Public Disclosure: Critical Cloudflare Bypass Vulnerability

– A Case Study in Origin Exposure

Title: How a Single DNS Record Can Nullify Your Entire Cloudflare Security Stack

Author: An Independent Security Researcher

Date: 9-22-25

Summary: This report details the discovery of a critical misconfiguration that completely bypasses Cloudflare's protections, including DDoS mitigation and WAF rules. The issue, a common DNS oversight, exposes the origin server IP address, allowing attackers to target infrastructure directly. This case study is published to highlight a widespread risk and encourage organizations to audit their own configurations.

The Discovery: A Map to the Castle Doors

During a review of publicly available DNS data for a domain protected by Cloudflare, a subdomain was found configured with an A record pointing directly to an origin server IP address, rather than to Cloudflare. This simple misconfiguration—often a forgotten development or testing record—effectively publishes a direct map to the backend infrastructure, entirely bypassing the protective proxy.

While accessing the IP directly returned a generic “Forbidden” error, this is a default server response, not a security measure. It simply confirms the server is reachable and awaiting correctly formatted instructions.

The Technical Breakdown: Bypassing the Shield

Cloudflare operates as a reverse proxy. Its security model depends on a fundamental rule: the origin server must only accept connections from Cloudflare's network.

This vulnerability invalidates that model. An attacker can send requests directly to the origin IP by adding a single HTTP header, bypassing all of Cloudflare's protections—DDoS mitigation, WAF, and rate limiting.

Proof of Concept:
The following command structure demonstrates the complete bypass. Cloudflare is entirely removed from the loop.

bash

curl -X POST http://[Origin-Server-IP]/api/v1/posts/search \
  -H "Host: app.example.com" \  # The critical header that enables the bypass
  -H "Content-Type: application/json" \
  -d '{"filters": {"content": {"like": "%a%"}}, "limit": 1000}'

With the correct Host header, the web server (Nginx/Apache) routes the request to the intended application. Cloudflare's security services are never engaged.

The Implication: A Free Pass for Attackers

This bypass has severe consequences:

• DDoS Attacks: An attacker can launch resource-intensive, application-layer attacks directly against the origin server, causing downtime while Cloudflare's metrics show no anomalous traffic.

• WAF Bypass: All Web Application Firewall rules are rendered ineffective. Attacks like SQL injection and cross-site scripting can be sent directly to the application without inspection.

• Zero-Day Exploitation: Unknown vulnerabilities can be probed for and exploited without any requests being logged or blocked by Cloudflare, severely hindering detection and forensic analysis.

The Universal Fix: A Two-Step Solution

This problem is common, and the solution is well-documented:

1. DNS Hygiene: Conduct regular audits of all DNS records. Ensure every domain and subdomain meant to be protected is pointed to Cloudflare, not to an origin IP.

2. Origin Firewall Rule: This is non-negotiable. The origin server’s firewall must be configured to reject all traffic on ports 80 (HTTP) and 443 (HTTPS) that does not originate from Cloudflare’s official IP ranges. This provides a network-level enforcement of the security model.

A Note on Disclosure

This issue is being disclosed publicly to highlight a critical and systemic risk in cloud security configurations. The goal is to encourage swift remediation across the industry and to educate the wider community on the importance of rigorous infrastructure hardening.

The lesson is clear: In modern cloud security, a single, seemingly minor oversight can dismantle an entire defense strategy. Vigilance and strict adherence to fundamental best practices are not optional.

---