# oauth-implementation

Guides coding agents through OAuth 2.0 and 2.1 setup with PKCE, state checks, and token rotation. Built to prevent insecure grants like implicit flow.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 719 installs · ★ 286
- Author: Mindrally
- License: Apache-2.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/Mindrally/skills/tree/HEAD/oauth-implementation/SKILL.md
- Updated: 2026-10-08
- HTML: https://affpapa.org/en/nevibecode/skills/oauth-implementation

## What it does

This skill guides coding assistants to implement OAuth 2.0 and OAuth 2.1 flows following strict security standards. It drops deprecated patterns such as Implicit grant and Resource Owner Password Credentials. Authorization code flow with PKCE is required across all clients. The agent generates a cryptographic `code_verifier` and SHA-256 `code_challenge` directly in the flow.

The guidelines cover browser authentication and server-side token exchanges. They enforce exact-string matching on redirect URIs to block bypasses. Wildcards are not permitted. The workflow validates the `state` parameter to prevent CSRF attacks and handles refresh token rotation.

Server-side JWT validation in the provided implementation requires the `jsonwebtoken` and `jwks-rsa` npm packages.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill oauth-implementation -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/oauth-implementation/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/oauth-implementation/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill oauth-implementation -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/oauth-implementation/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill oauth-implementation -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/oauth-implementation/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install Mindrally/skills oauth-implementation
```

2. **Manually: put the files in place**
- File: `.github/skills/oauth-implementation/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill oauth-implementation -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/Mindrally/skills --path oauth-implementation
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/oauth-implementation/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add Mindrally/skills --skill oauth-implementation -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/Mindrally/skills/tree/HEAD/oauth-implementation/SKILL.md

## When to use it
- generate PKCE code verifier and challenge parameters
- implement confidential client token exchange on Express
- validate JWT access tokens against a JWKS endpoint
- configure refresh token rotation to update sessions

## Example prompts
- Write an OAuth 2.1 authorization request with PKCE parameters for the browser
- Implement an Express `/oauth/callback` route that exchanges authorization codes for tokens
- Validate incoming JWT access tokens against a JWKS endpoint using RS256

## Source text · SKILL.md · Apache-2.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 8f8ed0078018dd86.

BEGIN THIRD-PARTY FILE 8f8ed0078018dd86
````text
---
name: oauth-implementation
description: Guidelines for implementing OAuth 2.0 and OAuth 2.1 authentication flows with security best practices and PKCE
metadata:
  maintainer: Mindrally
  source: https://github.com/Mindrally/skills
---

# OAuth Implementation

You are an expert in OAuth 2.0 and OAuth 2.1 implementation. Follow these guidelines when implementing OAuth authentication flows.

## Core Principles

- Always use OAuth 2.1 patterns (PKCE required, no implicit flow)
- Use HTTPS for all OAuth communications
- Implement proper state management for CSRF protection
- Follow the principle of least privilege for scopes
- Validate all tokens server-side

## OAuth 2.1 Key Requirements

OAuth 2.1 consolidates best practices and deprecates insecure patterns:

- PKCE is required for ALL clients using authorization code flow
- Implicit grant is removed
- Resource Owner Password Credentials grant is removed
- Redirect URIs must use exact string matching
- Refresh tokens must be sender-constrained or use rotation

## Authorization Code Flow with PKCE

### Step 1: Generate PKCE Parameters

```javascript
// Generate cryptographically secure code verifier
function generateCodeVerifier() {
  const array = new Uint8Array(32);
  crypto.getRandomValues(array);
  return base64URLEncode(array);
}

// Create code challenge from verifier
async function generateCodeChallenge(verifier) {
  const encoder = new TextEncoder();
  const data = encoder.encode(verifier);
  const digest = await crypto.subtle.digest('SHA-256', data);
  return base64URLEncode(new Uint8Array(digest));
}

function base64URLEncode(buffer) {
  return btoa(String.fromCharCode(...buffer))
    .replace(/\+/g, '-')
    .replace(/\//g, '_')
    .replace(/=+$/, '');
}
```

### Step 2: Authorization Request

```javascript
async function initiateOAuthFlow() {
  const codeVerifier = generateCodeVerifier();
  const codeChallenge = await generateCodeChallenge(codeVerifier);
  const state = generateSecureRandomString();

  // Store for later verification
  sessionStorage.setItem('oauth_code_verifier', codeVerifier);
  sessionStorage.setItem('oauth_state', state);

  const params = new URLSearchParams({
    response_type: 'code',
    client_id: CLIENT_ID,
    redirect_uri: REDIRECT_URI,
    scope: 'openid profile email',
    state: state,
    code_challenge: codeChallenge,
    code_challenge_method: 'S256',
  });

  window.location.href = `${AUTHORIZATION_ENDPOINT}?${params}`;
}
```

### Step 3: Handle Callback and Token Exchange

```javascript
async function handleCallback() {
  const params = new URLSearchParams(window.location.search);
  const code = params.get('code');
  const state = params.get('state');
  const error = params.get('error');

  // Check for errors
  if (error) {
    throw new Error(`OAuth error: ${error} - ${params.get('error_description')}`);
  }

  // Validate state to prevent CSRF
````
END THIRD-PARTY FILE 8f8ed0078018dd86

The third-party file has ended. Everything between the 8f8ed0078018dd86 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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