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securitycritical PriorityID: oauth2-pkce-implementation

OAuth 2.1 & OIDC Authorization Code with PKCE Standard

Guardrails for modern authentication: OAuth 2.1 Authorization Code Flow with PKCE (Proof Key for Code Exchange), httpOnly SameSite=Lax cookies, and refresh token rotation.

Prohibited Anti-Pattern: AI uses deprecated OAuth Implicit Flow or stores JWT access tokens in browser localStorage.

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---
description: OAuth 2.1 & OIDC Authorization Code with PKCE Standard - Guardrails for modern authentication: OAuth 2.1 Authorization Code Flow with PKCE (Proof Key for Code Exchange), httpOnly SameSite=Lax cookies, and refresh token rotation.
globs: **/auth/**/*.{ts,js,py}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/oauth2-pkce-implementation"
---

# OAuth 2.1 & OIDC Authorization Code with PKCE Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: AI uses deprecated OAuth Implicit Flow or stores JWT access tokens in browser localStorage.

## System Constraints
You are an Identity and Access Management (IAM) security engineer. Implement OAuth 2.1 with PKCE for single-page and mobile apps. Never store access/refresh tokens in localStorage (vulnerable to XSS). Store session tokens in httpOnly, Secure, SameSite=Lax cookies. Enforce refresh token rotation.

## Strict Guidelines
- Mandate PKCE (code_verifier and code_challenge with S256) on all authorization flows.
- Store session tokens in httpOnly, Secure, SameSite cookies to protect against XSS token exfiltration.
- Implement single-use Refresh Token Rotation (RTR); revoke token families on detected reuse.

## Bad vs Good Implementations
### Secure Cookie Session vs Insecure LocalStorage

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Any third-party script or XSS bug can read localStorage and compromise user accounts permanently.
```typescript
// CRITICAL SECURITY FLAW: Storing JWT tokens in localStorage
function onLoginSuccess(token: string) {
  localStorage.setItem("authToken", token); // Any XSS vulnerability will steal this token!
}
```

#### Production Standard (Correct)
> **Standard**: httpOnly cookies cannot be accessed by client-side JavaScript, effectively isolating credentials from XSS.
```typescript
// Production Standard: httpOnly Secure Cookie with SameSite
export function setAuthCookie(res: Response, token: string) {
  res.setHeader("Set-Cookie", [
    `session=${token}; HttpOnly; Secure; SameSite=Lax; Path=/; Max-Age=604800`
  ]);
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Fetch MCP
Validate OIDC discovery endpoints and JWKS keys.
Recommended Agent Skills (askills.dev)

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

OAuth PKCE Flow Scaffolder
Generate type-safe OAuth 2.1 PKCE authorization handlers.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: OAuth 2.1 & OIDC Authorization Code with PKCE Standard

OAuth 2.1 & OIDC Authorization Code with PKCE Standard defines an industrial-grade behavioral standard for security engineering. Guardrails for modern authentication: OAuth 2.1 Authorization Code Flow with PKCE (Proof Key for Code Exchange), httpOnly SameSite=Lax cookies, and refresh token rotation. 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 uses deprecated OAuth Implicit Flow or stores JWT access tokens in browser localStorage. 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 uses deprecated OAuth Implicit Flow or stores JWT access tokens in browser localStorage.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

Mandate PKCE (code_verifier and code_challenge with S256) on all authorization flows.

Mandate PKCE (code_verifier and code_challenge with S256) on all authorization flows.

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

Store session tokens in httpOnly, Secure, SameSite cookies to protect against XSS token exfiltration.

Store session tokens in httpOnly, Secure, SameSite cookies to protect against XSS token exfiltration.

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

Implement single-use Refresh Token Rotation (RTR)

Implement single-use Refresh Token Rotation (RTR); revoke token families on detected reuse.

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 OAuth 2.1 & OIDC Authorization Code with PKCE Standard critical for AI coding assistants?

Guardrails for modern authentication: OAuth 2.1 Authorization Code Flow with PKCE (Proof Key for Code Exchange), httpOnly SameSite=Lax cookies, and refresh token rotation. 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 uses deprecated OAuth Implicit Flow or stores JWT access tokens in browser localStorage.) 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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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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