Pythonorg Subdomain Hijacked: The Anatomy Of A GitHub Pages Takeover + Video

Introduction

Subdomain takeover vulnerabilities occur when a DNS record points to a service (like GitHub Pages, AWS S3, or Heroku) that no longer exists, allowing an attacker to claim the domain and host malicious content. This oversight can lead to phishing, cookie theft, or complete compromise of user trust. Recently, a security researcher discovered and responsibly disclosed such a flaw on python.org, highlighting how even the most reputable open-source projects can fall victim to misconfigurations. In this article, we dissect the technical steps behind subdomain takeover, provide actionable detection and mitigation techniques, and explore the python.org case as a real‑world example.

Learning Objectives

Understand the mechanics of subdomain takeover via GitHub Pages.
Learn to identify vulnerable subdomains using open‑source tools and manual checks.
Master the ethical exploitation process and responsible disclosure workflow.
Implement defensive measures to prevent such attacks in your own infrastructure.

You Should Know

1. What Is Subdomain Takeover?

Subdomain takeover exploits a dangling DNS record—typically a CNAME—that points to an external service (e.g., myapp.github.io) which is no longer provisioned. An attacker who registers the abandoned service can effectively control the subdomain. In the python.org case, the vulnerable subdomain had a CNAME pointing to python.github.io, but the corresponding GitHub Pages site was missing, allowing the researcher to claim it via a GitHub repository.

Key DNS concepts:

CNAME record: Aliases one domain to another.
NS record: Delegates a subdomain to another DNS server.
TXT record: Often used for verification by cloud providers.

2. How to Detect Vulnerable Subdomains

Detection begins with comprehensive subdomain enumeration. Use tools like subfinder, amass, or `assetfinder` to gather all subdomains of a target. Then check each for dangling CNAMEs.

Linux enumeration example:

 Install subfinder
go install -v github.com/projectdiscovery/subfinder/v2/cmd/subfinder@latest

Enumerate subdomains for python.org
subfinder -d python.org -o subdomains.txt

Check CNAME records using dig
while read sub; do
cname=$(dig CNAME $sub +short)
if [[ $cname == "github.io" ]]; then
echo "$sub -> $cname"
fi
done < subdomains.txt

Windows equivalent (PowerShell):

$subs = Get-Content .\subdomains.txt
foreach ($sub in $subs) {
$cname = Resolve-DnsName -Name $sub -Type CNAME -ErrorAction SilentlyContinue
if ($cname -and $cname.NameHost -like "github.io") {
Write-Host "$sub -> $($cname.NameHost)"
}
}

3. Verifying the Vulnerability (Manual Check)

Once a candidate is found, verify if the target service is truly unclaimed. For GitHub Pages, visit the CNAME target (e.g., python.github.io) – if you get a 404, it’s likely available. Then, attempt to create a GitHub repository with a custom domain.

Step‑by‑step verification:

1. Create a new public repository on GitHub.

Enable GitHub Pages from the repository settings, choosing the `main` branch.
In the “Custom domain” field, enter the vulnerable subdomain (e.g., vulnerable.python.org).
Save and wait for DNS propagation (may take a few minutes).
If GitHub accepts the domain and your content appears at vulnerable.python.org, the takeover is confirmed.

Important: Only perform this on domains you own or have explicit permission to test. Unauthorized testing is illegal.

4. Ethical Exploitation and Responsible Disclosure

After confirmation, immediately remove any malicious content and document the issue. Prepare a disclosure report including:
– The vulnerable subdomain and its DNS record.
– Steps to reproduce (without actually taking over).
– Potential impact (phishing, credential theft).
– Suggested fix (remove the DNS record or reclaim the service).

Example report snippet:

Subject: Subdomain Takeover on python.org (subdomain.example.python.org)

Hi team,

I discovered that subdomain.example.python.org has a CNAME pointing to unclaimed GitHub Pages. By creating a GitHub repository with that custom domain, I was able to serve content. I have since reverted the change. Please remove the dangling DNS record or set up the corresponding GitHub Pages site to mitigate this risk.

Thanks,
Researcher

Send to the organization’s security contact (e.g., [email protected]). In the python.org case, the team fixed it within hours and published a blog post acknowledging the finding.

5. Mitigation and Prevention

To protect your own domains:

Audit DNS records regularly: Use scripts to scan for dangling CNAMEs pointing to expired cloud services.
Implement verification tokens: Many providers (GitHub, AWS) allow you to add a TXT record to prove ownership before a custom domain becomes active.
Remove unused records: When decommissioning a service, delete its DNS entries immediately.
Monitor with tools: Use subjack, nuclei, or `dig` to continuously monitor your subdomains.

Example `subjack` command:

subjack -d python.org -t 100 -timeout 10 -o takeover.txt -ssl

6. Tools and Automation for Continuous Scanning

Automation is key to staying ahead. Integrate these tools into your CI/CD or periodic security checks:

| Tool | Purpose | Example Command |

|||–|

Windows batch script example:

@echo off
for /f %%i in (subdomains.txt) do (
nslookup -type=CNAME %%i | find "github.io" && echo %%i is vulnerable
)

7. Case Study: The Python.org Takeover

In early 2025, researcher Nirmal Thapa’s automated scanner flagged a python.org subdomain with a CNAME to python.github.io. Manual verification confirmed the GitHub Pages site was unclaimed. After responsibly reporting to [email protected], the Python Software Foundation swiftly removed the DNS record and published a post‑mortem. This incident underscores that even top‑tier open‑source projects are not immune, and proactive monitoring coupled with rapid disclosure can prevent real harm.

What Undercode Say

Key Takeaway 1: Automated scanning is indispensable for discovering hidden misconfigurations; manual follow‑up confirms true positives.
Key Takeaway 2: Responsible disclosure builds trust and strengthens the entire open‑source ecosystem—swift action by the Python team exemplifies best practices.

Analysis: Subdomain takeovers are often dismissed as low‑risk, but they can be leveraged for highly convincing phishing attacks or to distribute malware. The python.org case demonstrates that even a simple oversight can compromise a project’s reputation. Organizations must integrate DNS hygiene into their DevSecOps pipelines, using tools like those described above to continuously monitor for dangling records. Moreover, cloud providers should enforce ownership verification by default—GitHub, for instance, could require a TXT record before a custom domain becomes active, closing the window for takeovers entirely.

Prediction

As more companies adopt cloud‑native architectures, the number of dangling DNS records will increase, making subdomain takeovers a more frequent attack vector. We will see a rise in automated tools that both attackers and defenders use, and cloud providers will likely introduce mandatory verification steps to mitigate the risk. The python.org incident will serve as a textbook example in security training courses for years to come.

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