ID Token vs Access Token What Developers Often Get Wrong

16 November 2025

As a full-stack developer navigating the modern web, you've inevitably encountered OAuth 2.0 and OpenID Connect (OIDC). They are the bedrock of secure authentication and authorization for countless applications, from simple social logins to complex enterprise-level API security. Yet, a persistent and critical point of confusion remains: the fundamental difference between an ID Token and an Access Token. Many developers, even experienced ones, treat them interchangeably or misunderstand their distinct roles, leading to security vulnerabilities and flawed application logic.

This isn't just a matter of semantics. Misusing these tokens is like using your office key card to prove your identity at a bank. The key card grants you access to a specific building (authorization), but it says nothing verifiable about who you are (authentication). This guide, written from a developer's perspective, will dissect this crucial distinction. We will move beyond surface-level definitions to explore why these two tokens exist, who they are for, what they contain, and how they work together in a secure, robust system. By the end, you'll not only understand the theory but will be equipped to implement authentication and authorization correctly in your own projects.

Core Takeaway: An ID Token is for the Client Application to understand who the user is (Authentication). An Access Token is for the API/Resource Server to know if the client has permission to access a resource (Authorization). Never use an Access Token for authentication.

First, We Must Understand OAuth 2.0: The Framework of Authorization

Before we can even talk about ID Tokens, we must first have a rock-solid understanding of OAuth 2.0. The biggest mistake developers make is thinking OAuth 2.0 is an authentication protocol. It is not. The official RFC 6749 specification explicitly defines it as an authorization framework. Its primary goal is to solve the problem of delegated authority.

Imagine this pre-OAuth scenario: You want to use a third-party photo printing service that needs access to your photos on Google Photos. Without OAuth, you would have to give the printing service your Google username and password. This is a security nightmare. The service now has full access to your entire Google account—your Gmail, Google Drive, everything. It can store your credentials indefinitely, and if that service's database is breached, your Google credentials are stolen.

OAuth 2.0 was created to solve exactly this problem. It introduces a flow where you, the user (Resource Owner), can grant the printing service (Client) limited access to your photos (Resource) stored on Google's servers (Resource Server) via an Authorization Server, without ever sharing your credentials. The result of this flow is the Access Token.

The Key Players in the OAuth 2.0 Ecosystem

To grasp the flows, you must know the actors involved:

Resource Owner: You, the end-user who owns the data and can grant access to it.
Client: The third-party application (e.g., the photo printing service) that wants to access the Resource Owner's data.
Authorization Server: The server that authenticates the Resource Owner and issues Access Tokens after obtaining consent. This is the core engine of OAuth. (e.g., Google's login service).
Resource Server: The server that hosts the protected resources (e.g., the Google Photos API). It accepts and validates Access Tokens.

The Access Token: A "Key" for a Specific Door

The Access Token is the central artifact in OAuth 2.0. It is a string representing a specific set of permissions (called "scopes") that a client has been granted for a specific resource. Think of it as a temporary key card. It might grant you access to the 3rd floor (scope: `read:photos`) but not the 4th floor (scope: `delete:photos`), and it expires after a few hours.

Crucially, the Access Token is meant for one entity and one entity only: the Resource Server. The client application's job is simply to acquire this token from the Authorization Server and then present it in the `Authorization` header of an API request to the Resource Server.


GET /api/v1/photos HTTP/1.1
Host: photos.api.google.com
Authorization: Bearer [ACCESS_TOKEN_STRING]

The client application should treat the access token as an opaque string. It's not supposed to look inside it or try to interpret its contents. Its format is not standardized by the OAuth 2.0 spec; it could be a random string tied to a database record on the server, or it could be a self-contained structured format like a JSON Web Token (JWT). The point is, the client doesn't—and shouldn't—care. Its contract is simple: get the token, use the token. It's the Resource Server's job to validate it and determine if access should be granted.

A Deep Dive into OAuth 2.0 Grant Types

OAuth 2.0 is a framework, not a rigid protocol. It defines several different "grant types" (or flows) for obtaining an access token, each suited for different types of clients. Understanding the comparison of OAuth 2.0 Grant Types is essential for any developer working with API security.

