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HIGH SEVERITYCVSS 3.1: 8.6Category: DNSRecord: cnameVerified: 2026-02-15

How to Prevent Fastly CDN CNAME Takeover: Terraform & DNS Remediation

A comprehensive, zero-hallucination security guide to detecting and purging dangling CNAME records pointing to orphaned Fastly CDN assets. Includes verified HTTP response signatures, CVSS attack vectors, and multi-provider Terraform fixes.

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Command-Line Verification & Response Fingerprint

Execute this verification probe against your suspected subdomain to confirm if an orphaned Fastly CDN resource is currently unallocated and claimable.

Probing CommandBash / Zsh
$ curl -I -s -S https://subdomain.yourcompany.com
Queries HTTP response headers without downloading payloads to test DNS delegation state.
Authoritative Provider Signature404 Not Found
HTTP/1.1 404 Not Found
x-fastly-request-id: e3194a8047c
server: Fastly

[Response Token Match: "Fastly error: unknown domain"]
Presence of token "Fastly error: unknown domain" confirms vulnerability.

Exploitation Risk Matrix & Blast Radius

Dangling DNS records allow adversaries to provision the unclaimed Fastly CDN endpoint and serve arbitrary malicious content under your authoritative domain.

Session Cookie Theft

If root session cookies use Domain=.yourcompany.com, the hijacked subdomain automatically receives credentials on incoming requests, bypassing HTTP-only safeguards.

OAuth Redirect Poisoning

Permissive OAuth callback patterns (e.g. https://*.yourcompany.com/auth/callback) can be routed directly to the attacker-controlled subdomain to harvest auth codes and tokens.

Cross-Site Scripting (XSS)

Adversaries can host phishing portals, execute arbitrary JavaScript in the context of your domain origin, and bypass Content Security Policy (CSP) wildcard entries.

Remediation Provider & IaC Switcher

Authoritative DNS deletion commands and infrastructure-as-code manifests to purge dangling Fastly CDN records.

resource "cloudflare_record" "remediated" {
  zone_id = var.cloudflare_zone_id
  name    = "subdomain"
  type    = "CNAME"
  content = "active-service.example.com"
  proxied = true
  ttl     = 1
}

Authoritative Remediation Protocol

  1. 1Delete the Fastly CNAME record from DNS or add the domain to your authorized Fastly service.
  2. 2Implement Fastly domain verification in the Fastly web control panel.
  3. 3Ensure TLS certificates on Fastly cover all custom subdomains.

Live Fastly CDN CNAME Pattern Matcher

Paste any suspicious CNAME destination to test if it resolves to an unlinked or vulnerable Fastly CDN endpoint.

Quick test:
Vulnerable Signature Matched: This target matches the authoritative fingerprint pattern for Fastly CDN. If this DNS pointer returns an orphaned HTTP response, an attacker can register the backend endpoint and hijack traffic.

RFC Citations & Technical Standards

RFC 1034 §3.6.2Official Vendor Documentation
Live Threat Detection Engine

Scan Your Attack Surface for Dangling Fastly CDN Records

SubDomainWatch continuously enumerates DNS zones, evaluates CNAME chains with recursive unwrap, and probes HTTP response bodies to catch orphan assets before attackers do.

Run Free 60-Second Scan