OAuth2’s Three-Legged Flow, also known as the **Authorization Code grant**, is a secure method that allows an application to access APIs on behalf of a user without ever handling their password. In this flow, a user logs in through Cisco’s identity service and grants the application permission to act on their behalf with scoped access tokens. This pattern is widely used across Cisco APIs such as Webex and other DevNet endpoints that require user-centric authorization. It is an important topic for the **350-901 DEVCOR** exam, where candidates must demonstrate understanding of API authentication and authorization. Mastering this flow not only helps with real integrations but also aligns with key objectives of Cisco’s certification blueprint.



### **Conceptual Overview**

At its core, the OAuth2 Three-Legged Flow involves four main participants: the user, the client application, the authorization server  and the resource server. The client redirects the user to the authorization server, where the user logs in and explicitly consents to the permissions (scopes) the application is requesting. After approval, the client receives an **authorization code**, which it then exchanges securely for an **access token** and often a **refresh token**.

This flow is preferred when an application needs to perform actions on behalf of a user, as it keeps user credentials safe and enforces fine-grained access control through scopes. Compared to client credentials - which are limited to machine-to-machine interactions - the three-legged flow supports interactive user consent and is essential for integrations that require personalized access, such as meeting or messaging actions in Webex.



### **Step-by-Step Implementation**

To start, you first **register your application** in the Cisco DevNet Portal. During registration, specify your **redirect URI**, then securely store the provided **Client ID** and **Client Secret**. These values uniquely identify your app and allow it to request authorization on behalf of users.

Next, construct the **authorization URL** that redirects your user to Cisco’s identity service. It typically looks like:

https://api.webex.com/v1/oauth2/authorize?

 response_type=code

 &client_id={CLIENT_ID}

 &redirect_uri={YOUR_REDIRECT_URI}

 &scope={SCOPES}



The scopes you include determine what actions your app can perform  and user consent prompts will reflect these permissions.

Once the user logs in and approves the request, Cisco will redirect back to your application’s redirect URI with an authorization code appended. Your backend application should listen on that endpoint, capture the returned code  and verify the state parameter (used to protect against CSRF attacks).

To complete the flow, exchange the authorization code for tokens by making a **POST** request to:

https://api.webex.com/v1/oauth2/token



Include the grant_type=authorization_code, the returned code, your client credentials  and redirect URI. The response will include an access token for API calls and generally a refresh token for obtaining new access tokens later, without requiring the user to log in again.



### **Common Debug Scenarios**

Even experienced developers run into issues when implementing OAuth2. One frequent problem is an Invalid Redirect URI error. This happens if the redirect URI provided in the authorization request doesn’t exactly match the one you registered in the DevNet portal - even small differences like a trailing slash or case variation can trigger this.

Another error you might encounter is Unauthorized Client. This typically indicates that your client secret is incorrect or that the requested scopes aren’t valid for your application configuration. Double-check that the scopes are properly encoded in the URL and that your secret isn’t being exposed to the browser.

If no authorization code is returned, it could mean the user denied the access or that your request omitted the required state parameter. Using state helps prevent cross-site request forgery (CSRF) and should always be included in your request URL for security.

When debugging, a simple checklist helps:

Make sure the redirect URI in your request matches the registered value exactly.
 Confirm that the scopes you request are supported by the Cisco API you’re targeting.
 Log HTTP status codes and full responses for the token exchange - this often reveals subtle issues with your POST body or headers.



### **Post-Authorization: Using Tokens**

Once you receive an access_token, include it in API calls using the standard Bearer scheme:

Authorization: Bearer {access_token}



This token serves as proof that your application was authorized by the user to act on their behalf. When the access token expires - which often happens within minutes - you can use the **refresh token** to request a new access token without redirecting the user again. This helps maintain long-lasting sessions in your application and offers a smoother experience for users who interact frequently with Cisco APIs.



### **Strengthening Your Cisco 350-901 DEVCOR Exam Preparation**

For candidates preparing for the 350-901 DEVCOR exam, a solid grasp of OAuth2 flows and API authentication is essential because the “Using APIs” domain explicitly tests these concepts, including the OAuth2 three-legged Authorization Code grant. The official exam topics outline that candidates must understand secure API access mechanisms, error handling  and API integration fundamentals as part of application development and security.

To support study plans, many learners use **[Cisco 350-901 DEVCOR Exam Dumps](https://www.certshero.com/cisco/350-901)** to practice how multiple-choice and scenario-based questions are phrased around these topics. They can help you identify common question patterns and reinforce your understanding of key areas like token exchange sequences, error responses  and when to apply specific OAuth2 grant types in real exam scenarios.

### **FAQ - Real Questions People Ask**

**What is the OAuth2 Three-Legged Flow used for in Cisco APIs?**
 It’s used when an application must access APIs on behalf of a user with explicit consent, keeping user credentials secure while enforcing scoped access through tokens.

**Why do I need to match the redirect URI exactly?**
 OAuth2 servers like Cisco’s compare the provided redirect URI in the request to the registered value character-for-character. Any mismatch will cause the request to fail.

**How do refresh tokens help?**
 Refresh tokens allow your application to obtain new access tokens after the current one expires, without user re-login, enabling long-running sessions.

**Does the DEVCOR exam require coding knowledge for OAuth2?**
 The exam focuses more on concepts and sequences rather than specific code snippets, so understanding the flow and common error scenarios is more important than memorizing syntax. 



Implementing and Debugging OAuth2 Three-Legged Flow for Cisco APIs (350-901 DEVCOR)