How we ensure Cloudflare customers aren’t affected by Let’s Encrypt’s certificate chain change

Let’s Encrypt, a publicly trusted certificate authority (CA) that Cloudflare uses to issue TLS certificates, has been relying on two distinct certificate chains. One is cross-signed with IdenTrust, a globally trusted CA that has been around since 2000, and the other is Let’s Encrypt’s own root CA, ISRG Root X1. Since Let’s Encrypt launched, ISRG Root X1 has been steadily gaining its own device compatibility.

On September 30, 2024, Let’s Encrypt’s certificate chain cross-signed with IdenTrust will expire. After the cross-sign expires, servers will no longer be able to serve certificates signed by the cross-signed chain. Instead, all Let’s Encrypt certificates will use the ISRG Root X1 CA.

Most devices and browser versions released after 2016 will not experience any issues as a result of the change since the ISRG Root X1 will already be installed in those clients’ trust stores. That’s because these modern browsers and operating systems were built to be agile and flexible, with upgradeable trust stores that can be updated to include new certificate authorities.

The change in the certificate chain will impact legacy devices and systems, such as devices running Android version 7.1.1 (released in 2016) or older, as those exclusively rely on the cross-signed chain and lack the ISRG X1 root in their trust store. These clients will encounter TLS errors or warnings when accessing domains secured by a Let’s Encrypt certificate. We took a look at the data ourselves and found that, of all Android requests, 2.96% of them come from devices that will be affected by the change. That’s a substantial portion of traffic that will lose access to the Internet. We’re committed to keeping those users online and will modify our certificate pipeline so that we can continue to serve users on older devices without requiring any manual modifications from our customers.

A better Internet, for everyone

In the past, we invested in efforts like “No Browsers Left Behind” to help ensure that we could continue to support clients as SHA-1 based algorithms were being deprecated. Now, we’re applying the same approach for the upcoming Let’s Encrypt change.

We have made the decision to remove Let’s Encrypt as a certificate authority from all flows where Cloudflare dictates the CA, impacting Universal SSL customers and those using SSL for SaaS with the “default CA” choice.

Starting in June 2024, one certificate lifecycle (90 days) before the cross-sign chain expires, we’ll begin migrating Let’s Encrypt certificates that are up for renewal to use a different CA, one that ensures compatibility with older devices affected by the change. That means that going forward, customers will only receive Let’s Encrypt certificates if they explicitly request Let’s Encrypt as the CA.

The change that Let’s Encrypt is making is a necessary one. For us to move forward in supporting new standards and protocols, we need to make the Public Key Infrastructure (PKI) ecosystem more agile. By retiring the cross-signed chain, Let’s Encrypt is pushing devices, browsers, and clients to support adaptable trust stores.

However, we’ve observed changes like this in the past and while they push the adoption of new standards, they disproportionately impact users in economically disadvantaged regions, where access to new technology is limited.

Our mission is to help build a better Internet and that means supporting users worldwide. We previously published a blog post about the Let’s Encrypt change, asking customers to switch their certificate authority if they expected any impact. However, determining the impact of the change is challenging. Error rates due to trust store incompatibility are primarily logged on clients, reducing the visibility that domain owners have. In addition, while there might be no requests incoming from incompatible devices today, it doesn’t guarantee uninterrupted access for a user tomorrow.

Cloudflare’s certificate pipeline has evolved over the years to be resilient and flexible, allowing us to seamlessly adapt to changes like this without any negative impact to our customers.  

How Cloudflare has built a robust TLS certificate pipeline

Today, Cloudflare manages tens of millions of certificates on behalf of customers. For us, a successful pipeline means:

  • Customers can always obtain a TLS certificate for their domain
  • CA related issues have zero impact on our customer’s ability to obtain a certificate
  • The best security practices and modern standards are utilized
  • Optimizing for future scale
  • Supporting a wide range of clients and devices
  • Every year, we introduce new optimizations into our certificate pipeline to maintain the highest level of service. Here’s how we do it…

    Ensuring customers can always obtain a TLS certificate for their domain

    Since the launch of Universal SSL in 2014, Cloudflare has been responsible for issuing and serving a TLS certificate for every domain that’s protected by our network. That might seem trivial, but there are a few steps that have to successfully execute in order for a domain to receive a certificate:

  • Domain owners need to complete Domain Control Validation for every certificate issuance and renewal.
  • The certificate authority needs to verify the Domain Control Validation tokens to issue the certificate.
  • CAA records, which dictate which CAs can be used for a domain, need to be checked to ensure only authorized parties can issue the certificate.
  • The certificate authority must be available to issue the certificate.
  • Each of these steps requires coordination across a number of parties — domain owners, CDNs, and certificate authorities. At Cloudflare, we like to be in control when it comes to the success of our platform. That’s why we make it our job to ensure each of these steps can be successfully completed.

