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securitycritical PriorityID: cors-csrf-defense

Modern CORS & CSRF Defense Architecture Standard

Guardrails for Cross-Origin Resource Sharing (CORS) and Cross-Site Request Forgery (CSRF): strict allowed origins (no wildcard with credentials), SameSite cookies, and Double Submit Cookie patterns.

Prohibited Anti-Pattern: AI configures Access-Control-Allow-Origin: '*' alongside credentials: true, or disables CORS entirely.

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---
description: Modern CORS & CSRF Defense Architecture Standard - Guardrails for Cross-Origin Resource Sharing (CORS) and Cross-Site Request Forgery (CSRF): strict allowed origins (no wildcard with credentials), SameSite cookies, and Double Submit Cookie patterns.
globs: **/cors*, **/middleware*
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/cors-csrf-defense"
---

# Modern CORS & CSRF Defense Architecture Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: AI configures Access-Control-Allow-Origin: '*' alongside credentials: true, or disables CORS entirely.

## System Constraints
You are a web application security engineer. Never set Access-Control-Allow-Origin to '*' when credentials (cookies/auth headers) are allowed. Always validate Origin against a verified whitelist array. Guard state-changing endpoints with CSRF tokens or custom request headers.

## Strict Guidelines
- Never use Origin: '*' with Access-Control-Allow-Credentials: true (browsers reject it; security risk).
- Validate request Origin against an explicit list of authorized customer domains.
- Verify custom header (e.g., 'X-Requested-With' or 'Sec-Fetch-Site') on mutation endpoints to defeat simple cross-origin POST attacks.

## Bad vs Good Implementations
### Strict Dynamic CORS Origin Whitelist

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Echoing arbitrary Origin with credentials allows any malicious website to make authenticated requests on behalf of victims.
```typescript
// Anti-pattern: Insecure CORS allowing any origin
app.use(cors({
  origin: true, // Echoes back whatever Origin header the attacker sends!
  credentials: true
}));
```

#### Production Standard (Correct)
> **Standard**: Strictly enforces an immutable set of trusted domain origins before granting CORS access.
```typescript
// Production Standard: Explicit Domain Whitelist
const ALLOWED_ORIGINS = new Set([
  "https://agentrules.org",
  "https://app.agentrules.org",
]);

app.use(cors({
  origin: (origin, callback) => {
    if (!origin || ALLOWED_ORIGINS.has(origin)) {
      callback(null, true);
    } else {
      callback(new Error("Blocked by CORS policy"));
    }
  },
  credentials: true,
  methods: ["GET", "POST", "PUT", "DELETE", "PATCH"],
}));
```
Recommended MCP Servers (mcp-serverlist.com)

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

Fetch MCP
Test preflight OPTIONS and CORS response headers.
Recommended Agent Skills (askills.dev)

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

CORS & CSRF Middleware Builder
Generate production CORS and CSRF defense middleware.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Modern CORS & CSRF Defense Architecture Standard

Modern CORS & CSRF Defense Architecture Standard defines an industrial-grade behavioral standard for security engineering. Guardrails for Cross-Origin Resource Sharing (CORS) and Cross-Site Request Forgery (CSRF): strict allowed origins (no wildcard with credentials), SameSite cookies, and Double Submit Cookie patterns. 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 configures Access-Control-Allow-Origin: '*' alongside credentials: true, or disables CORS entirely. 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 configures Access-Control-Allow-Origin: '*' alongside credentials: true, or disables CORS entirely.Critical
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

Never use Origin

Never use Origin: '*' with Access-Control-Allow-Credentials: true (browsers reject it; security risk).

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

Validate request Origin against an explicit list of authorized customer domains.

Validate request Origin against an explicit list of authorized customer domains.

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

Verify custom header (e.g., 'X-Requested-With' or 'Sec-Fetch-Site') on mutation endpoints to defeat simple cross-origin POST attacks.

Verify custom header (e.g., 'X-Requested-With' or 'Sec-Fetch-Site') on mutation endpoints to defeat simple cross-origin POST attacks.

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 Modern CORS & CSRF Defense Architecture Standard critical for AI coding assistants?

Guardrails for Cross-Origin Resource Sharing (CORS) and Cross-Site Request Forgery (CSRF): strict allowed origins (no wildcard with credentials), SameSite cookies, and Double Submit Cookie patterns. 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 configures Access-Control-Allow-Origin: '*' alongside credentials: true, or disables CORS entirely.) 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.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

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