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CRITICAL SEVERITYCVSS 3.1: 9.1Category: PAASRecord: cnameVerified: 2026-02-15

How to Prevent Fly.io App Platform CNAME Takeover: Terraform & DNS Remediation

A comprehensive, zero-hallucination security guide to detecting and purging dangling CNAME records pointing to orphaned Fly.io App Platform 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 Fly.io App Platform 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
server: Fly/edge

[Response Token Match: "404 Not Found - Fly.io"]
Presence of token "404 Not Found - Fly.io" confirms vulnerability.

Exploitation Risk Matrix & Blast Radius

Dangling DNS records allow adversaries to provision the unclaimed Fly.io App Platform 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 Fly.io App Platform 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 CNAME pointing to fly.dev in DNS.
  2. 2Re-issue `fly certs add <domain>` in your authenticated Fly.io application.
  3. 3Audit allocated Fly Anycast IP addresses.

Live Fly.io App Platform CNAME Pattern Matcher

Paste any suspicious CNAME destination to test if it resolves to an unlinked or vulnerable Fly.io App Platform endpoint.

Quick test:
Vulnerable Signature Matched: This target matches the authoritative fingerprint pattern for Fly.io App Platform. 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 Fly.io App Platform 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