    We ensure that every certificate issuance and renewal requires minimal effort from our customers. To get a certificate, a domain owner has to complete Domain Control Validation (DCV) to prove that it does in fact own the domain. Once the certificate request is initiated, the CA will return DCV tokens which the domain owner will need to place in a DNS record or an HTTP token. If you’re using Cloudflare as your DNS provider, Cloudflare completes DCV on your behalf by automatically placing the TXT token returned from the CA into your DNS records. Alternatively, if you use an external DNS provider, we offer the option to Delegate DCV to Cloudflare for automatic renewals without any customer intervention.

    Once DCV tokens are placed, Certificate Authorities (CAs) verify them. CAs conduct this verification from multiple vantage points to prevent spoofing attempts. However, since these checks are done from multiple countries and ASNs (Autonomous Systems), they may trigger a Cloudflare WAF rule which can cause the DCV check to get blocked. We made sure to update our WAF and security engine to recognize that these requests are coming from a CA to ensure they’re never blocked so DCV can be successfully completed.

    Some customers have CA preferences, due to internal requirements or compliance regulations. To prevent an unauthorized CA from issuing a certificate for a domain, the domain owner can create a Certification Authority Authorization (CAA) DNS record, specifying which CAs are allowed to issue a certificate for that domain. To ensure that customers can always obtain a certificate, we check the CAA records before requesting a certificate to know which CAs we should use. If the CAA records block all of the CAs that are available in Cloudflare’s pipeline and the customer has not uploaded a certificate from the CA of their choice, then we add CAA records on our customers’ behalf to ensure that they can get a certificate issued. Where we can, we optimize for preference. Otherwise, it’s our job to prevent an outage by ensuring that there’s always a TLS certificate available for the domain, even if it does not come from a preferred CA.

    Today, Cloudflare is not a publicly trusted certificate authority, so we rely on the CAs that we use to be highly available. But, 100% uptime is an unrealistic expectation. Instead, our pipeline needs to be prepared in case our CAs become unavailable.

    Ensuring that CA-related issues have zero impact on our customer’s ability to obtain a certificate

    At Cloudflare, we like to think ahead, which means preventing incidents before they happen. It’s not uncommon for CAs to become unavailable — sometimes this happens because of an outage, but more commonly, CAs have maintenance periods every so often where they become unavailable for some period of time.

    It’s our job to ensure CA redundancy, which is why we always have multiple CAs ready to issue a certificate, ensuring high availability at all times. If you’ve noticed different CAs issuing your Universal SSL certificates, that’s intentional. We evenly distribute the load across our CAs to avoid any single point of failure. Plus, we keep a close eye on latency and error rates to detect any issues and automatically switch to a different CA that’s available and performant. You may not know this, but one of our CAs has around 4 scheduled maintenance periods every month. When this happens, our automated systems kick in seamlessly, keeping everything running smoothly. This works so well that our internal teams don’t get paged anymore because everything just works.

    Adopting best security practices and modern standards  

    Security has always been, and will continue to be, Cloudflare’s top priority, and so maintaining the highest security standards to safeguard our customer’s data and private keys is crucial.

    Over the past decade, the CA/Browser Forum has advocated for reducing certificate lifetimes from 5 years to 90 days as the industry norm. This shift helps minimize the risk of a key compromise. When certificates are renewed every 90 days, their private keys remain valid for only that period, reducing the window of time that a bad actor can make use of the compromised material.

    We fully embrace this change and have made 90 days the default certificate validity period. This enhances our security posture by ensuring regular key rotations, and has pushed us to develop tools like DCV Delegation that promote automation around frequent certificate renewals, without the added overhead. It’s what enables us to offer certificates with validity periods as low as two weeks, for customers that want to rotate their private keys at a high frequency without any concern that it will lead to certificate renewal failures.

    Cloudflare has always been at the forefront of new protocols and standards. It’s no secret that when we support a new protocol, adoption skyrockets. This month, we will be adding ECDSA support for certificates issued from Google Trust Services. With ECDSA, you get the same level of security as RSA but with smaller keys. Smaller keys mean smaller certificates and less data passed around to establish a TLS connection, which results in quicker connections and faster loading times.

    Optimizing for future scale

    Today, Cloudflare issues almost 1 million certificates per day. With the recent shift towards shorter certificate lifetimes, we continue to improve our pipeline to be more robust. But even if our pipeline can handle the significant load, we still need to rely on our CAs to be able to scale with us. With every CA that we integrate, we instantly become one of their biggest consumers. We hold our CAs to high standards and push them to improve their infrastructure to scale. This doesn’t just benefit Cloudflare’s customers, but it helps the Internet by requiring CAs to handle higher volumes of issuance.