Grant Type	Client Type	Description	Pros	Cons & Security Notes
Authorization Code Grant	Web Apps (Confidential Clients)	The most common and secure flow. The user is redirected to the Authorization Server, logs in, consents, and is redirected back to the client with an `authorization_code`. The client's back-end then securely exchanges this code for an access token and (optionally) a refresh token.	Extremely secure. The access token is never exposed to the user's browser. Supports refresh tokens.	Requires a back-end component to handle the code-for-token exchange. The standard for web applications.
Authorization Code with PKCE	Mobile & Single-Page Apps (Public Clients)	An extension of the Authorization Code grant. Before the flow starts, the client generates a secret (`code_verifier`) and a transformed version (`code_challenge`). The `code_challenge` is sent in the initial request. When exchanging the `authorization_code`, the client must also send the original `code_verifier`. This prevents authorization code interception attacks.	The current best practice for all clients, including SPAs and mobile apps. Mitigates CSRF and code interception risks.	Slightly more complex to implement than the original, but libraries handle this well. It has essentially made the Implicit Grant obsolete.
Implicit Grant (Legacy)	Single-Page Apps (Public Clients)	(Largely deprecated) A simplified flow where the access token is returned directly to the client in the URL fragment after the user authenticates and consents.	Simple to implement as it has no back-end requirement.	Considered insecure now. The access token is exposed in the browser's URL and history. It does not support refresh tokens, leading to poor user experience or security compromises. Do not use this for new applications. Use Authorization Code with PKCE instead.
Client Credentials Grant	Machine-to-Machine (M2M)	Used for non-interactive, back-end service communication. The client authenticates itself directly with the Authorization Server using its `client_id` and `client_secret` to get an access token. There is no end-user involvement.	Simple and perfect for server-to-server API calls where the client is the "user."	The `client_secret` must be stored securely. Not applicable for scenarios involving a human user. The token represents the client itself, not a user.
Resource Owner Password Credentials Grant (Legacy)	Trusted First-Party Apps	(Highly discouraged) The client directly collects the user's username and password and sends them to the Authorization Server to get an access token.	Useful for migrating legacy applications that use username/password to an OAuth 2.0 system.	Breaks the core principle of delegated authority. The client sees the user's credentials. It should only be used for highly trusted, first-party applications where redirect-based flows are not possible.

For modern application development, the choice is clear: use the Authorization Code grant with the PKCE (Proof Key for Code Exchange) extension for any client involving a user, whether it's a traditional web app, a Single-Page Application (SPA), or a native mobile app. For machine-to-machine communication, use the Client Credentials grant.

The Authentication Gap

So, we've gone through the OAuth 2.0 flow. Our client application now has an access token. It can successfully call the Resource Server (e.g., the Google Photos API) to fetch photos. We have achieved Authorization. But a critical question remains: who is the user that authorized this?

The client application has no standard, reliable way of knowing. Sure, it could call a `/userinfo` or `/me` endpoint on the Resource Server using the access token, but:

This requires an extra network round trip, adding latency.
The format of the response from this endpoint is not standardized. Google's `/me` endpoint will look different from GitHub's, which will look different from Facebook's.
Most importantly, the client only has the access token. It cannot verify that the token was genuinely issued for itself or that the user is who they say they are without asking the Authorization Server again.

This is the fundamental gap. OAuth 2.0 provides the "what" (what the client can do) but not the "who" (who the user is). This is where OpenID Connect (OIDC) comes in. This is the source of the difference between ID Token and Access Token.

Enter OpenID Connect (OIDC): The Identity Layer on Top of OAuth 2.0

OpenID Connect (OIDC) is not a replacement for OAuth 2.0. It is a thin identity layer that sits directly on top of it. Think of it as OAuth 2.0++, a superset. An OIDC-compliant provider is an OAuth 2.0 provider, but one that provides additional, standardized features for authentication.

OIDC's primary goal is to provide a standard, verifiable way for a client to learn about the identity of the end-user. It achieves this by introducing a new artifact: the ID Token.

How does it work? It cleverly extends the OAuth 2.0 flows you already know. To turn a standard OAuth 2.0 request into an OIDC request, the client adds a specific scope to the authorization request: openid.

