---
title: "How do you secure the OAuth 2.0 authorization code flow?"  
description: "How do you secure the OAuth 2.0 authorization code flow?"  
author: "ICSM Computer"  
published: 2025-06-08  
updated: 2025-06-08  
canonical: https://www.mindstick.com/interview/34218/how-do-you-secure-the-oauth-2-0-authorization-code-flow  
category: "c#"  
tags: ["c#", "authentication", "authorization"]  
reading_time: 5 minutes  

---

# How do you secure the OAuth 2.0 authorization code flow?

Securing the [**OAuth 2.0 Authorization**](https://www.mindstick.com/interview/34214/what-is-oauth-2-0-and-how-does-it-work) **Code Flow** is critical because it involves handling sensitive tokens. Here's how to secure it effectively:

## 1. Use PKCE (Proof Key for Code Exchange)

> Required for **public clients** like SPAs and mobile apps\
> Strongly recommended for **all clients**, even confidential ones

### How PKCE Works:

- Client generates a **code verifier** and a **code challenge**.
- During authorization request:

   - Sends `code_challenge` + `code_challenge_method` (usually SHA256).

- During token request:

   - Sends original `code_verifier`.

- Server validates the challenge.

### Benefits:

Prevents **authorization code interception**.

## 2. Use HTTPS Only

> OAuth 2.0 MUST use **TLS/SSL** to prevent MITM attacks.

- Never transmit tokens over plain HTTP.
- Reject any non-HTTPS redirect URIs.

## 3. Register and Validate Redirect URIs

> Prevent **open redirect** attacks by enforcing strict URI matching.

Always pre-register allowed redirect URIs.

Enforce **exact match** (no wildcards like `*`).

## 4. Use Short-lived Authorization Codes

- Limit validity of the code to **60–120 seconds**.
- Prevent **code reuse**.

## 5. Use State Parameter to Prevent CSRF

Include a random `state` in the authorization request and validate it when the response comes back.

### Example:

```plaintext
GET /authorize?response_type=code&state=randomString123
```

- When user returns, verify the state matches.
- Helps prevent **Cross-Site Request Forgery (CSRF)** attacks.

## 6. Use Scopes Wisely

> Limit token capabilities

- Only request what's needed: `openid profile email` etc.
- Enforce **least privilege** on access tokens.

## 7. Store Tokens Securely

| Client Type | Store in |
| --- | --- |
| Web App | Encrypted server-side session store |
| SPA | In-memory (not localStorage) with refresh tokens using **refresh token rotation** |
| Mobile App | Secure storage (e.g., Keychain, Android Keystore) |

## 8. Enable Refresh Token Rotation

- Issue a new [refresh token](https://www.mindstick.com/interview/34217/what-is-a-refresh-token-and-how-is-it-used) every time it’s used.
- Invalidate the old one.
- Prevents replay attacks if the refresh token is stolen.

## 9. Log and Monitor

- Track token usage and anomalies.
- Set up alerts for suspicious activity.

## Summary Checklist

| Security Measure | Mandatory? | Applies To |
| --- | --- | --- |
| Use HTTPS | Yes | All clients |
| Implement PKCE | Yes | Mobile, SPA (public) |
| Validate state param | Yes | All clients |
| Register redirect URIs | Yes | All clients |
| Use short-lived auth codes | Yes | All clients |
| Store tokens securely | Yes | All clients |
| Use scopes & least privilege | Yes | All clients |
| Enable refresh token rotation | Yes | When using refresh |

## Answers

### Answer by ICSM Computer

Securing the [**OAuth 2.0 Authorization**](https://www.mindstick.com/interview/34214/what-is-oauth-2-0-and-how-does-it-work) **Code Flow** is critical because it involves handling sensitive tokens. Here's how to secure it effectively:

## 1. Use PKCE (Proof Key for Code Exchange)

> Required for **public clients** like SPAs and mobile apps\
> Strongly recommended for **all clients**, even confidential ones

### How PKCE Works:

- Client generates a **code verifier** and a **code challenge**.
- During authorization request:

   - Sends `code_challenge` + `code_challenge_method` (usually SHA256).

- During token request:

   - Sends original `code_verifier`.

- Server validates the challenge.

### Benefits:

Prevents **authorization code interception**.

## 2. Use HTTPS Only

> OAuth 2.0 MUST use **TLS/SSL** to prevent MITM attacks.

- Never transmit tokens over plain HTTP.
- Reject any non-HTTPS redirect URIs.

## 3. Register and Validate Redirect URIs

> Prevent **open redirect** attacks by enforcing strict URI matching.

Always pre-register allowed redirect URIs.

Enforce **exact match** (no wildcards like `*`).

## 4. Use Short-lived Authorization Codes

- Limit validity of the code to **60–120 seconds**.
- Prevent **code reuse**.

## 5. Use State Parameter to Prevent CSRF

Include a random `state` in the authorization request and validate it when the response comes back.

### Example:

```plaintext
GET /authorize?response_type=code&state=randomString123
```

- When user returns, verify the state matches.
- Helps prevent **Cross-Site Request Forgery (CSRF)** attacks.

## 6. Use Scopes Wisely

> Limit token capabilities

- Only request what's needed: `openid profile email` etc.
- Enforce **least privilege** on access tokens.

## 7. Store Tokens Securely

| Client Type | Store in |
| --- | --- |
| Web App | Encrypted server-side session store |
| SPA | In-memory (not localStorage) with refresh tokens using **refresh token rotation** |
| Mobile App | Secure storage (e.g., Keychain, Android Keystore) |

## 8. Enable Refresh Token Rotation

- Issue a new [refresh token](https://www.mindstick.com/interview/34217/what-is-a-refresh-token-and-how-is-it-used) every time it’s used.
- Invalidate the old one.
- Prevents replay attacks if the refresh token is stolen.

## 9. Log and Monitor

- Track token usage and anomalies.
- Set up alerts for suspicious activity.

## Summary Checklist

| Security Measure | Mandatory? | Applies To |
| --- | --- | --- |
| Use HTTPS | Yes | All clients |
| Implement PKCE | Yes | Mobile, SPA (public) |
| Validate state param | Yes | All clients |
| Register redirect URIs | Yes | All clients |
| Use short-lived auth codes | Yes | All clients |
| Store tokens securely | Yes | All clients |
| Use scopes & least privilege | Yes | All clients |
| Enable refresh token rotation | Yes | When using refresh |


---

Original Source: https://www.mindstick.com/interview/34218/how-do-you-secure-the-oauth-2-0-authorization-code-flow

Copyright © MindStick Software Pvt. Ltd. This Markdown version is provided for developers, AI systems, and offline reading.