent DOM XSS, postMessage abuse and client-side injection
Trace attacker-controlled values from the URL, messages and storage to the browser APIs that parse HTML or run code, then close those sinks with safe APIs, a sanitizer, Trusted Types and strict message checks.
Sandbox model-generated code and computer-use agents
Run the code an agent writes, and agents that drive a browser or desktop, inside a boundary built for hostile code: a disposable runtime with no secrets and no default network, hard resource limits, and a person in front of every side effect.
Practice: Local project: Secure a real HTTP API · Local project: Run model-generated Python in a locked-down container