Cloudflare has resolved a security vulnerability in its certificate validation logic that could temporarily bypass web application firewall (WAF) protections. The issue, affecting ACME (Automatic Certificate Management Environment), was reported by security researchers late last year through Cloudflare’s bug bounty program. Cloudflare has implemented a fix, no action is required from customers, and there is no evidence that the vulnerability was exploited in the wild.
ACME is a protocol used to automate SSL/TLS certificate issuance and renewal. Cloudflare uses it to manage certificates for customers at the edge of its network, ensuring HTTPS connections remain secure.
The vulnerability affected requests to ACME HTTP-01 challenge paths (/.well-known/acme-challenge/*), which Cloudflare uses to verify domain ownership. Under certain conditions, the system incorrectly disabled some WAF protections and allowed challenge requests to reach customer origin servers when they should have been blocked.
While this did not allow certificate issuance or access to sensitive Cloudflare infrastructure, it created a temporary gap in security feature enforcement for affected paths.
Cloudflare addressed the flaw by updating its edge logic so that security features are only disabled for actively managed ACME challenges. Requests that do not match a valid Cloudflare-managed challenge are now fully processed by the WAF.
Even widely used systems like ACME can have subtle issues at scale, making ongoing security review and monitoring of automated infrastructure essential.
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