When the Authorization Server sees the openid scope, it knows the client wants to perform authentication, not just authorization. As a result, when the client exchanges the authorization code, the Authorization Server returns not only an `access_token` but also an `id_token`.


// Typical response from the Token Endpoint in an OIDC flow
{
  "access_token": "a_very_long_opaque_string_for_the_api",
  "token_type": "Bearer",
  "expires_in": 3599,
  "refresh_token": "another_long_secret_string",
  "scope": "openid profile email",
  "id_token": "a_very_long.jwt_string.for_the_client" 
}

Now our client has two distinct tokens. The `access_token` serves the exact same purpose as before: it is to be sent to the Resource Server (API) to access protected resources. The new `id_token` has a completely different purpose and audience. It is a JSON Web Token (JWT) that contains identity information about the end-user. This token is intended to be consumed and validated by the Client Application.

The Core of OIDC: The ID Token

The ID Token is the prize of the OIDC flow. It is a cryptographically signed assertion from the Authorization Server about an authentication event. It states that a specific user was authenticated at a specific time and provides basic profile information about them. It is always represented as a JWT.

A JWT has three parts, separated by dots: `header.payload.signature`.

Header: Contains metadata about the token, such as the signing algorithm used (`alg`, e.g., `RS256`) and the token type (`typ`, which is `JWT`).
Payload (Claims): Contains the actual information about the user and the authentication event. These are called "claims."
Signature: A cryptographic signature created using the header, the payload, and a secret key held by the Authorization Server. The client can use the Authorization Server's public key to verify this signature, proving that the token is authentic and has not been tampered with.

Let's look at the decoded payload (the claims) of a typical ID Token:


{
  "iss": "https://accounts.google.com", // Issuer: Who issued the token.
  "sub": "110169484474386276334", // Subject: A unique, stable identifier for the user.
  "aud": "1234567890-abcdefg.apps.googleusercontent.com", // Audience: Who the token is intended for (your client_id).
  "exp": 1678886400, // Expiration Time: When the token expires (as a Unix timestamp).
  "iat": 1678882800, // Issued At Time: When the token was issued (as a Unix timestamp).
  "nonce": "a_random_unguessable_string", // Nonce: A value provided by the client in the initial request to mitigate replay attacks.
  "name": "John Doe",
  "email": "john.doe@example.com",
  "email_verified": true,
  "picture": "https://example.com/johndoe.jpg"
}

These claims provide the client with everything it needs for authentication:

`iss` (Issuer): Identifies the Authorization Server that issued the token. The client MUST validate that this is the expected issuer.
`sub` (Subject): The user's unique ID. This is the primary identifier and should be used to key the user's account in the client's database. It is stable and will not change.
`aud` (Audience): Identifies the client for which this token was issued. This is critical. The client MUST validate that its own `client_id` is present in the `aud` claim. This prevents a malicious client from tricking a user into logging in and then forwarding that user's ID Token to your application.
`exp` (Expiration Time) & `iat` (Issued At Time): Standard claims for validating the token's lifetime. The client must ensure the token is not expired.
`nonce` (Number used once): An optional but highly recommended security measure. The client generates a random, unique string and includes it in the initial authorization request. The Authorization Server then includes this same nonce in the ID Token. The client must validate that the `nonce` in the token matches the one it originally sent. This prevents replay attacks where an attacker could capture an ID Token from a previous flow and try to use it to log into the client.

With these claims, the client can, without any further network calls, perform a series of checks locally:

Verify the JWT's signature using the provider's public key.
Verify that the `iss` claim matches the provider.
Verify that the `aud` claim contains its own `client_id`.
Verify that the token is not expired (`exp` claim).
Verify the `nonce` to prevent replay attacks.

If all these checks pass, the client can trust the information in the payload. It knows with cryptographic certainty that the user with the subject ID `11016...` has successfully authenticated with `https://accounts.google.com`, and this token was intended for this specific client. The client can now create a session for "John Doe" and consider him logged in. Authentication is complete.

The Ultimate Showdown: ID Token vs. Access Token

Now that we have explored both tokens in detail, let's put them side-by-side. This table crystallizes the difference between ID Token and Access Token and should serve as your definitive reference.

