With the proliferation of APIs driving modern digital experiences, the methods we use to secure these critical interfaces deserve careful scrutiny. Traditional approaches like simple API keys and OAuth 2.0's client secret model have served us well, but growing security threats demand more robust solutions. This post explores why organizations should transition to asymmetric authentication methods like mTLS and private key JWT – approaches that dramatically reduce the risk of credential theft and unauthorized access.
The Inherent Vulnerabilities of Simple API Keys
Traditional API keys represent the most basic form of authentication – a single string that grants access to your services. While straightforward to implement, they present significant security challenges:
Storage and Management Risks
API keys are often generated and stored in plaintext, creating an immediate vulnerability if no further encryption is applied. Once generated, the responsibility for secure storage falls entirely on developers who might resort to insecure practices – from environment variables to, in worst-case scenarios, sticky notes on monitors.
Limited Security Capabilities
Simple API keys function primarily as identifiers rather than robust security mechanisms. They provide limited authentication capabilities and should never be used as the sole method for securing important resources. When leaked, an API key essentially hands over your digital identity to attackers.
Widespread Exposure Vectors
API key leaks frequently occur through accidental exposure in public code repositories, hard-coded into applications, or inadvertently shared through third-party tools. This is such a common security failure that dedicated scanning tools and malicious bots continuously search public repositories for exposed API keys to exploit.
The consequences are severe: unauthorized access, data breaches, financial losses, and exploitation of services – all while potentially generating massive usage charges on your account. According to some cybersecurity experts, API key hacking represents a silent yet growing threat to enterprise security, potentially leading to massive data breaches and unauthorized system access.
The Limitations of OAuth 2.0's Client Secret Model
Traditional OAuth 2.0 implementations rely on a client_id and client_secret pair for application authentication. While more structured than simple API keys, this approach still relies on a fundamental security weakness – a shared secret.
The Shared Secret Problem
The client_secret is essentially a password known only to the application and the authorization server. Like any shared secret, it becomes a liability the moment it's exposed. Once leaked, there's nothing preventing an attacker from impersonating your legitimate application. One challenge is that API keys are generic, not tied to a particular developer (and in some cases, a particular application) - this means that several parties can have access and the consequent ability to use keys in an unauthorized and often undetected manner.
Bearer Token Vulnerabilities
In traditional OAuth implementations, the resulting access tokens function as "bearer tokens" – whoever possesses the token can use it, regardless of whether they're the intended recipient. This creates significant risk if tokens are intercepted or leaked.
Compliance Challenges
In PCI DSS v4, clause 8.6.3 requires cycling of passwords for application and system accounts - if an organization's API falls within PCI scope, the client_secret would be captured by this requirement. The implications for organizations that have no automated capability - whether as resource host or resource consumer - are significant. Raidiam have worked with several customers to implement authentication methods which remove dependence on a shared secret, and provide for automated credential cycling.
→ Case Study: Discover how Raidiam accelerated onboarding and enhanced security for a leading card issuer.
The Asymmetric Authentication Alternative
Asymmetric authentication methods like mTLS and private key JWT offer a fundamentally more secure approach by eliminating shared secrets and establishing cryptographic proof of client identity.
mTLS: Two-Way Trust Verification
Mutual TLS (mTLS) extends standard TLS by requiring both parties to authenticate each other using X.509 digital certificates. Unlike traditional TLS where only the server proves its identity, mTLS requires clients to also present their own certificate during the TLS handshake.
This approach delivers several key benefits:
- Elimination of shared secrets: Authentication relies on public key cryptography rather than shared passwords8.
- Strong client identity verification: The client must possess the private key corresponding to its certificate, which is significantly harder to steal than a text-based secret, mainly because it isn't passed in an authentication request (unlike a client secret) and consequently never leaves the client domain.
- Certificate-bound access tokens: Perhaps most powerfully, mTLS allows for binding access tokens to specific client certificates, ensuring that even if access tokens are somehow leaked, they cannot be used without the corresponding private key.
Private Key JWT: Asymmetric Client Authentication
The private_key_jwt method delivers similar security benefits through a different mechanism. Instead of using certificates during the TLS handshake, clients generate and sign a JSON Web Token (JWT) with their private key. The authorization server then verifies this signature using the client's registered public key.
Key advantages include:
- No shared secrets: The client authenticates using its private key while the server verifies using the public key – no shared passwords exist to be compromised.
- Stronger repudiation: The asymmetric key signing provides better assurance of the client's origin after signature verification.
- Simplified key management: The authorisation server need not store client secrets, only public keys.
Certificate Binding: The Advanced Security Layer
Both mTLS and private_key_jwt enable powerful additional security through token binding – cryptographically tying access tokens to specific clients.
With mTLS token binding, the authorization server includes confirmation of the client's certificate thumbprint within the issued token. When the token is presented to a resource server, that server can verify not only that the token is valid, but that it's being presented by the same client it was issued to.
