Guide to machine to machine (M2M) authentication

M2M authentication allows software and devices to have their own unique identities, similar to how employees are assigned unique user accounts. But instead of a person entering a password or approving a multifactor authentication (MFA) prompt, a client workload must prove that it controls the cryptographic credential attached to its assigned identity.

Modern IT architectures span hybrid cloud and multicloud environments that use and interact with thousands of nonhuman identities (NHIs)—digital identities that belong to software and devices instead of a human being. NHIs include microservices, daemons, application programming interfaces (APIs), continuous integration/continuous delivery (CI/CD) jobs, Kubernetes workloads, IoT devices, cron jobs and more recently, artificial intelligence (AI) agents.

These identities now significantly outnumber human users. Estimates range from 45:1 to 92:1. The services and platforms that live within an IT environment make many of their communication and data requests without a human being getting involved. Microservices call APIs, deployment pipelines modify cloud resources, scheduled jobs process sensitive data, agents invoke tools and devices send telemetry.

M2M authentication establishes a verifiable identity at each of these trust boundaries, helping ensure that machines accept requests only from legitimate, authorized workloads.

In enterprise computing environments, devices and systems are constantly making autonomous calls, and every recipient must be able to verify which workload is calling before it communicates with backend services. Machine-to-machine authentication is the intermediary process that helps autonomous systems verify NHIs and establish M2M communication.

Consider a service named “billing API” calling a customer profile API. In a zero trust ecosystem, the customer profile API cannot trust the request just because it originated from the same Kubernetes cluster or corporate network. Network settings might be misconfigured. An attacker who compromised one workload might already be inside the network. So, M2M authentication tools require the billing API to present verifiable evidence that it’s actually the billing API.

Authenticating each workload enables IT teams to implement a core zero trust principle: never trust, always verify. Without strong M2M authentication, any system that can reach an endpoint might be able to impersonate a legitimate service, steal data, trigger privileged operations or move freely through the environment.

M2M authentication also enables safe automation at cloud, Internet of Things (IoT) and AI scale, which requires interactions to run safely without a human being approving every call. M2M authentication systems require workloads to prove their identity to an identity provider (IdP)—such as Auth0—or platform-native identity service. In return, workloads receive short-lived, narrowly scoped, revocable credentials that dictate when and how they can interact with the network.

This dynamic reduces the risks that come with using embedded API keys or long-lived shared secrets and makes key rotation and revocation more manageable.

An effective M2M authentication design separates authentication—which asks, “which workload is calling?”—from authorization—which asks, “what is the authenticated workload allowed to do?”

Authentication does not inherently grant access. It establishes a reliable client name or identity to which access rules can be applied. Authentication is usually a prerequisite for machine-to-machine (M2M) authorization. A system must verify the client’s identity before it can grant the client access to anything.

M2M authorization occurs after the target system knows the client’s identity. It evaluates policies, scopes, roles, claims and contextual conditions to decide whether the identity can perform the requested action.

Authentication facilitates trustworthy attribution, and authorization enables least-privilege access, so enterprise IT architectures depend on both capabilities.

M2M authentication typically involves four components.

1. A workload client: The application, job, device, AI or service that makes the request.
2. A credential or key: A secret, private key, certificate, hardware-bound device key or federated workload identity assertion used to prove control of an identity.
3. An identity or authorization server: The system that validates workload credentials and issues a proof of identity (often an access token).
4. A resource server: The target—or recipient—API, service, message broker or cloud control plane.

At a high level, M2M communication and authentication flows follow the same basic sequence.

• The workload proves its identity to an authority. The workload client presents some credential or key, along with its client ID, to an identity or authorization service that the environment trusts.

• The authority validates the workload and issues a proof of identity. The identity or authorization service validates the credential and, if security policies allow, issues a temporary, audience-bound proof of identity.

• The workload calls the target service with that proof. The authorization service attaches proof of identity to the workload to contact a protected API, service or resource.

• The target service verifies the proof and enforces authorization. The resource server validates the proof, checking the signature, issuer, audience and expiry time. Then, it applies authorization rules (scopes, roles, policies) to decide whether the requested operation is permitted.

Let’s use the example of a company’s nightly backup system. Every night at 11p.m., backup servers need to copy data from the company’s file server to secure backup storage. There are no employees sitting at a workstation to approve data requests at that hour, so the backup process uses M2M authentication.

First, the backup job identifies itself. Then, it requests permission to communicate with the file server by presenting its credential to the company’s IdP. It asks for a short-lived access token or refresh token to use the backup storage service.

The IdP checks the credential, confirming that the request is really from “backup-service.” It also checks whether the service is permitted to write backups to the designated storage location. If approved, the backup job receives a token that is valid for a limited time and intended only for the backup storage system.

The job connects to the backup storage service, sending the necessary files along with the token. The storage service confirms that the token is authentic, came from the company’s trusted identity system, has not expired and was issued specifically for backup storage.

The storage service checks permissions and verifies that “backup-service” is allowed to upload backup files (but not, for example, delete all historical backups or access unrelated production data). If the job passes all checks, the request is allowed and the storage service stores the files. If the token is expired, forged, meant for a different system or lacks the necessary permission, the storage service rejects the request and the files.