Attribute	ID Token	Access Token
Primary Purpose	Authentication: To prove that a user has been authenticated.	Authorization: To grant access to a specific resource (API).
Intended Audience	The Client Application. It is consumed and validated by the application that initiated the login flow.	The Resource Server (API). It is consumed and validated by the API being called.
Standardized Format	Always a JWT (JSON Web Token). Its structure and a set of standard claims are defined by the OIDC specification.	No standardized format. Can be a JWT, but can also be an opaque string (a "bearer" token). The client should not care about its internal structure.
Content / Claims	Contains identity claims about the user (`sub`, `name`, `email`) and metadata about the authentication event (`iss`, `aud`, `exp`, `iat`, `nonce`).	If it's a JWT, it contains authorization information like scopes (`scp`), user ID (`sub`), and client ID (`cid`). Its content is primarily for the Resource Server's access control logic.
How It's Used	The client decodes and validates it to identify the user, create a session, and display personalized information (e.g., "Welcome, John Doe").	The client attaches it to the `Authorization: Bearer [token]` header of an API request to a Resource Server.
Source Protocol	Defined by OpenID Connect (OIDC).	Defined by OAuth 2.0.
Can it be used for API access?	Absolutely not. A Resource Server should reject any request that uses an ID Token as a bearer token. The `aud` claim will not match the Resource Server's identifier.	Yes, this is its only purpose.
Can it be used for user authentication?	Yes, this is its only purpose. After validation, the client knows who the user is.	Absolutely not. A client must not decode an access token to identify the user. Its audience is the API, not the client, and its format isn't guaranteed. Using it for authentication is a critical security flaw.

A simple way to remember: The ID Token is for the front door of your application to let the user in. The Access Token is the keycard the user gives your application to open specific, locked rooms (API endpoints) on their behalf.

Putting It All Together: How to Implement Social Login

Let's walk through a concrete example of a "Login with Google" button on a web application, which beautifully illustrates how OIDC and OAuth 2.0 work in concert. This is a common question for developers: how to implement social login correctly.

Scenario: A web app ("MyWebApp") wants to let users log in with their Google account and also access their Google Calendar events.

Step 1: The User Initiates Login. The user clicks "Login with Google". MyWebApp's front-end redirects the user's browser to Google's Authorization Server endpoint with specific parameters.
```
https://accounts.google.com/o/oauth2/v2/auth?
 response_type=code&
 client_id=YOUR_CLIENT_ID.apps.googleusercontent.com&
 scope=openid%20profile%20email%20https://www.googleapis.com/auth/calendar.readonly&
 redirect_uri=https://mywebapp.com/auth/callback&
 state=RANDOM_CSRF_TOKEN&
 nonce=RANDOM_NONCE_VALUE&
 code_challenge=PKCE_CHALLENGE&
 code_challenge_method=S256
    
```
Notice the scopes: `openid`, `profile`, `email` are standard OIDC scopes requesting an ID Token with profile info. `https://www.googleapis.com/auth/calendar.readonly` is an OAuth 2.0 scope requesting authorization to read calendar data. We also include security parameters like `state`, `nonce`, and PKCE challenge.
Step 2: User Authentication and Consent. Google's server receives the request. The user, if not already logged into Google, enters their credentials. Google then presents a consent screen: "MyWebApp would like to: See your name, email, and profile picture. View your calendars." The user clicks "Allow".
Step 3: Redirection with Authorization Code. Google redirects the user's browser back to the `redirect_uri` specified by MyWebApp, including an `authorization_code` and the original `state` value in the query parameters.
```
https://mywebapp.com/auth/callback?code=AUTHORIZATION_CODE&state=RANDOM_CSRF_TOKEN
    
```

Step 4: Code-for-Tokens Exchange. MyWebApp's back-end server receives this request. It first validates that the `state` value matches the one it generated in Step 1 to prevent CSRF attacks. Then, it makes a direct, server-to-server POST request to Google's token endpoint.


