CDN Bypass Techniques

TL;DR

Network security filters often block known malicious IPs or domains. Content Delivery Networks (CDNs) can help bypass these blocks by serving content from a wide range of IP addresses, making it harder for filters to consistently identify and block your traffic. This guide explains how attackers use CDNs to evade network security measures.

Understanding the Problem

Many networks rely on IP address blacklists or domain name blocking to prevent access to harmful content. However, CDNs distribute content across numerous servers globally. If a CDN’s IP addresses are blocked, attackers can switch to different IPs within the same CDN, effectively bypassing the filter.

Bypassing Network Security Filters with CDNs

1. Identify a CDN: Determine if the target website uses a CDN. Tools like What’s My DNS or browser developer tools (Network tab) can reveal the CDN provider.
   • Check the response headers for fields like Server, X-Cache, or Via which often indicate a CDN.
   • Look at the IP addresses serving content; multiple IPs from different geographic locations suggest a CDN.
2. CDN IP Address Enumeration: Once you know the CDN, find its IP address ranges.
   • Use online databases like BGP Looking Glass to query for the CDN’s ASN (Autonomous System Number) and associated IP blocks.
   • Tools like dig or nslookup can also help identify IPs.

     dig +trace targetdomain.com

3. IP Address Rotation: The core bypass technique is to use different IP addresses from the CDN for each request.
   • If you control the client, rotate through a list of known CDN IPs in your code or configuration.
   • Use proxy servers that can switch between CDN IPs automatically.
4. Subdomain Takeover (Advanced): If a subdomain is not properly configured, an attacker might be able to take it over and point it to their own server hosted on the CDN.
   • Check for DNS misconfigurations using tools like crt.sh or online subdomain enumeration tools.
   • If a takeover is possible, the attacker can serve malicious content through the compromised subdomain via the CDN.
5. Cache Poisoning (Advanced): Exploit vulnerabilities in the CDN’s caching mechanism to inject malicious content.
   • This is highly complex and requires finding specific weaknesses in the CDN’s infrastructure.
   • Successful cache poisoning can lead to widespread distribution of harmful content through the CDN.

Mitigation Strategies

1. Reputation-Based Blocking: Block entire ASNs (Autonomous System Numbers) associated with known CDNs, but be aware this can cause false positives.
2. GeoIP Filtering: Restrict access based on the geographic location of the IP address. This is useful if you only expect traffic from specific regions.
3. Traffic Analysis: Monitor network traffic for unusual patterns or anomalies that might indicate CDN bypass attempts.
4. Web Application Firewall (WAF): Implement a WAF to inspect HTTP/HTTPS traffic and block malicious requests before they reach the server.
5. DNS Security Extensions (DNSSEC): Use DNSSEC to ensure the integrity of DNS records, preventing subdomain takeovers.