What Is mTLS? A 4-Minute Look at TLS Mutual Authentication
Mutual TLS authentication is a win-win scenario for digital security — here’s what it is and how it aids your company’s zero-trust initiatives
Using transport layer security (TLS) on its own isn’t enough to protect your most trusted resources. If you want to add another layer of security to your systems, mutual TLS (mTLS) ensures only legitimate users can connect.
Mutual TLS authentication has gained traction over the past decade. It provides a higher level of security for your company and its most sensitive data. Some popular companies that use mTLS include Mastercard, which uses mutual TLS authentication to help secure its developer APIs, and Skype for Business Server, which uses it to secure server-to-server communications and IMs.
But what is TLS mutual authentication and how does it improve the security of your internal sites and resources? Get ready to start the clock…
Let’s hash it out.
What Is Mutual Authentication in TLS (mTLS)?
Mutual TLS authentication, also known as two-way authentication, is the process of two parties verifying each other’s identities to establish a secure, encrypted TLS connection. These authenticating entities can be users, devices, and servers. This differs from a standard TLS connection in which only the server’s identity is verified.
This identification verification process occurs when a website and the web client connecting to it (e.g., a browser like Chrome or Firefox) present their verified digital identities to the other before fully connecting. By “verified digital identities,” we mean X.509 digital certificates, and by “digital certificates,” we mean:
- Website security certificates (SSL/TLS certificates) for the server
- Device certificates, or user or client authentication certificates for the connecting entity.
Here’s a quick overview of how this process works:

The Role of Digital Certificates in Secure Communications
These digital certificates are critical elements of public key infrastructure (PKI), which makes secure online communications possible via otherwise potentially insecure networks.
They provide public key signatures from the trusted certification authorities (CAs) that issued them. Depending on how each certificate is used, the CAs are either public (for external connections) or private (for internal uses only).
How mTLS Works
This TLS authentication process, known as a TLS handshake, takes place on the backend when a user connects to a website. Traditionally, this one-way authentication process involves a browser verifying that the web server it’s connecting to is legitimate before establishing a secure connection.
Mutual TLS authentication takes this a step further by having the connecting user’s web client authenticate to the browser to provide two-way authentication.
Here’s a visual overview that demonstrates the differences between these two processes:

Want a closer look at how the TLS mutual authentication process works in the TLS handshake? Say no more:

Why Companies Use mTLS
It all boils down to two key points: passwordless authentication and stringent security. Password security can be tricky: security misconfigurations can happen, users and password admins don’t always follow best practices (e.g., not using password salting before hashing secrets), and other things can go wrong.
mTLS provides companies with more robust authentication and digital security than traditional usernames and passwords can provide. We’ve all seen what happens when passwords are stored insecurely:
- A $600,000 Reminder to Not Save Your Passwords on Post-It Notes
- Don’t Let These Password Cracking Attacks Catch You Off Guard
- Compromised Credentials: 7 Ways to Fight Credential Attacks
Using PKI digital certificates to authenticate users, devices, and applications means there are no passwords for bad guys to phish or steal via breaches and leaks.
Where You’ll Typically Find It in Use
Organizations often use these digital certificates to secure their internal APIs, apps, and sensitive systems. Mutual TLS is commonly used to secure the authenticated connections for a variety of internal systems, endpoints, and other resources, including:
- APIs, web apps, and microservices
- Hybrid or cloud-based services
- IoT devices, mobile devices, and users
- Physical (ID cards) and remote access (Wi-Fi and VPN)
mTLS Makes Zero Trust Possible…
Mutual TLS authentication is at the heart of a zero-trust architecture and strategy. It ensures that only verified, authorized users within your organization can access the resources you’ve intended them to use.
Let’s suppose Henry from HR wants to access your customer database. When you have mTLS and other access controls properly set up, he won’t be able to access it without you first assigning access privileges (authorization) and then proving his identity is legitimate (authentication).
Simply put, mTLS goes hand-in-hand with your identity and access management initiatives by authenticating the traffic both ways (i.e., to and from your most sensitive resources).
… But It Only Works If You Securely Manage Your Certificates
If you don’t properly manage and store your PKI assets (i.e., the cryptographic keys corresponding to your certificates), then mTLS won’t help. For that to happen, you have to closely track and manage each certificate and its keys so you know:
- How many you have
- Where they are
- Who manages them
- Who has access to them
- when they’re set to expire
Wrapping Up TLS Mutual Authentication
That’s it! We hope you’ve found this quick overview useful for understanding what mTLS is and how businesses are using it to secure access to their systems and data. It’s an important element of your zero-trust strategy and architecture that can’t be ignored.
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