How we mitigated a vulnerability in Cloudflare’s ACME validation logic
Jan 19, 2026Hrushikesh Deshpande, Andrew Mitchell, Leland Garofalo
Summary
This post details a vulnerability in Cloudflare's ACME validation logic where WAF features were disabled for ACME HTTP-01 challenge requests, potentially allowing them to reach the origin when they should have been blocked. The mitigation involved a code change that ensures WAF features are only disabled when the request matches a valid ACME HTTP-01 challenge token for the hostname, and Cloudflare is serving the challenge response.
More from the Website Security & Threat Management trail
- We tested our own WAF with frontier AI models. Here’s what we found
- When scanners miss the attack: how Cloudflare Client-Side Security protects storefronts
- Introducing automatic remediation policies with Cloudflare CASB
- Introducing context-aware vulnerability discovery and remediation with Cloudflare Managed Defense and OpenAI Daybreak Models
- Introducing Adaptive Intelligence: undermining the economics of every bot attack