CloudFront Origin & MITM Risk

TL;DR

Yes, a Man-in-the-Middle (MITM) attack could be facilitated by configuring a CloudFront distribution with an origin pointing to a domain you don’t control. However, it’s not automatic and relies on several vulnerabilities and misconfigurations. Simply adding the origin isn’t enough; attackers need to exploit DNS, TLS/SSL certificates, or other weaknesses.

Understanding the Risk

CloudFront acts as a Content Delivery Network (CDN), caching content closer to users. When you set an origin, CloudFront fetches content from that source. If the origin isn’t yours, and you don’t have proper security measures in place, an attacker could intercept traffic.

Steps to Understand & Mitigate the Risk

1. DNS Spoofing/Cache Poisoning:

• An attacker could try to manipulate DNS records so that users are directed to a malicious server instead of your CloudFront distribution. This is the most common initial attack vector.
• Mitigation: Use DNSSEC (Domain Name System Security Extensions) to digitally sign your DNS records, making them harder to tamper with. Regularly monitor your DNS records for unauthorized changes. Consider using a reputable DNS provider with robust security features.

TLS/SSL Certificate Issues:

• If the origin domain doesn’t have a valid TLS/SSL certificate, or if it uses a self-signed certificate that isn’t trusted by browsers, users will see warnings and could be vulnerable to interception.
• Mitigation: Always ensure your origin has a valid, trusted TLS/SSL certificate issued by a Certificate Authority (CA). CloudFront can handle SSL termination for you if configured correctly.

Origin Server Vulnerabilities:

• If the origin server itself is vulnerable to attacks (e.g., SQL injection, cross-site scripting), an attacker could compromise it and inject malicious content that CloudFront then distributes.
• Mitigation: Regularly patch and update your origin servers with the latest security fixes. Implement strong access controls and web application firewalls (WAFs).

CloudFront Configuration Errors:

• Incorrectly configured CloudFront settings can create vulnerabilities. For example, allowing HTTP traffic when HTTPS is preferred.
• Mitigation: Review your CloudFront configuration regularly. Enforce HTTPS-only communication. Use appropriate cache policies and origin access controls. Consider using AWS WAF to protect against common web attacks.

Origin Access Identity (OAI) & Signed URLs/Cookies:

• If you’re trying to restrict access to content on your origin server, but the OAI is misconfigured or not used correctly, attackers might be able to bypass those restrictions.
• Mitigation: Use Origin Access Identity (OAI) to control access to your S3 buckets or custom origins. Implement signed URLs or cookies for restricted content.

Example Scenario & Mitigation

Let’s say an attacker controls evil-domain.com and you configure CloudFront with this as the origin.

1. Attacker Setup: The attacker sets up a server at evil-domain.com that mimics your website but contains malicious code.
2. DNS Poisoning (Attack): They attempt to poison DNS records so users are directed to evil-domain.com instead of your CloudFront distribution.
3. CloudFront Fetches Malicious Content: If the DNS poisoning is successful, CloudFront fetches content from evil-domain.com and caches it.
4. Users Receive Malicious Content: Users accessing your website through CloudFront receive the attacker’s malicious code.

Mitigation Steps in this scenario:

• Implement DNSSEC to protect against DNS poisoning.
• Monitor your DNS records for changes.
• Use HTTPS-only communication and ensure the origin server has a valid TLS/SSL certificate.
• Consider using AWS WAF to detect and block malicious requests.

Important Considerations

• Never use an origin domain you don’t control unless absolutely necessary, and even then, take extreme precautions.
• Regularly audit your CloudFront configurations for security vulnerabilities.
• Monitor your website traffic for suspicious activity.