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Introduction
Subdomain takeovers are a critical cybersecurity threat where attackers hijack unused or misconfigured subdomains to launch phishing attacks, distribute malware, or steal sensitive data. In this article, we’ll explore how to identify, exploit (ethically), and mitigate subdomain takeovers, along with verified commands and techniques used by bug bounty hunters.
Learning Objectives
- Understand how subdomain takeovers occur
- Learn how to detect vulnerable subdomains using OSINT tools
- Secure your organization’s DNS and cloud configurations
You Should Know
1. Identifying Vulnerable Subdomains with Subfinder
Command:
subfinder -d example.com -o subdomains.txt
What It Does:
Subfinder is an OSINT tool that discovers subdomains by querying multiple sources like search engines, SSL certificates, and DNS records.
Step-by-Step Guide:
1. Install Subfinder:
go install -v github.com/projectdiscovery/subfinder/v2/cmd/subfinder@latest
2. Run the scan:
subfinder -d example.com -o subdomains.txt
3. Review the output for dangling subdomains (pointing to expired cloud services).
2. Checking DNS Records for Takeover Risks
Command:
dig CNAME vulnerable.example.com
What It Does:
Checks if a subdomain has a CNAME record pointing to a third-party service (e.g., AWS S3, GitHub Pages) that no longer exists.
Step-by-Step Guide:
1. Use `dig` to query DNS:
dig CNAME test.example.com
2. If the response shows a “NXDOMAIN” error, the subdomain may be vulnerable.
3. Automating Subdomain Takeover Detection with Nuclei
Command:
nuclei -l subdomains.txt -t takeovers/
What It Does:
Nuclei scans subdomains for known takeover patterns (e.g., unused AWS buckets, Heroku apps).
Step-by-Step Guide:
1. Install Nuclei:
go install -v github.com/projectdiscovery/nuclei/v2/cmd/nuclei@latest
2. Run the scan:
nuclei -l subdomains.txt -t ~/nuclei-templates/takeovers/
4. Exploiting a Vulnerable Subdomain (Proof-of-Concept)
Command:
curl -X PUT -d "Hacked" http://vulnerable-s3.example.com/test.txt
What It Does:
If an S3 bucket is misconfigured, this command uploads a file, proving takeover potential.
Step-by-Step Guide:
- Identify an unclaimed S3 bucket via DNS records.
2. Test write access:
curl -X PUT -d "Test" http://bucket.example.com/test.txt
3. If successful, report the issue immediately.
5. Mitigating Subdomain Takeovers in AWS
Command:
aws s3api put-bucket-policy --bucket my-bucket --policy file://policy.json
What It Does:
Applies a restrictive bucket policy to prevent unauthorized access.
Step-by-Step Guide:
1. Create a `policy.json` file:
{
"Version": "2012-10-17",
"Statement": [{
"Effect": "Deny",
"Principal": "",
"Action": "s3:",
"Resource": "arn:aws:s3:::my-bucket/",
"Condition": {"StringNotEquals": {"aws:SourceIp": ["YOUR_IP"]}}
}]
}
2. Apply the policy:
aws s3api put-bucket-policy --bucket my-bucket --policy file://policy.json
What Undercode Say
- Key Takeaway 1: Subdomain takeovers are preventable with proper DNS hygiene and cloud configuration audits.
- Key Takeaway 2: Automated tools like Subfinder and Nuclei streamline detection, but manual verification is crucial.
Analysis:
Bug bounty hunters like Isha Sangpal highlight the importance of proactive scanning. A single unmonitored subdomain can lead to reputational damage, data breaches, or legal consequences. Organizations should implement continuous monitoring and enforce strict cloud resource policies.
Prediction
As cloud adoption grows, subdomain takeovers will remain a top attack vector. Future exploits may leverage AI-driven reconnaissance tools, making automated defense mechanisms essential. Companies investing in DevSecOps and bug bounty programs will mitigate risks effectively.
References:
🎯Let’s Practice For Free:
IT/Security Reporter URL:
Reported By: Isha Sangpal – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
