
# SharedArrayBuffer, Spectre, and Cross-Origin Isolation Concerns

## 1. Introduction to SharedArrayBuffer

### Overview

- `SharedArrayBuffer` is a JavaScript object that enables **shared memory** access between the main thread and Web Workers.
- Unlike `ArrayBuffer`, the memory is **not copied** between threads—allowing **true parallelism**.
- Paired with `Atomics`, it allows low-level memory synchronization (e.g., locks, waits).

### Example Use

```js
const sab = new SharedArrayBuffer(1024);
const sharedArray = new Uint8Array(sab);
sharedArray[0] = 42;
```

## 2. Browser Security Requirements

### Security Headers Required to Use SharedArrayBuffer

Modern browsers **restrict access** to `SharedArrayBuffer` due to Spectre-class vulnerabilities.

The following **HTTP headers must be set** to enable it:

```
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
```

### HTTPS Requirement

- Must be served over **HTTPS** (except `localhost` for dev).
- These headers enforce **cross-origin isolation**.

### Role of CORS

- CORS **alone is not sufficient**.
- However, embedded resources (like scripts and iframes) must still include proper CORS headers if they are to be loaded in a cross-origin isolated context.

## 3. Spectre Vulnerability

### What is Spectre?

- A class of **side-channel attacks** exploiting **speculative execution** in CPUs.
- Allows an attacker to read arbitrary memory from the same address space.

### Affected Architectures

- Intel, AMD, ARM — essentially **all modern processors**.

### Why It's Still a Concern

- It's a **hardware flaw**, not just a software bug.
- Can't be fully fixed in software without performance penalties.
- New Spectre **variants** (e.g., v2, RSB, BranchScope) continue to emerge.

## 4. Mitigations and Current Limitations

### Browser Mitigations

- **Restricted precision** for `performance.now()`.
- **Disabled or gated** access to `SharedArrayBuffer`.
- **Reduced or removed** fine-grained timers.

### OS/Hardware Mitigations

- **Kernel Page Table Isolation (KPTI)**
- **Microcode updates**
- **Retpoline** compiler mitigations

### Developer Responsibilities

- Avoid sharing sensitive data across threads unless necessary.
- Use **constant-time cryptographic functions**.
- Assume timing attacks are **still possible**.
- Opt into **cross-origin isolation** only when absolutely required.

## 5. Practical Development Notes

### Using SharedArrayBuffer Safely

- Ensure the site is **cross-origin isolated**:
  - Serve all resources with appropriate **CORS policies** (`Cross-Origin-Resource-Policy`, `Access-Control-Allow-Origin`)
  - Set the required **COOP/COEP headers**
- Validate support using:

```js
if (window.crossOriginIsolated) {
    // Safe to use SharedArrayBuffer
}
```

### Testing and Fallback

- Provide fallbacks to `ArrayBuffer` if isolation is not available.
- Document use cases clearly (e.g., high-performance WebAssembly applications or real-time audio/video processing).

## 6. Summary of Concerns and Advisements

| Topic                          | Concern / Consideration                             | Advisory                                               |
|-------------------------------|------------------------------------------------------|--------------------------------------------------------|
| Shared Memory                 | Can expose sensitive data across threads            | Use only in cross-origin isolated environments         |
| Spectre Vulnerabilities       | Still viable, evolving with new attack vectors      | Do not assume complete mitigation; minimize attack surfaces |
| Cross-Origin Isolation        | Required for `SharedArrayBuffer`                    | Must serve with COOP/COEP headers + HTTPS              |
| CORS                          | Not sufficient alone                                 | Must combine with full isolation policies              |
| Developer Security Practices | Timing attacks and shared state remain risky        | Favor safer primitives; avoid unnecessary complexity   |