    And now, with Let’s Encrypt shortening their chain of trust, we’re going to add an additional improvement to our pipeline — one that will ensure the best device compatibility for all.

    Supporting all clients — legacy and modern

    The upcoming Let’s Encrypt change will prevent legacy devices from making requests to domains or applications that are protected by a Let’s Encrypt certificate. We don’t want to cut off Internet access from any part of the world, which means that we’re going to continue to provide the best device compatibility to our customers, despite the change.

    Because of all the recent enhancements, we are able to reduce our reliance on Let’s Encrypt without impacting the reliability or quality of service of our certificate pipeline. One certificate lifecycle (90 days) before the change, we are going to start shifting certificates to use a different CA, one that’s compatible with the devices that will be impacted. By doing this, we’ll mitigate any impact without any action required from our customers. The only customers that will continue to use Let’s Encrypt are ones that have specifically chosen Let’s Encrypt as the CA.

    What to expect of the upcoming Let’s Encrypt change

    Let’s Encrypt’s cross-signed chain will expire on September 30th, 2024. Although Let’s Encrypt plans to stop issuing certificates from this chain on June 6th, 2024, Cloudflare will continue to serve the cross-signed chain for all Let’s Encrypt certificates until September 9th, 2024.

    90 days or one certificate lifecycle before the change, we are going to start shifting Let’s Encrypt certificates to use a different certificate authority. We’ll make this change for all products where Cloudflare is responsible for the CA selection, meaning this will be automatically done for customers using Universal SSL and SSL for SaaS with the “default CA” choice.

    Any customers that have specifically chosen Let’s Encrypt as their CA will receive an email notification with a list of their Let’s Encrypt certificates and information on whether or not we’re seeing requests on those hostnames coming from legacy devices.

    After September 9th, 2024, Cloudflare will serve all Let’s Encrypt certificates using the ISRG Root X1 chain. Here is what you should expect based on the certificate product that you’re using:

    Universal SSL

    With Universal SSL, Cloudflare chooses the CA that is used for the domain’s certificate. This gives us the power to choose the best certificate for our customers. If you are using Universal SSL, there are no changes for you to make to prepare for this change. Cloudflare will automatically shift your certificate to use a more compatible CA.

    Advanced Certificates

    With Advanced Certificate Manager, customers specifically choose which CA they want to use. If Let’s Encrypt was specifically chosen as the CA for a certificate, we will respect the choice, because customers may have specifically chosen this CA due to internal requirements, or because they have implemented certificate pinning, which we highly discourage.

    If we see that a domain using an Advanced certificate issued from Let’s Encrypt will be impacted by the change, then we will send out email notifications to inform those customers which certificates are using Let’s Encrypt as their CA and whether or not those domains are receiving requests from clients that will be impacted by the change. Customers will be responsible for changing the CA to another provider, if they chose to do so.

    SSL for SaaS

    With SSL for SaaS, customers have two options: using a default CA, meaning Cloudflare will choose the issuing authority, or specifying which CA to use.

    If you’re leaving the CA choice up to Cloudflare, then we will automatically use a CA with higher device compatibility.

    If you’re specifying a certain CA for your custom hostnames, then we will respect that choice. We will send an email out to SaaS providers and platforms to inform them which custom hostnames are receiving requests from legacy devices. Customers will be responsible for changing the CA to another provider, if they chose to do so.

    Custom Certificates

    If you directly integrate with Let’s Encrypt and use Custom Certificates to upload your Let’s Encrypt certs to Cloudflare then your certificates will be bundled with the cross-signed chain, as long as you choose the bundle method “compatible” or “modern” and upload those certificates before September 9th, 2024. After September 9th, we will bundle all Let’s Encrypt certificates with the ISRG Root X1 chain. With the “user-defined” bundle method, we always serve the chain that’s uploaded to Cloudflare. If you upload Let’s Encrypt certificates using this method, you will need to ensure that certificates uploaded after September 30th, 2024, the date of the CA expiration, contain the right certificate chain.

    In addition, if you control the clients that are connecting to your application, we recommend updating the trust store to include the ISRG Root X1. If you use certificate pinning, remove or update your pin. In general, we discourage all customers from pinning their certificates, as this usually leads to issues during certificate renewals or CA changes.

    Conclusion

    Internet standards will continue to evolve and improve. As we support and embrace those changes, we also need to recognize that it’s our responsibility to keep users online and to maintain Internet access in the parts of the world where new technology is not readily available. By using Cloudflare, you always have the option to choose the setup that’s best for your application.

    For additional information regarding the change, please refer to our developer documentation.

    Source:: CloudFlare