What Happens During Azure AD Token Validation: A Complete Guide

Purpose of Azure AD Token Validation

Ensures that access tokens and ID tokens are authentic and unaltered
Validates the identity of users, applications, or services accessing protected resources
Prevents unauthorized access by verifying claims and token integrity
Supports secure communication between clients and APIs using OAuth 2.0 and OpenID Connect

Note: Azure AD token validation is critical for any application or API that relies on Microsoft identity for authentication and authorization.

Prerequisites for Token Validation

An Azure AD tenant and registered application
A valid access token or ID token issued by Azure AD
Knowledge of your application's client_id and tenant_id
Access to public signing keys (via the Azure AD OpenID Connect metadata endpoint)
A supported library or SDK for token validation (e.g., Microsoft.Identity.Web, MSAL, or JWT libraries)

Warning: Never trust tokens without validating them, even if they appear to be from Azure AD.

Step-by-Step Guide: How Azure AD Token Validation Works

1. Decode the Token

Parse the JWT token to extract header, payload (claims), and signature.

2. Retrieve Signing Keys

Obtain the public keys from Azure AD's OpenID Connect metadata endpoint (e.g., OpenID Configuration).

3. Validate the Signature

Verify the token's signature using the public key to ensure it was issued by Azure AD and not tampered with.

4. Check Token Claims

Audience (aud): Should match your application's client ID or API identifier
Issuer (iss): Should match Azure AD's expected issuer URL
Expiration (exp): Ensure the token has not expired
Not Before (nbf): Ensure the token is active
Scope/roles: Confirm required permissions are present

5. Accept or Reject the Token

If all validations pass, grant access. Otherwise, reject the request and return an appropriate error.

Usage Examples

1. Validating Tokens in a .NET Core Web API

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options => {
        options.Authority = "https://login.microsoftonline.com/{tenant_id}/v2.0";
        options.Audience = "{client_id}";
    });

2. Token Validation in Node.js (Express + jsonwebtoken)

const jwt = require('jsonwebtoken');
const jwksClient = require('jwks-rsa');

const client = jwksClient({
  jwksUri: 'https://login.microsoftonline.com/{tenant_id}/discovery/v2.0/keys'
});

function getKey(header, callback) {
  client.getSigningKey(header.kid, function(err, key) {
    var signingKey = key.publicKey || key.rsaPublicKey;
    callback(null, signingKey);
  });
}

jwt.verify(token, getKey, options, callback);

3. Securing a REST API Endpoint

GET /api/orders HTTP/1.1
Authorization: Bearer eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIs...

The API validates the token before processing the request.

4. Authenticating Git Operations with Azure Repos

git clone https://<organization>@dev.azure.com/<organization>/_git/<repo>
# Git uses a Personal Access Token (PAT) or OAuth token, validated by Azure AD

Azure DevOps validates the token before granting repository access.

5. Validating Tokens in Python (Flask + PyJWT)

import jwt
from jwt.algorithms import RSAAlgorithm

public_key = RSAAlgorithm.from_jwk(jwk_json)

payload = jwt.decode(token, public_key, algorithms=['RS256'], audience='client_id')

Security Best Practices

Always validate tokens on the server side, never on the client
Use HTTPS for all communications to prevent token interception
Check all critical claims: aud, iss, exp, nbf, scope
Regularly update your libraries and dependencies to address vulnerabilities
Implement token revocation and lifetime policies as needed
Log failed validation attempts for audit and monitoring

Warning: Never accept tokens with invalid signatures or expired claims. Doing so exposes your application to security risks.

Feature Comparison: Access Token vs. ID Token

Feature	Access Token	ID Token
Purpose	Authorize access to APIs	Authenticate the user
Audience (`aud`)	API/Application	Client Application
Contains User Claims	Limited	Yes
Used By	APIs	Client Apps
Issued For	Resource Access	User Authentication

Frequently Asked Questions

The request is rejected, and the client must obtain a new token from Azure AD.

Yes, as long as you have the public signing keys and the token is not revoked. However, for critical scenarios, check token revocation status online.

Use official libraries such as Microsoft.IdentityModel.Tokens for .NET, jsonwebtoken for Node.js, and PyJWT for Python.

Implement token revocation checks by validating against Azure AD's token revocation endpoints or using short-lived tokens with refresh mechanisms.

Access tokens are validated by APIs to authorize resource access, while ID tokens are validated by client apps to authenticate users.