Attempts to Bypass CDNs, (Wed, Dec 3rd)
Source SANS ISC Handlers Diary
Description
Currently, in order to provide basic DDoS protection and filter aggressive bots, some form of Content Delivery Network (CDN) is usually the simplest and most cost-effective way to protect a web application. In a typical setup, DNS is used to point clients to the CDN, and the CDN will then forward the request to the actual web server. There are a number of companies offering services like this, and cloud providers will usually have solutions like this as well.
Impact
For European organizations, this threat poses a significant risk to the availability and integrity of web applications. If attackers successfully bypass CDNs, they can launch direct DDoS attacks against origin servers, potentially causing service outages and degraded performance. This can disrupt business operations, damage reputation, and lead to financial losses. Additionally, bypassing CDN protections may expose origin servers to aggressive bots and automated attacks that CDNs typically filter, increasing the risk of exploitation of other vulnerabilities. Organizations relying on CDNs for compliance with security standards may face regulatory scrutiny if protections fail. The impact is heightened for sectors with critical online services such as finance, e-commerce, government, and healthcare, which are prevalent across Europe. The increased attack activity also indicates a growing trend that could lead to more sophisticated bypass techniques, necessitating proactive defense measures.
Defensive guidance
1. Implement strict origin server access controls by allowing inbound traffic only from the official IP ranges of the CDN providers used. Regularly update these IP lists to accommodate changes. 2. Use custom, randomized headers or tokens that are inserted by the CDN and verified by the origin server to authenticate legitimate traffic. Avoid relying on generic CDN-identifying headers that can be easily spoofed. 3. Employ network-level filtering such as firewall rules and access control lists (ACLs) to block direct access attempts from unauthorized IP addresses. 4. Monitor web server logs for unusual headers or traffic patterns that mimic CDN headers but originate from unexpected sources. 5. Engage with CDN providers to understand their recommended best practices for origin protection and implement any available security features such as origin cloaking or private network connections. 6. Conduct regular penetration testing and threat hunting exercises focused on CDN bypass techniques to identify and remediate weaknesses. 7. Consider deploying Web Application Firewalls (WAFs) with rules tailored to detect and block forged CDN header attempts. 8. Educate security teams about the evolving tactics attackers use to bypass CDNs and update incident response plans accordingly.
Summary
Content Delivery Networks (CDNs) are widely used to provide DDoS protection and filter aggressive bots by acting as intermediaries between clients and origin web servers. Typically, DNS resolves to the CDN, which then forwards requests to the origin server, shielding it from direct attacks. However, a critical weakness exists if attackers can discover the actual IP addresses of the origin servers. By bypassing the CDN and sending requests directly to these IPs, attackers can circumvent the CDN's protections. Recent threat intelligence from SANS ISC indicates an uptick in attempts to exploit this weakness. Attackers are including forged CDN-specific headers such as 'Cf-Warp-Tag-Id' (Cloudflare Warp VPN), 'X-Fastly-Request-Id' (Fastly), and 'X-Akamai-Transformed' (Akamai) in their requests to mimic legitimate CDN traffic and evade detection. Some headers like 'X-T0Ken-Inf0' and 'Xiao9-' prefixed headers have unclear purposes but may be used similarly. The challenge for defenders is that some CDNs have large and dynamic IP ranges, making IP-based filtering complex. Additionally, relying on generic CDN-identifying headers is insecure since attackers can easily spoof them. More robust methods include restricting origin server access strictly to known CDN IP ranges and implementing custom headers with randomized tokens verified by the origin server. The observed increase in such bypass attempts suggests attackers are actively probing for weaknesses in CDN configurations to launch direct attacks on origin infrastructure.
References
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