Cloudflare recently patched a security vulnerability within its Automatic Certificate Management Environment validation logic that allowed potential attackers to bypass web application firewall protections. By exploiting a flaw in how the edge network handled specific validation paths, unauthorized users could have gained direct access to customer origin servers despite existing security controls.
The issue centered on the HTTP-01 challenge process, which certificate authorities use to verify domain ownership before issuing SSL certificates. Normally, Cloudflare disables its firewall for these specific requests to ensure the validation process isn't blocked by its own security rules. However, the system failed to verify if the token in a request actually belonged to the specific domain being accessed, allowing a token from one zone to bypass the firewall for a completely different zone.
If an attacker possessed a valid token for any Cloudflare-managed domain, they could have crafted a request to the sensitive validation path of a target server. Because the system recognized the token as valid within the broader network, it would automatically disable firewall features. This error essentially created a hole in the perimeter, allowing arbitrary traffic to reach the origin server without being inspected or blocked by the web application firewall.
Researchers who discovered the flaw noted that it could have been used for extensive reconnaissance or to access sensitive files on origin servers across the entire Cloudflare network. Because the tokens involved can be deterministic and long-lived, the risk for potential data exposure was significant. Despite these risks, the company stated that their internal investigation found no evidence that the vulnerability was successfully exploited by malicious actors prior to the fix.
To address the security gap, Cloudflare updated its validation logic in late 2025 to ensure that firewall rules are only suspended when a request matches a valid token specifically associated with the requested hostname. This change ensures that domain validation continues to function automatically without leaving customer servers vulnerable to unauthorized access through the certificate renewal path.
Source: Cloudflare Fixes ACME Validation Bug Allowing WAF Bypass to Origin Servers