Earlier this year, a group of external researchers identified and reported a vulnerability in Cloudflare’s SSL for SaaS v1 (Managed CNAME) product offering through Cloudflare’s bug bounty program. We officially deprecated SSL for SaaS v1 in 2021; however, some customers received extensions for extenuating circumstances that prevented them from migrating to SSL for SaaS v2 (Cloudflare for SaaS). We have continually worked with the remaining customers to migrate them onto Cloudflare for SaaS over the past four years and have successfully migrated the vast majority of these customers. For most of our customers, there is no action required; for the very small number of SaaS v1 customers, we will be actively working to help migrate you to SSL for SaaS v2 (Cloudflare for SaaS).

Back in 2017, Cloudflare announced SSL for SaaS, a product that allows SaaS providers to extend the benefits of Cloudflare security and performance to their end customers. Using a “Managed CNAME” configuration, providers could bring their customer’s domain onto Cloudflare. In the first version of SSL for SaaS (v1), the traffic for Custom Hostnames is proxied to the origin based on the IP addresses assigned to the zone. In this Managed CNAME configuration, the end customers simply pointed their domains to the SaaS provider origin using a CNAME record. The customer’s origin would then be configured to accept traffic from these hostnames. 

While SSL for SaaS v1 enabled broad adoption of Cloudflare for end customer domains, its architecture introduced a subtle but important security risk – one that motivated us to build Cloudflare for SaaS. 

As adoption scaled, so did our understanding of the security and operational limitations of SSL for SaaS v1. The architecture depended on IP-based routing and didn’t verify domain ownership before proxying traffic. That meant that any custom hostname pointed to the correct IP could be served through Cloudflare — even if ownership hadn’t been proven. While this produced the desired functionality, this design introduced risks and created friction when customers needed to make changes without downtime. 

A malicious CF user aware of another customer's Managed CNAME (via social engineering or publicly available info), could abuse the way SSL for SaaS v1 handles host header redirects through DNS manipulation and Man-in-The-Middle attack because of the way Cloudflare serves the valid TLS certificate for the Managed CNAME.

For regular connections to Cloudflare, the certificate served by Cloudflare is determined by the SNI provided by the client in the TLS handshake, while the zone configuration applied to a request is determined based on the host-header of the HTTP request.

In contrast, SSL for SaaS v1/Managed CNAME setups work differently. The certificate served by Cloudflare is still based on the TLS SNI, but the zone configuration is determined solely based on the specific Cloudflare anycast IP address the client connected to.

For example, let’s assume that 192.0.2.1 is the anycast IP address assigned to a SaaS provider. All connections to this IP address will be routed to the SaaS provider's origin server, irrespective of the host-header in the HTTP request. This means that for the following request:

$ curl --connect-to ::192.0.2.1 https://www.cloudflare.com

The certificate served by Cloudflare will be valid for www.cloudflare.com, but the request will not be sent to the origin server of www.cloudflare.com. It will instead be sent to the origin server of the SaaS provider assigned to the 192.0.2.1 IP address.

While the likelihood of exploiting this vulnerability is low and requires multiple complex conditions to be met, the vulnerability can be paired with other issues and potentially exploit other Cloudflare customers if:

1. The adversary is able to perform DNS poisoning on the target domain to change the IP address that the end-user connects to when visiting the target domain

2. The adversary is able to place a malicious payload on the Managed CNAME customer’s website, or discovers an existing cross-site scripting vulnerability on the website

To address these challenges, we launched SSL for SaaS v2 (Cloudflare for SaaS) and deprecated SSL for SaaS v1 in 2021. Cloudflare for SaaS transitioned away from IP-based routing towards a verified custom hostname model. Now, custom hostnames must pass a hostname verification step alongside SSL certificate validation to proxy to the customer origin. This improves security by limiting origin access to authorized hostnames and reduces downtime through hostname pre-validation, which allows customers to verify ownership before traffic is proxied through Cloudflare.

When Cloudflare for SaaS became generally available, we began a careful and deliberate deprecation of the original architecture. Starting in March 2021, we notified all v1 users of the then upcoming sunset in favor of v2 in September 2021 with instructions to migrate. Although we officially deprecated Managed CNAME, some customers were granted exceptions and various zones remained on SSL for SaaS v1. Cloudflare was notified this year through our Bug Bounty program that an external researcher had identified the SSL for SaaS v1 vulnerabilities in the midst of our continued efforts to migrate all customers.

