T09 · Insecure Skill Coding Practices
- Location
security.md:35- Finding
Incomplete CDN Firewall Allowlist Can Cause Origin Denial of Service
- Content
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Vulnerability Details
File Location:
security.md, lines 35–38
Vulnerability Type: Incomplete and unsafe firewall configuration
Risk Level: Highbash # Only allow CDN IPs (example for Cloudflare) # Get current IPs: https://www.cloudflare.com/ips/ iptables -A INPUT -p tcp --dport 443 -s 103.21.244.0/22 -j ACCEPT iptables -A INPUT -p tcp --dport 443 -j DROPTechnical Analysis
The executable example permits HTTPS traffic from only one Cloudflare IPv4 network and then unconditionally drops all other inbound TCP traffic to port 443. Cloudflare operates multiple IPv4 and IPv6 network ranges, so this configuration is not a complete provider allowlist.
The commands also append rules without validating the existing firewall policy, rule order, required monitoring sources, administrative access paths, or a rollback mechanism. Consequently, the effective behavior may vary by host and can immediately disrupt legitimate traffic.
Attack Path
- An operator copies the documented commands to an origin server.
- The firewall permits port 443 only for
103.21.244.0/22. - The unconditional DROP rule rejects requests from every other source.
- Legitimate CDN edge nodes using other Cloudflare IPv4 or IPv6 ranges cannot reach the origin.
- Requests routed through those edge nodes fail, producing regional or widespread service disruption.
- If legitimate traffic is repeatedly forced through excluded CDN ranges, the incomplete rule set can amplify the availability impact.
Impact Assessment
This issue does not grant an attacker additional system privileges or direct data access. Its primary impact is availability: it can cause partial or complete origin denial of service, regional delivery failures, failed health checks, monitoring loss, and operational lockout from services using port 443.
The scope includes the origin host on which the commands are applied and every CDN-served application dep ...[truncated 22 chars]
- Remediation
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Remediation Suggestions
- Do not publish a single provider network as a complete allowlist.
- Retrieve the provider's authoritative current IPv4 and IPv6 ranges and validate the response before generating firewall rules.
- Build the complete replacement policy in a dedicated chain, then activate it atomically to avoid partially applied rules.
- Preserve explicitly required access for health checks, monitoring, management systems, and emergency recovery.
- Verify rule ordering and the host's default policies before applying changes.
- Test the policy in a staging environment and confirm connectivity through multiple CDN regions.
- Configure an automatic rollback or out-of-band recovery mechanism before enforcement.
- Regularly synchronize the allowlist because CDN address ranges can change.
- Where possible, use authenticated origin pulls in addition to network allowlisting.