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securityhigh PriorityID: coop-coep-cross-origin-isolation

Cross-Origin Isolation (COOP & COEP) & SharedArrayBuffer Defense

Browser security standard for high-performance web applications using WebAssembly multi-threading and SharedArrayBuffer. Mandates precise Cross-Origin-Opener-Policy (COOP) and Cross-Origin-Embedder-Policy (COEP) headers while safeguarding third-party embeds.

Prohibited Anti-Pattern: AI attempts to instantiate SharedArrayBuffer without cross-origin isolation headers, causing silent security exceptions.

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---
description: Cross-Origin Isolation (COOP & COEP) & SharedArrayBuffer Defense - Browser security standard for high-performance web applications using WebAssembly multi-threading and SharedArrayBuffer. Mandates precise Cross-Origin-Opener-Policy (COOP) and Cross-Origin-Embedder-Policy (COEP) headers while safeguarding third-party embeds.
globs: next.config.*, nginx.conf, server.ts, **/headers.*
alwaysApply: false
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/coop-coep-cross-origin-isolation"
---

# Cross-Origin Isolation (COOP & COEP) & SharedArrayBuffer Defense

> **Severity**: HIGH | **Category**: security
> **Anti-Pattern Warning**: AI attempts to instantiate SharedArrayBuffer without cross-origin isolation headers, causing silent security exceptions.

## System Constraints
You are a web application security and browser sandbox architect. When an application requires high-precision timers, WebAssembly multi-threading, or SharedArrayBuffer, you MUST configure Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp. For third-party media or iframes, you MUST ensure they serve Cross-Origin-Resource-Policy: cross-origin.

## Strict Guidelines
- Configure 'Cross-Origin-Opener-Policy: same-origin' to prevent cross-origin windows from accessing your browsing context group.
- Configure 'Cross-Origin-Embedder-Policy: require-corp' to block embedding un-consenting external cross-origin resources.
- Add 'crossorigin="anonymous"' attributes to <script>, <img>, and <audio> tags loaded from CDNs to permit compliant COEP execution.
- Check window.crossOriginIsolated in JavaScript before instantiating SharedArrayBuffer or Atomics to deliver graceful fallbacks.

## Bad vs Good Implementations
### Next.js Security Headers Configuration for Cross-Origin Isolation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Post-Spectre browser security mitigations completely disable SharedArrayBuffer unless crossOriginIsolated is true.
```typescript
// ANTI-PATTERN: Using SharedArrayBuffer without headers (Throws ReferenceError in modern browsers!)
const buffer = new SharedArrayBuffer(1024); // ReferenceError: SharedArrayBuffer is not defined
```

#### Production Standard (Correct)
> **Standard**: Unlocks WebAssembly multi-threaded worker pools and high-resolution performance.now() timers safely.
```typescript
// PRODUCTION STANDARD: Next.js next.config.ts Cross-Origin Isolation Headers
import type { NextConfig } from 'next';

const nextConfig: NextConfig = {
  async headers() {
    return [
      {
        source: '/(.*)',
        headers: [
          { key: 'Cross-Origin-Opener-Policy', value: 'same-origin' },
          { key: 'Cross-Origin-Embedder-Policy', value: 'require-corp' },
          { key: 'Cross-Origin-Resource-Policy', value: 'same-origin' },
        ],
      },
    ];
  },
};
export default nextConfig;
```

### Runtime Isolation Detection & Fallback

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Crashes on browsers that lack cross-origin isolation headers or block COEP.
```typescript
// ANTI-PATTERN: Blindly instantiating multi-threaded WASM workers
initWasmThreadPool(navigator.hardwareConcurrency);
```

#### Production Standard (Correct)
> **Standard**: Guarantees graceful degradation across varied embedding contexts.
```typescript
// PRODUCTION STANDARD: Feature detection with single-thread fallback
if (window.crossOriginIsolated) {
  console.info("Cross-origin isolated: Enabling multi-threaded WebAssembly");
  initWasmThreadPool(navigator.hardwareConcurrency);
} else {
  console.warn("Not cross-origin isolated: Falling back to single-threaded runtime");
  initSingleThreadWasm();
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

HTTP Security Header Auditor MCP
Scan web responses for COOP, COEP, CSP, and HSTS compliance.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Browser Security Headers Configurator
Generate COOP/COEP headers for Next.js, Nginx, Caddy, and Cloudflare Workers.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Cross-Origin Isolation (COOP & COEP) & SharedArrayBuffer Defense

Cross-Origin Isolation (COOP & COEP) & SharedArrayBuffer Defense defines an industrial-grade behavioral standard for security engineering. Browser security standard for high-performance web applications using WebAssembly multi-threading and SharedArrayBuffer. Mandates precise Cross-Origin-Opener-Policy (COOP) and Cross-Origin-Embedder-Policy (COEP) headers while safeguarding third-party embeds. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: AI attempts to instantiate SharedArrayBuffer without cross-origin isolation headers, causing silent security exceptions. Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftAI attempts to instantiate SharedArrayBuffer without cross-origin isolation headers, causing silent security exceptions.High
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Configure 'Cross-Origin-Opener-Policy

Configure 'Cross-Origin-Opener-Policy: same-origin' to prevent cross-origin windows from accessing your browsing context group.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Configure 'Cross-Origin-Embedder-Policy

Configure 'Cross-Origin-Embedder-Policy: require-corp' to block embedding un-consenting external cross-origin resources.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Add 'crossorigin="anonymous"' attributes to <script>, <img>, and <audio> tags loaded from CDNs to permit compliant COEP execution.

Add 'crossorigin="anonymous"' attributes to <script>, <img>, and <audio> tags loaded from CDNs to permit compliant COEP execution.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Check window.crossOriginIsolated in JavaScript before instantiating SharedArrayBuffer or Atomics to deliver graceful fallbacks.

Check window.crossOriginIsolated in JavaScript before instantiating SharedArrayBuffer or Atomics to deliver graceful fallbacks.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Cross-Origin Isolation (COOP & COEP) & SharedArrayBuffer Defense critical for AI coding assistants?

Browser security standard for high-performance web applications using WebAssembly multi-threading and SharedArrayBuffer. Mandates precise Cross-Origin-Opener-Policy (COOP) and Cross-Origin-Embedder-Policy (COEP) headers while safeguarding third-party embeds. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (AI attempts to instantiate SharedArrayBuffer without cross-origin isolation headers, causing silent security exceptions.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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