POST /oauth2/v4/token HTTP/1.1
Host: www.googleapis.com
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code&
code=AUTHORIZATION_CODE&
redirect_uri=https://mywebapp.com/auth/callback&amp;
client_id=YOUR_CLIENT_ID&
client_secret=YOUR_CLIENT_SECRET&
code_verifier=ORIGINAL_PKCE_VERIFIER

Step 5: Receiving the Tokens. Google validates everything—the code, client secret, redirect URI, and the PKCE verifier. If all is correct, it returns a JSON object containing both the `id_token` and the `access_token`.
```
{
  "access_token": "ya29.A0AR...FOR_CALENDAR_API",
  "expires_in": 3599,
  "scope": "...",
  "token_type": "Bearer",
  "id_token": "eyJhbGciOiJ...THIS_IS_THE_ID_TOKEN"
}
    
```
Step 6: Processing the Tokens (The Crucial Part). Now MyWebApp's back-end has two distinct tokens and must handle them correctly.
- The ID Token: The back-end will parse and validate the `id_token`. It checks the signature, issuer, audience, expiration, and nonce. If valid, it extracts the `sub` (Google's user ID), `email`, and `name`. It can now look up this user in its own database (or create a new user account if one doesn't exist) and establish a session for the user. The user is now logged in to MyWebApp.
- The Access Token: The back-end should encrypt and store this `access_token` (and any `refresh_token`) securely, associating it with the user's account. It should not attempt to read its contents.
Step 7: Using the Access Token. Later, when the user navigates to the "My Calendar" page in MyWebApp, the application's back-end retrieves the stored `access_token` for that user. It then makes a server-side API call to the Google Calendar API, including the token in the Authorization header.
```
GET /calendar/v3/calendars/primary/events HTTP/1.1
Host: www.googleapis.com
Authorization: Bearer ya29.A0AR...FOR_CALENDAR_API
    
```
Google's Calendar API (the Resource Server) validates the access token and, if valid, returns the user's calendar data. MyWebApp then renders this data to the user.

This flow perfectly demonstrates the separation of concerns. The ID Token was used once, at login time, by the client to establish the user's identity. The Access Token is used potentially many times, by the client on behalf of the user, to access a protected API.

Under the Hood: How JWT (JSON Web Token) Works

Since both ID Tokens and, often, Access Tokens are JWTs, a deep understanding of how JWT (JSON Web Token) works is non-negotiable for a modern developer. A JWT is a compact, URL-safe means of representing claims to be transferred between two parties. It's an open standard (RFC 7519).

The Three Parts of a JWT

A JWT looks like this: `xxxxx.yyyyy.zzzzz`

1. The Header (xxxxx)

This is a Base64Url-encoded JSON object that describes the token and the cryptographic operations applied to it.


// Decoded Header
{
  "alg": "RS256", // Algorithm: RSA Signature with SHA-256. This is an asymmetric algorithm.
  "typ": "JWT",   // Type: Always "JWT".
  "kid": "1a2b3c" // Key ID: An optional hint indicating which key was used to sign the token.
}

The `alg` claim is the most important. `RS256` (asymmetric) is common and secure, meaning the token is signed with a private key and verified with a public key. Another common algorithm is `HS256` (symmetric), where the token is signed and verified with the same shared secret. Asymmetric algorithms are generally preferred for public-facing authorization servers.

2. The Payload (yyyyy)

This is a Base64Url-encoded JSON object containing the claims. Claims are statements about an entity (typically, the user) and additional metadata. There are three types of claims:

Registered Claims: A set of predefined claims that are not mandatory but recommended. We've seen these: `iss` (issuer), `sub` (subject), `aud` (audience), `exp` (expiration time), `iat` (issued at), `jti` (JWT ID).
Public Claims: These are claims that can be defined at will by those using JWTs. To avoid collisions, they should be defined in the IANA JSON Web Token Registry or be specified as a URI that contains a collision-resistant namespace.
Private Claims: Custom claims created to share information between parties that agree on using them. For example: `{"https://mywebapp.com/roles": ["admin", "editor"]}`.

3. The Signature (zzzzz)

This is the part that provides security. The signature is created by taking the encoded header, the encoded payload, a secret (for HS256) or a private key (for RS256), and signing them with the algorithm specified in the header.

