Service Accounts vs API Keys vs OAuth Tokens: What Should Developers Use?

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Service Accounts vs API Keys vs OAuth Tokens: What Should Developers Use?

TTTech TeamPublished inAccessibility·APIs·Security·App Development·Mobile Web Development·
August 24, 2026
·Updated:August 25, 2026

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When the mobile client communicates with the backend, the backend communicates with third-party services, and, increasingly, the autonomous agents communicate with everything together.

API traffic now makes up more than 83 percent of all web traffic. How a client proves its identity has quietly become one of the most common decisions a backend developer runs into. There are three key terms behind such decisions, namely API keys, OAuth tokens, and service accounts. They are often used as synonyms when people discuss the issue casually. 

However, they do not address the same issue, and the confusion between them is among the main reasons for “API2:2023 Broken Authentication”

API Keys vs OAuth Tokens vs Service Accounts: What is the Difference?

To start with the comparison of use cases, the terminology should be sorted out, since these are completely different layers of the stack.

The differences between API keys – Oauth Tokens and Service Accounts

1. API key: This is a static, long-living credential transmitted via the header or the query parameter. An API key serves as both the caller’s identifier and a secret that identifies the caller, where the server verifies whether the passed string matches any value stored in the database.
2. OAuth 2.0 access token:OAuth 2.0 is a standardization, defined in RFC 6749, that is used for the issuance of short-lived, cryptographically verifiable credentials that possess the set of delegated rights in a certain scope, usually presented in the form of a JWT according to RFC 9068 with its expiry date, scopes, and audience included in the payload.
3. **Service account: **is an identity within the Identity and Access Management (IAM) system, representing the identity of an automated application or process that uses either a static key or a short-lived OAuth token as proof of its identity. There are actually two questions here: WHO is asking and HOW it proves its identity.

API Keys: Simple Authentication for API Usage

API keys are the simplest and most popular kind of API authentication mechanism used. Opaque strings, randomly generated and issued to clients by servers upon signing up, accompany each request thereafter.

How API Key Authentication Works

The server verifies the key through a database lookup, or by hashing, to determine the application/project/user that owns the API key. Since it is a shared secret, API keys must be transmitted

The standard method of transmitting the key would be through an HTTP header, either Authorization (with bearer token) or a custom X-API-Key header.

Don’t transmit your API keys as URL parameters. Query strings are routinely recorded in logs by proxies and servers.

const apiKey = process.env.ORDERS_API_KEY;

const response = await fetch(“https://api.example.com/v1/orders“, {

    “Authorization”: Bearer ${apiKey},

    “Content-Type”: “application/json”

When To Use API Keys In Development

Though having several shortcomings, using API keys is justified in cases where server-to-server communication within a secure network occurs, and there is a need to monitor which project makes requests for billing purposes.

Here, authentication does not come first, but rather tracking which project requested data. 

However, many older services still do not have any modern identity management systems installed; therefore, in some cases, it is totally acceptable to stick to API keys due to the costliness of the changeover.

The Security Risks of API Keys

While the same features make API keys convenient to use, they also make them vulnerable. Fixed. Persistent.
And often without any predefined permission boundaries. So that would mean that a stolen key will remain active for an indefinite period unless manually revoked.

The numbers are not tri65 million secrets were found hard-coded in public repositories on GitHub, an increase of 34% over the year before

Even more concerning, 64% of the valid secrets identified in 2022 were still active as of January 2026. Such secrets are discouraged by NIST SP 800-63B for high-assurance systems.

OAuth 2.0 Tokens: Delegated Access for Users and Applications

OAuth 2.0 (RFC 6749) has been developed in order to address the problem of static secrets with access tokens having short lifetimes.

Many people have misunderstood this and think that it has something to do with login.

In reality, however, it is all about the process of authorization or defining the scope of access for the client application.

