What is SSL Pinning, and Why It’s Important?

SSL certificates secure the connection between servers and applications, making it an excellent means of reducing cyberattacks. However, it can be harmful if your SSL certificate is out-of-date or vulnerable to hackers.

SSL Pinning addresses this potential weakness of relying solely on certificates issued by a trusted CA. It prevents Man-in-the-Middle (MITM) attacks by hardcoding a server’s SSL/TLS certificate public key directly into an app.

But What is SSL pinning? This article will explain SSL pinning, including its varieties and how it works. But first, you must understand the basics.

What is SSL Pinning?

SSL pinning is a security technique used in mobile & web applications and devices to enhance the security of HTTPS connections. The application embeds an SSL/TLS certificate or public key directly within its code.

The application or device will contact the server for data whenever a user requests. The app then compares the hardcoded public key with the server’s certificate. If the key matches, secure connections will be made, and users can access data.

Types of SSL Pinning

SSL Pinning types allow you to pin an application’s certificate or public key to the application. However, which method to choose depends on your specific needs.

#1. Static Pinning

Static SSL pinning is where you hardcode a certificate or public key in the application. It improves the security of server-app connections and ensures users’ trust by only connecting a valid server. If the certificate or public key embedded in the app does not match the one presented by the server connection, it will not be secure, so the app will not establish communication.

#2. Dynamic Pinning

Dynamic pinning is an approach in which you don’t pin a certificate or public key to an app. Instead, the application will simply create a cache of the SSL certificate the server presents for the first time during an SSL handshake.

So, each time an app connects with the server, the same cached certificate or public key is used for validation. The certificates or security keys match to ensure secure communication between the server and the app. If you want to apply either of these types of SSL pinning, you can take two approaches.

Techniques Used in SSL Pinning

Depending on your approach, you can apply different types of SSL pinning. Two prominent approaches that businesses use are certificate pinning and public key pinning.

#1. SSL Certificate Pinning

This method is straightforward in implementation but has a drawback requiring continuous updates. SSL certificates have expiration dates; you must update the pinned one each time you renew the SSL certificate.

In this type of SSL pinning, you can pin the entire certificate into the application. This will allow you to embed the SSL certificate and ensure secure connections. During the runtime, the client side will compare the server’s certificate with the one embedded in the app to allow the connection.

Types of SSL Certificate Pinning

SSL certificate pinning can be categorized based on the type of certificates you pin.

#1. Leaf Certificate Pinning 

Pinning the leaf certificate increases the secure communication for specific domains or entities. However, Leaf certificates have a shorter expiry, so you must update the certificate regularly.

#2. Intermediate Certificate Pinning 

Intermediate certificates are crucial for CA trust. Pinning an intermediate certificate ensures the CA’s trustworthiness and validates it across the chain of trust.

#3. Root Certificate Pinning 

A Root certificate pinning is the process of embedding the public key of the root certificate; if the certificates do not match during validation, it searches for the appropriate CA in the trust chain until a trusted root certificate is reached.

#2. Public Key Pinning

Public key pinning is a process where a security key is embedded instead of an SSL certificate. This addresses the downside of certificate pinning, which requires updating each time certificates are renewed.

Public key pinning involves storage of the public key of a trusted server stored on the app. During the SSL/TLS handshake the stored public key is matched with the one that server offers for successful connection.

If the public key from server and other once stored in app files matches, secure connection is established. If it doesn’t match, the connection is terminated.

How SSL Pinning Works?

SSL pinning leverages the principle of questioning the certificate and its validation claims for each connection. It is a process where applications do trust certificates but only specific ones.

Apps have a list of pre-determined and trustworthy certificates. It will validate a server’s SSL certificates, compare them with the list, and validate. Here is a step-by-step process of SSL pinning.

Step 1: Embedding the certificate in the application

In this step, you embed a copy of the server’s SSL certificate or public key to the application during development. This allows the app to compare the embedded certificate or public key with the original one.

Step 2: Establishing the connection between the server and the app

The application tries to establish a secure connection with the server through validation. The server presents an SSL certificate, and the app matches the embedded one to establish a connection.

Step 3: The validation process

If there is no SSL pinning, validation occurs on a fundamental level. The certificate is considered valid if it is issued by a trusted Certificate Authority (CA). But if a public key or certificate is pinned, it’s compared with the server certificate as an additional verification step.

Step 4: Final decision on connection

The connection is deemed secure if the embedded certificate or public key matches the server certificate or public key.

Benefits of SSL pinning

SSL pinning comes with various advantages. Let’s explore some of these benefits in detail:

1. Enhanced Security

   SSL pinning can improve security by inserting the certificate of the respective server or its public key into our application early enough so that if an attacker attempts to push a fake certificate through a system, the application can recognize that it has to use just those initially loaded certificates.

   Such a move blocks out any unauthorized entrants while safeguarding communications from alterations or unauthorized access, thus raising our system’s degree of resistance to online dangers.

2. Mitigation of Certificate Spoofing

   Certificate spoofing occurs when a hacker presents a fake SSL certificate to make the system trust an unauthorized entity, allowing them to intercept and manipulate secure communications.