The formula is essentially:


const signature = HMACSHA256(
  base64UrlEncode(header) + "." + base64UrlEncode(payload),
  secret
);

When the recipient (the Client for an ID Token, the Resource Server for an Access Token) gets the JWT, it can re-compute the signature using the header, payload, and the public key/secret it knows. If the computed signature matches the signature part of the JWT, the recipient knows two things:

Authenticity: The token was genuinely created by the sender (who is the only one with the private key/secret).
Integrity: The header and payload have not been altered in transit. Any modification would invalidate the signature.

Important Security Note: The payload of a JWT is Base64Url-encoded, not encrypted. Anyone who intercepts a JWT can easily decode the header and payload and read their contents. Therefore, you must never store sensitive information (like passwords, social security numbers, etc.) in a JWT's payload. JWTs provide integrity and authenticity, not confidentiality. If you need confidentiality, you should use a standard like JWE (JSON Web Encryption).

OAuth 2.0 Best Practices from a Security Perspective

Implementing these protocols correctly is paramount for your application's security. Here are some essential OAuth 2.0 best practices from a security perspective, which also apply to OIDC.

Always Use PKCE: As stated before, use the Authorization Code grant with PKCE for all public clients (SPAs, mobile apps). This is the current industry best practice.
Validate Everything in an ID Token: Do not just decode the ID token and trust its contents. You MUST validate the signature, issuer (`iss`), audience (`aud`), expiration (`exp`), and nonce. Most mature JWT libraries provide a single function to do all of this.
Secure Token Storage on the Client: This is a highly debated topic.
- Local Storage / Session Storage: Do not store tokens here. They are accessible via JavaScript, making them vulnerable to XSS (Cross-Site Scripting) attacks. An attacker who injects malicious JS into your page can steal the tokens.
- HTTP-Only, Secure Cookies: This is a much better option for traditional web apps. The token is stored in a cookie that cannot be accessed by JavaScript, mitigating XSS risks. You must also set the `Secure` flag (so it's only sent over HTTPS) and the `SameSite` attribute (usually to `Lax` or `Strict`) to mitigate CSRF attacks.
- In-Memory: For SPAs, storing tokens in memory within a JavaScript closure is the most secure client-side option against XSS. However, the tokens are lost on a page refresh, requiring a mechanism (like using a hidden iframe with a refresh token stored in a secure cookie) to silently re-authenticate.
Use Short-Lived Access Tokens: Access tokens should have a short lifetime (e.g., 15-60 minutes). This minimizes the damage if a token is leaked.
Use Refresh Tokens for Long-Lived Sessions: To maintain a long session without constantly asking the user to log in, use refresh tokens. Refresh tokens are long-lived, powerful credentials used to obtain new access tokens. They must be stored very securely (e.g., in your back-end database or in a secure, HTTP-Only cookie) and should only be sent to the token endpoint, never to a Resource Server. Implement refresh token rotation for added security.
Use the `state` and `nonce` Parameters: The `state` parameter is crucial for preventing CSRF attacks in redirect-based flows. The `nonce` parameter is crucial for preventing replay attacks in OIDC. Always use both.
Use Scopes for Least Privilege: Don't request more permissions than your application needs. If you only need to read a user's profile, only request the `openid profile` scopes, not scopes that grant write access to their files.

Conclusion: The Right Token for the Right Job

The confusion between the ID Token and the Access Token stems from a misunderstanding of their underlying protocols. OAuth 2.0 is about Authorization—granting access—and its currency is the Access Token, a key for an API. OpenID Connect is about Authentication—proving identity—and its currency is the ID Token, a verifiable statement for the client about the user.

As a developer, your mandate is clear:

Use an OIDC flow (by including the `openid` scope) to handle user login.
When you receive the tokens, use the ID Token on the client-side to authenticate the user and establish a session. Validate it thoroughly.
Use the Access Token to make calls to your protected APIs (Resource Servers). Treat it as an opaque string and pass it along in the `Authorization` header.

By respecting this fundamental separation of concerns, you will build applications that are not only more robust and maintainable but also significantly more secure. Getting this right is not just an implementation detail; it is a hallmark of a developer who truly understands modern application security architecture.