OAuth 2.0 Flow Overview

Different types of access depend on whether a human is in the flow or not. When a human operates the application (be it a mobile application or a single-page application), the OAuth 2.0 Authorization Code grant is used, and Proof Key for Code Exchange (PKCE) extension is applied according to RFC 9700.

Where a back-end application cannot redirect a user, the Client Credentials grant type (RFC 6749, Section 4.4) comes into play.

The client authenticates to the authorization server using the client_id and client_secret and receives an access token which is valid only for a short time period. The client can use the secret as part of a JWT assertion with signature by key pair thanks to RFC 7523.

Using an OAuth Access Token

The resulting token is almost always a JWT, standardized by RFC 9068. In a machine-to-machine setup, using it takes two steps: request a token, then attach it to the API call.

const params = new URLSearchParams();

params.append(‘grant_type’, ‘client_credentials’);

params.append(‘client_id’, process.env.OAUTH_CLIENT_ID);

params.append(‘client_secret’, process.env.OAUTH_CLIENT_SECRET);

params.append(‘scope’, ‘orders:read’);

const tokenResponse = await fetch(“https://auth.example.com/oauth/token“, {

  headers: { ‘Content-Type’: ‘application/x-www-form-urlencoded’ },

const { access_token } = await tokenResponse.json();

const orders = await fetch(“https://api.example.com/v1/orders“, {

  headers: { ‘Authorization’: Bearer ${access_token} }

Scopes, Expiration, and Refresh Tokens

Three things separate OAuth tokens from API keys. 

1. First, they’re short-lived by design – the exp claim from RFC 7519 marks the exact moment a token stops working, often somewhere between 5 and 60 minutes.
2. Second, they carry a scope claim limiting what they can actually do, so a token requested for orders: read can’t write or delete records even if it leaks.
3. Third, RFC 9068 tokens include an aud claim naming the exact API they’re meant for, which blocks replay against a different service.

For machine-to-machine flows, refresh tokens are simply not relevant at all.

According to RFC 6749 section 4.4.3, the client already possesses its own credentials and only needs to request new tokens.

Service Accounts: Giving Applications Their OwnIdentity

API keys and OAuth tokens are credentials and protocols, respectively. Service accounts are at a completely different level; they are identities in the IAM system of cloud providers representing applications, workloads, containers, or processes.

What Is a Service Account?

Service accounts have to be created because delegating human credentials to the background process is fundamentally against security principles.

Humans move jobs and change their passwords. Microservices written by humans require fewer permissions compared to those of a human who writes them.

The traditional approach to authentication was wrong. People had to create service accounts, create a static JSON key, and then put it in an environment variable – practically turning one well-structured identity into an API key.

With modern practice, it is substituted by Workload Identity Federation, where the runtime creates tokens automatically.

Authenticating a Service-to-Service Request

In cloud native infrastructure such as GCP, AWS, Azure, or Kubernetes, the developer uses ADC or IRSA.

The cloud provider provides the credentials temporarily to the runtimeyone store secrets manually

Keyless Service Account Access

const { SecretManagerServiceClient } = require(‘@google-cloud/secret-manager’);

// Credentials are picked up automatically from the runtime (ADC).

// No key file or secret string appears anywhere in this code.

const client = new SecretManagerServiceClient();

const [version] = await client.accessSecretVersion({

  name: ‘projects/my-project/secrets/db-password/versions/latest’

const password = version.payload.data.toString(‘utf8’);

GitHub Actions uses the same pattern with AWS. The runner signs an OIDC JWT instead of using a stored access key, and AWS hands back a short-lived session token tied to an IAM role.

Why Least Privilege Matters

A service account should hold only the exact permissions its job calls for, mapped through central IAM policy rather than left up to application code. Skip this step and the blast radius widens.

But also, it shows up more and more with autonomous AI agents these days.

Researchers call it the confused deputy problem: an agent’s over-broad access lets an attacker trick it into reading production values and posting them somewhere public. The request looks authorized. Technically, it is.