   SSL pinning addresses this by locking the client to accept only a specific certificate or a certificate signed by a particular authority. If a hacker attempts to spoof a certificate, the system will reject it as it is not the designated certificate, significantly enhancing security.

3. Protection Against Reverse Engineering

   SSL pinning helps protect against reverse engineering, a technique used by hackers to analyze and replicate software.  Hard-coding the server’s certificate or public key into the application, a unique, unchangeable identity for the server is created. This proactive measure can alert you to potential attacks before they cause harm.

4. Defense Against API Abuse

   SSL pinning provides an extra layer of security against API abuse. Attackers exploit API weaknesses to gain unauthorized access and steal data. With certificate pinning, the app only communicates with the designated server, reducing the risk of MITM attacks.

5. Improved Trustworthiness

   SSL pinning can enhance an application’s trustworthiness by reassuring users that their data is handled securely. By binding the application to a specific certificate or public key, an extra level of security is demonstrated, which can increase user confidence and loyalty.

However, you must ensure that the certificates you use for the pinning are secure. The SSL pin can be damaged if your certificates are stolen or compromised. So, you must ensure that the certificate management mechanism is in place and works optimally.

Challenges and Limitations of SSL Pinning

Although SSL pinning increases security, it also has some disadvantages.

1. Certificate Changes

   The biggest challenge is handling changes in certificates. They are bound to change under various circumstances, such as expiring or revoking. Here, the app pins the certificate, and if it changes eventually, the application might break until it is updated—a service disruption that can frustrate users greatly.

2. Increased Maintenance

   SSL pinning increases maintenance requirements. You must keep tabs on expiration and renew the certificate in time. This means updating the app regularly to update on such changes, a process that can be time-consuming.

3. Deployment Complexity

   SSL pinning adds complexity to the deployment process. Having the correct certificates pinned and appropriately deployed on all the app’s environments (development, testing, production) becomes crucial. So, make sure to avoid such deployment complexity.

4. Limited Flexibility

   The downside to SSL pinning is that it can decrease flexibility. For example, if you need to change to another certificate authority or update your SSL configuration on the server, the pinned certificates will not be valid anymore. In such a case, a new app version would be needed to update the pinned certificates.

What are the Alternatives to SSL Pinning?

SSL pinning is an effective approach but is not the only option. There are other methods that you can use for better data security in your apps.

1. Certificate Transparency

   Certificate transparency is an approach where a public log provides all the issued certificates. This helps in improving the transparency and sets accountability for certificate issuance process.

   These logs have comprehensive information on all the CAs which are trustworthy and the certificates they have issued. So if your certificate is not in this log it will be deemed invalid or fraudulent.

2. HTTP Strict Transport Security (HSTS)

   HSTS is a web server directive that enables a secure connection between the website and the browser. When a user accesses the website, hackers can gain control of the connection, blocking 301 responses. HSTS asks the browser to redirect itself to HTTPS before responding.

   This means hackers can’t access a browser session and hack it using a 301 response because HSTS enforces HTTPS site-wide. This enhances the security of your websites just like SSL pinning but without its complexities.

3. OCSP Stapling

   Online Certificate Status Protocol Stapling or OCSP stapling is an alternate process allowing you to verify certificate validity dynamically. Organizations can check whether their certificates are valid through an assertive digital signature provided by servers. Here, you don’t need to rely only on CA’s credibility to ensure certificate validity because the digital signature does that for you.

Best Practices for SSL Pinning

SSL pinning is a technique to improve a mobile app’s security. It only trusts a selected number of certificates to ensure better security. It helps prevent man-in-the-middle attacks. Below are the best practices for implementing SSL pinning:

1. Use Strong Cryptographic Algorithms

   Use robust cryptographic algorithms when implementing SSL pinning. Hence, always go for SHA-256 over SHA-1. With more robust algorithms, better security is ensured, and even if an attacker tries to forge certificates, it’s harder to accomplish.

2. Pin the Public Key, Not the Certificate

   Pin the public key and not the certificate. A certificate can expire and subsequently be reissued, while a public key remains the same. By pinning the public key, there’s less need for frequent updates.

3. Include a Backup Key

   Always include a backup key to ensure your app works even if your primary key is compromised or updated. It provides an alternate layer of security and reliability.

4. Test Thoroughly

   Test your SSL pinning throughout the process. Simulate various scenarios, such as certificate expiration, revocation, and man-in-the-middle attacks, and ensure that your app behaves appropriately in each case.

5. Stay Updated

   Keep the pinned certificates/public keys up to date. Monitor any changes made in the server’s SSL/TLS configuration continuously and update the app. Information on SSL pinning must be kept current to ensure security.

6. Limit the Number of Pinned Certificates

   Pinning too many certificates could increase the risk of pinning failures. Focus only on the most critical ones to balance security and usability.

7. Use a Reliable Library

   Several libraries are available, which significantly eases the implementation. Choose a library that is well-maintained and widely used.

8. Monitor and Log

   Regular monitoring helps identify and follow up on issues promptly. It can also help detect any unusual activity indicating an attempted attack.

Conclusion

SSL pinning is a powerful security strategy that protects your apps from malicious attacks by verifying the integrity of the connection. Although it can be challenging to implement, its benefits are undeniable. Choosing the right option for your platform and updating certificates regularly for maximum security is essential.