Whereas M2M authentication verifies the identity of a workload, user authentication verifies the identity of a human being interacting with a software application.

User authentication normally involves interactive steps, such as a single sign-on (SSO) login, a passkey, an MFA challenge or device approval.

Enterprises can use a range of M2M authentication patterns to verify NHIs.

API keys—which enable APIs to identify a caller—use static client secrets to grant long-lived access to whoever possesses the secret.

Using API keys is the simplest M2M authentication pattern. The client typically includes an opaque, pre-provisioned character string—the secret—in an HTTP header or as a parameter in a query string. The receiving API gateway verifies that the secret matches an enabled credential. The API then maps the key to a client identity, permissions, rate limits and other policy attributes.

This authentication approach is suitable for simple internal integrations, prototypes and lower-risk legacy APIs, because they do not require granular permissions. However, with static API keys, possession is sufficient for use, so any machine with the key can impersonate the workload until the key is revoked or expires.

With the OAuth 2.0 client credentials grant flow, a confidential client authenticates to an authorization server’s token endpoint and requests an access token for a specific resource server, audience or scope. 

The workload client presents its client ID and a certificate or secret. The authorization server issues and signs a short-lived token (often a JSON Web Token), which the workload then presents to the target API. The target API validates the token and authorizes the requested operation based on the included permission information.

OAuth 2.0 client credentials—a standard pattern for service-to-service APIs—work well for internal APIs, software as a service (SaaS) integrations and platform ecosystems, because it separates credential validation from API authorization. This configuration lets teams upgrade security protocols without disrupting APIs. The token issuance mechanism can also mature independently of the API contract.

Furthermore, with the OAuth 2.0 client credentials flow, a stolen token cannot be used without the corresponding certificate or key, so it’s a harder pattern for hackers to abuse.

To complete this M2M authentication pattern, the client creates a short-lived JSON Web Token and signs the token with a private key.

JWTs comprise a header, a payload and a signature. Typically, they identify the client as their issuer and subject and the authorization server or token endpoint as their audience. They also include the token’s expiration time and often carry a unique token identifier to help prevent replay.

To initiate the authentication process, the client essentially says, “I am X service, and I am requesting a token from this authorization server right now,” and signs off on the message. The resulting signed message is called a JWT client assertion.

The authorization server uses public key infrastructure (PKI) to verify the client assertion, and if verification succeeds, the server issues an access token and authentication is complete.

Private key JWT client assertion provides a stronger version of the OAuth client credentials grant type that avoids transmitting a reusable client secret. This pattern is useful in environments where secret distribution is either difficult or undesirable (multitenant platforms, Kubernetes pods, cross-cloud workloads, B2B integrations).

Furthermore, the authorization server only requires the public key. If the authorization server were compromised, hackers can obtain the public key, but a public key cannot create a valid signature or be feasibly converted into the matching private key. Therefore, the attacker still cannot generate their own client assertion.

Mutual TLS authenticates both sides of an M2M TLS connection. With ordinary TLS, the client validates the authorization or resource server’s certificate. mTLS adds an additional step where the server requests and verifies the client certificate in return. This process enables servers to establish the calling workload’s identity during the transport handshake.

mTLS gives every connecting system a verifiable machine identity and can encrypt connections at the same time. mTLS enables services to trust only specific clients with X.509 certificates issued by a trusted certificate authority, instead of inherently trusting clients from the internal network or that know an IP address.

The mTLS authentication pattern is particularly effective for identity and access management (IAM) in east-west service traffic, service meshes, high-assurance internal systems and B2B integrations.

Workload identity federation is a secretless M2M authentication pattern where clients use a trusted identity from their execution environment to obtain temporary credentials for another system. Instead of storing an OAuth certificate or static API key, the client obtains a short-lived, cryptographically signed identity token from its platform at runtime. The client exchanges the token with a target IdP, security token service or authorization server.

For example, a Kubernetes pod receives a projected service account token from a Kubernetes API server. A cloud provider or external IdP validates the token against a configured trust relationship and issues temporary cloud credentials (for cloud IAM systems) or an API access token.

Workload identity federation works well for Kubernetes, cloud workloads, CI/CD pipelines, cross-cloud automation and cross-organization access. These systems create lots of short-lived, distributed, high-privilege execution points. Using long-lived credentials at each execution point would increase the likelihood of credential leakage, make credential rotation more difficult, and potentially give hackers persistent access.

With workload identity federation, there is no credential to handle.

Signed request M2M authentication binds authentication to an individual HTTP request.

The client creates a canonical representation of key request components, such as HTTP method, URI path, query parameters, authorization headers, timestamp, nonce and body hash. It then uses HMACs to compute a cryptographic signature over the representation. The resource server reconstructs the same canonical request and verifies the client’s signature before accepting the call.

HMAC-based request signing uses a symmetric secret or key shared by the client and verifier, which helps protect signed request components from tampering and limit credential replay. It’s commonly used for cloud control planes, webhooks, payment integrations and APIs where request integrity is as important as caller authentication.