This directly addresses one of the most significant weaknesses in traditional OAuth implementations – the risk of token theft and reuse. Even if an attacker somehow obtains a valid access token, they cannot use it without also possessing the private key corresponding to the bound certificate.
Implementation Considerations
Adopting asymmetric authentication methods does require careful planning:
- Certificate management: Organizations need robust processes for certificate issuance, renewal, and revocation.
- Infrastructure adjustments: Support for mTLS may require updates to load balancers, API gateways, and other infrastructure components.
- Client support: Client applications must be updated to support these authentication methods.
However, the benefits far outweigh these challenges. Modern cloud platforms and API management solutions increasingly provide built-in support for asymmetric authentication, significantly reducing implementation complexity.
Conclusion
As API security threats continue to evolve, organizations must move beyond simple API keys and shared secrets. Asymmetric authentication methods like mTLS and private_key_jwt represent the security-focused future of API authentication – eliminating shared secrets, establishing strong client identity, and binding access rights to cryptographic proof of possession.
By embracing these more robust authentication methods, organizations can significantly reduce their risk of credential theft, unauthorized access, and data breaches while building a more resilient digital infrastructure for the future.
Implementing these advanced authentication methods is not just a security best practice – it's increasingly becoming a competitive necessity in a world where data breaches and API vulnerabilities carry enormous financial and reputational costs.
Further Reading:
- https://www.techtarget.com/searchsecurity/tip/API-keys-Weaknesses-and-security-best-practices
- https://www.oauth.com/oauth2-servers/client-registration/client-id-secret/
- https://www.form3.tech/blog/engineering/buckle-up-your-mtls-with-oauth-2-0-client-authentication
- https://docs.secureauth.com/ciam/en/private-key-jwt-client-authentication.html
- https://cloudentity.com/developers/blog/mtls_vs_dpop/
- https://learn.liferay.com/w/dxp/headless-delivery/using-oauth2/json-web-tokens-jwts/using-private-key-jwt
- https://support.anthropic.com/en/articles/9767949-api-key-best-practices-keeping-your-keys-safe-and-secure
- https://auth0.com/docs/get-started/authentication-and-authorization-flow/authenticate-with-mtls
- https://escape.tech/blog/how-to-secure-api-secret-keys/
- https://tetrate.io/learn/what-is-mtls/
- https://www.networkpoppins.com/blog/api-key-hacking-the-silent-threat-to-enterprise-cybersecurity
- https://docs.secureauth.com/ciam/en/oauth-2-0-mutual-tls-client-authentication--mtls-.html
- https://www.legitsecurity.com/aspm-knowledge-base/api-key-security-best-practices
- https://learn.microsoft.com/en-us/windows-server/security/token-binding/introducing-token-binding
- https://fidoalliance.org/fido-technote-the-growing-role-of-token-binding/
- https://www.keepersecurity.com/blog/2025/01/07/what-are-api-vulnerabilities/
- https://security.stackexchange.com/questions/112615/should-the-oauth-client-id-be-kept-confidential
- https://docs.secureauth.com/ciam/en/oauth-2-0-mutual-tls-client-authentication--mtls-.html
- https://developer.transmitsecurity.com/guides/user/auth_fapi_private_key_jwt/
- https://owasp.org/www-project-api-security/
- https://stackoverflow.com/questions/33436767/why-i-shouldnt-keep-client-secret-in-mobile-app-in-oauth-2-0-authorization-cod
- https://konghq.com/blog/engineering/zero-trust-oauth-2-0-mtls-client-authentication
- https://is.docs.wso2.com/en/7.0.0/guides/authentication/oidc/private-key-jwt-client-auth/
- https://www.wiz.io/academy/what-is-api-security
- https://learn.microsoft.com/en-us/entra/identity-platform/v2-oauth2-client-creds-grant-flow
- https://cloudentity.com/developers/blog/mtls_vs_dpop/
- https://www.authlete.com/kb/oauth-and-openid-connect/client-authentication/client-auth-private-key-jwt/
- https://www.okta.com/blog/2023/06/a-leap-forward-in-token-security-okta-adds-support-for-dpop/
- https://docs.criipto.com/verify/guides/privatekey-jwt/
- https://help.sap.com/docs/btp/sap-business-technology-platform/retrieving-access-tokens-with-mutual-transport-layer-security-mtls
- https://security.stackexchange.com/questions/206775/oauth2-client-credentials-flow-with-assertion-jwt-how-to-share-the-clients-pub
- https://www.ubisecure.com/access-management/private_key_jwt-or-client_secret/
- https://www.scottbrady.io/oauth/client-authentication
- https://auth0.com/docs/secure/application-credentials
- https://connect2id.com/learn/oauth-2-1
- https://build.fhir.org/ig/HL7/smart-app-launch/client-confidential-asymmetric.html