The majority of customers have successfully migrated to the modern v2 setup. For those few that require more time to migrate, we've implemented compensating controls to limit the potential scope and reach of this issue for the remaining v1 users. Specifically:

• This feature is unavailable for new customer accounts, and new zones within existing customer accounts, to configure via the UI or API

• Cloudflare actively maintains an allowlist of zones & customers that currently use the v1 service

We have also implemented WAF custom rules configurations for the remaining customers such that any requests targeting an unauthorized destination will be caught and blocked in their L7 firewall.

The architectural improvement of Cloudflare for SaaS not only closes the gap between certificate and routing validation but also ensures that only verified and authorized domains are routed to their respective origins—effectively eliminating this class of vulnerability.

There is no action necessary for Cloudflare customers, with the exception of remaining SSL for SaaS v1 customers, with whom we are actively working to help migrate. While we move to the final phases of sunsetting v1, Cloudflare for SaaS is now the standard across our platform, and all current and future deployments will use this secure, validated model by default.

As always, thank you to the external researchers for responsibly disclosing this vulnerability. We encourage all of our Cloudflare community to submit any identified vulnerabilities to help us continually improve upon the security posture of our products and platform.

We also recognize that the trust you place in us is paramount to the success of your infrastructure on Cloudflare. We consider these vulnerabilities with the utmost concern and will continue to do everything in our power to mitigate impact. Although we are confident in our steps to mitigate impact, we recognize the concern that such incidents may induce. We deeply appreciate your continued trust in our platform and remain committed not only to prioritizing security in all we do, but also acting swiftly and transparently whenever an issue does arise.

In an era where digital threats evolve faster than ever, cybersecurity isn't just a back-office concern — it's a critical business priority. At Cloudflare, we understand the responsibility that comes with operating in a connected world. As part of our ongoing commitment to security and transparency, Cloudflare is proud to have joined the United States Cybersecurity and Infrastructure Security Agency’s (CISA) “Secure by Design” pledge in May 2024. 

By signing this pledge, Cloudflare joins a growing coalition of companies committed to strengthening the resilience of the digital ecosystem. This isn’t just symbolic — it's a concrete step in aligning with cybersecurity best practices and our commitment to protect our customers, partners, and data. 

A central goal in CISA’s Secure by Design pledge is promoting transparency in vulnerability reporting. This initiative underscores the importance of proactive security practices and emphasizes transparency in vulnerability management — values that are deeply embedded in Cloudflare’s Product Security program. ​We believe that openness around vulnerabilities is foundational to earning and maintaining the trust of our customers, partners, and the broader security community.

Transparency in vulnerability reporting is essential for building trust between companies and customers. In 2008, Linus Torvalds noted that disclosure is inherently tied to resolution: “So as far as I'm concerned, disclosing is the fixing of the bug”, emphasizing that resolution must start with visibility. While this mindset might apply well to open-source projects and communities familiar with code and patches, it doesn’t scale easily to non-expert users and enterprise users who require structured, validated, and clearly communicated disclosures regarding a vulnerability’s impact. Today’s threat landscape demands not only rapid remediation of vulnerabilities but also clear disclosure of their nature, impact and resolution. This builds trust with the customer and contributes to the broader collective understanding of common vulnerability classes and emerging systemic flaws.

Common Vulnerabilities and Exposures (CVE) is a catalog of publicly disclosed vulnerabilities and exposures. Each CVE includes a unique identifier, summary, associated metadata like the Common Weakness Enumeration (CWE) and Common Platform Enumeration (CPE), and a severity score that can range from None to Critical. 

The format of a CVE ID consists of a fixed prefix, the year of the disclosure and an arbitrary sequence number ​​like CVE-2017-0144. Memorable names such as "EternalBlue"  (CVE-2017-0144)  are often associated with high-profile exploits to enhance recall.

As an authorized CVE Numbering Authority (CNA), Cloudflare can assign CVE identifiers for vulnerabilities discovered within our products and ecosystems. Cloudflare has been actively involved with MITRE's CVE program since its founding in 2009. As a CNA, Cloudflare assumes the responsibility to manage disclosure timelines ensuring they are accurate, complete, and valuable to the broader industry. 

Cloudflare issues CVEs for vulnerabilities discovered internally and through our Bug Bounty program when they affect open source software and/or our distributed closed source products.

The findings are triaged based on real-world exploitability and impact. Vulnerabilities without a plausible exploitation path, in addition to findings related to test repositories or exposed credentials like API keys, typically do not qualify for CVE issuance.

We recognize that CVE issuance involves nuance, particularly for sophisticated security issues in a complex codebase (for example, the Linux kernel). Issuance relies on impact to users and the likelihood of the exploit, which depends on the complexity of executing an attack. The growing number of CVEs issued industry-wide reflects a broader effort to balance theoretical vulnerabilities against real-world risk. 