Service Accounts vs API Keys vs OAuth Tokens

With the mechanics separated out, the differences line up pretty cleanly side by side.

1. What it represents: an API key is a static credential tied to a project. An OAuth token is a signed, delegated grant of permissions. A service account is a non-human identity inside an IAM system.
2. API keys generally don’t expire unless revoked by hand. OAuth tokens expire in minutes to hours through the exp claim. Service account sessions expire quickly and reissue on their own – that’s the expiration piece sorted.
3. Permission granularity: API keys are usually all-or-nothing. OAuth tokens carry fine-grained scope and aud claims. Service accounts get fine-grained roles through IAM policy.
4. Rotation: API keys need manual rotation. OAuth tokens rotate by protocol design. Service account credentials rotate automatically, with no key ever stored.
5. Main risk: API keys risk source code leaks. OAuth tokens risk theft and replay, addressed with DPoP or mTLS. Service accounts risk over-broad IAM roles and confused deputy attacks.

When to Use API Keys?

API keys should only be used on public services that are low risk where calling party identification is required for either rate limiting purposes or for charging but not for the purpose of securing sensitive data. 

API keys must not protect any sensitive user data or the internal control plane.

When to Use OAuth Tokens?

According to OAuth best practices, it can be used when the microservice requires crossing of a trust boundary, when the microservice is connecting to a third-party SaaS vendor, and when the application is acting on behalf of an actual human user.

When to Use Service Accounts

Use service accounts, specifically Workload Identity Federation, whenever an application talks natively to cloud platforms. 

Reading from an S3 bucket, writing to Cloud SQL, pulling a secret from a key vault – these are all cases where the environment’s own keyless identity beats managing credentials manually.

Common Credential and Authentication Mistakes

Even with well-documented standards available, the same mistakes keep showing up. Most of them, honestly, are easy to avoid once you know what to look for.

1. Hardcoding a secret directly in

This guarantees a leak the moment that code reaches version control.

// Bad: hardcoded and permanently exposed in git history

const STRIPE_API_KEY = “sk_live_1234567890abcdef”;

Pulling the key from an environment variable is a step up, but it’s not a full answer. Environment variables can still end up in logs, memory dumps, or a rogue process running on the same machine.

Production systems need a dedicated secrets manager — HashiCorp Vault or AWS Secrets Manager, for instance — with the workload authenticating to it using its own identity.

// Better, though still not a complete secrets strategy

const STRIPE_API_KEY = process.env.STRIPE_API_KEY;

2. Skipping scopes on an OAuth request 

This is the second most common mistake. Asking for a token without a scope hands back far more access than the client actually needs.

// Good: the token is limited to exactly what’s needed

3. Proxying a third-party API without usage limits 

rounds out the list. When an app authenticates once with an upstream vendor and calls it on behalf of every user, fake accounts can hijack that shared access and run up the bill fast.

Multi-tenant isolation, along with tying token creation to a verified account, closes that gap.

How Should Developers Choose?

How to choose between API keys – Oauth Tokens and Service Accounts

Honestly, it all depends on the system architecture, the trust boundary that needs to be traversed, and whether a human being is involved in the exchange.

1. Communicating with cloud providers (AWS, GCP, Azure, Kubernetes)? Use a service account via Workload Identity Federation and ADC; no need for static JSON keys.
2. **Backend microservices communicating with other services or SaaS applications? **Use Client Credentials grant from OAuth and scope it strictly according to RFC 9068. In finance or health care industries, use sender-constrained tokens via DPoP (RFC 9449) or mTLS (RFC 8705).
3. **Performing actions on behalf of the actual user? **Use Authorization Code grant from OAuth with PKCE according to RFC 9700. Neither an API key nor a service account can convey consent in the same way as this protocol.
4. Building a public, self-serve API for outside developers? API keys work here, with strict rate limits, anomaly checks, and a working key-rotation path in place.

Regardless of which one you pick, API security overall is moving away from static secrets and toward short-lived, cryptographically bound credentials.

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