In scenarios where Cloudflare was impacted by a vulnerability, but the root cause was within another CNA’s scope of products, Cloudflare will not assign the CVE. Instead, Cloudflare may choose other mediums of disclosure, like blog posts.

Our disclosure process begins with internal evaluation of severity and scope, and any potential privacy or compliance impacts. When necessary, we engage our Legal and Security Incident Response Teams (SIRT). For vulnerabilities reported to Cloudflare by external entities via our Bug Bounty program, our standard disclosure timeline is within 90 days. This timeline allows us to ensure proper remediation, thorough testing, and responsible coordination with affected parties. While we are committed to transparent disclosure, we believe addressing and validating fixes before public release is essential to protect users and uphold system security. For open source projects, we also issue security advisories on the relevant GitHub repositories. Additionally, we encourage external researchers to publish/blog about their findings after issues are remediated. Full details and process of Cloudflare’s external researcher/entity disclosure policy can be found via our Bug Bounty program policy page

To date, Cloudflare has issued and disclosed multiple CVEs. Because of the security platforms and products that Cloudflare builds, vulnerabilities have primarily been in the areas of denial of service, local privilege escalation, logical flaws, and improper input validation. Cloudflare also believes in collaboration and open sources of some of our software stack, therefore CVEs in these repositories are also promptly disclosed.

Cloudflare disclosures can be found here. Below are some of the most notable vulnerabilities disclosed by Cloudflare:

Cloudflare quiche (through version 0.19.1/0.20.0) was affected by an unlimited resource allocation vulnerability causing rapid increase of memory usage of the system running a quiche server or client.

A remote attacker could take advantage of this vulnerability by repeatedly sending an unlimited number of 1-RTT CRYPTO frames after previously completing the QUIC handshake.

Exploitation was possible for the duration of the connection, which could be extended by the attacker.

quiche 0.19.2 and 0.20.1 are the earliest versions containing the fix for this issue.

The Cloudflare WordPress plugin was found to be vulnerable to improper authentication. The vulnerability enables attackers with a lower privileged account to access data from the Cloudflare API.

The issue has been fixed in version >= 4.12.3 of the plugin

The Cloudflare WARP client for Windows assigns loopback IPv4 addresses for the DNS servers, since WARP acts as a local DNS server that performs DNS queries securely. However, if a user is connected to WARP over an IPv6-capable network, the WARP client did not assign loopback IPv6 addresses but rather Unique Local Addresses, which under certain conditions could point towards unknown devices in the same local network, enabling an attacker to view DNS queries made by the device.

This issue was patched in version 2023.7.160.0 of the WARP client (Windows).

An improper privilege management vulnerability in Cloudflare WARP for Windows allowed file manipulation by low-privilege users. Specifically, a user with limited system permissions could create symbolic links within the C:\ProgramData\Cloudflare\warp-diag-partials directory. When the "Reset all settings" feature is triggered, the WARP service — running with SYSTEM-level privileges — followed these symlinks and may delete files outside the intended directory, potentially including files owned by the SYSTEM user.

This vulnerability affected versions of WARP prior to 2024.12.492.0.

Cloudflare’s mutual TLS implementation caused a vulnerability in the session resumption handling. The underlying issue originated from BoringSSL’s process to resume TLS sessions. BoringSSL stored client certificates, which were reused from the original session (without revalidating the full certificate chain) and the original handshake's verification status was not re-validated. 

While Cloudflare was impacted by the vulnerability, the root cause was within NGINX's implementation, making F5 the appropriate CNA to assign the CVE. This is an example of alternate mediums of disclosure that Cloudflare sometimes opt for. This issue was fixed as per guidance from the respective CVE — please see our blog post for more details.

Irrespective of the industry, if your organization builds software, we encourage you to familiarize yourself with CISA’s “Secure by Design” principles and create a plan to implement them in your company. The CISA Secure by Design pledge is built around seven security goals, prioritizing the security of customers, and challenges organizations to think differently about security. 

As we continue to enhance our security posture, Cloudflare remains committed to enhancing our internal practices, investing in tooling and automation, and sharing knowledge with the community. CVE transparency is not a one-time initiative — it’s a sustained effort rooted in openness, discipline, and technical excellence. By embedding these values in how we design, build and secure our products, we aim to meet and exceed expectations set out in the CISA pledge and make the Internet more secure, faster and reliable!

For more updates on our CISA progress, review our related blog posts. Cloudflare has delivered five of the seven CISA Secure by Design pledge goals, and we aim to complete the remainder of the pledge goals in May 2025.