Unearthing Hidden Assets: The Power of Active DNS Brute Forcing in Subdomain Discovery

Aayush kumar
2 min readJul 9, 2024

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Hello everyone, this is my second write-up. I want to keep it small and simple. As we all know, while doing recon, there are two popular ways to find subdomains: passive and active. Passive means we collect subdomains from different websites like crt.sh or use tools like Subfinder or Amass to get a list of subdomains. Active DNS brute forcing means we actively try to brute force to find more subdomains.

My take on doing Active DNS Brute Forcing as a BB hunter

See, this is my personal take on this. Feel free to comment if you have other ideas or disagree with what I say. Over time, I’ve seen that many top bug bounty hunters have automation set up to constantly look for any changes in the production side, whether it be making a subdomain active for some time, a change in a feature, or an update in a JavaScript file. Hunting on an asset that is completely new can give you an edge.

However, you have to keep in mind that active DNS brute forcing is resource-intensive. If you plan to do it on your home router, make sure you have a good internet plan, or your router might crash. A VPS is highly preferred.

I was once looking at a heavily tested target, so I started my recon process and began to enumerate subdomains. I usually don’t do active DNS brute forcing much, but this time I decided to try it. During my search for unique results, I found a unique subdomain. This led me to a subdomain where Laravel Debug Mode was enabled, leaking some environment variables containing very sensitive information. I reported it and got a bounty.It was in a out of scope asset but the company awarded me with a bonus since the severity was good.

What tool or command I used?

I used pure dns by d3mondev , make sure to give him a follow https://x.com/d3mondev?lang=en

Command Used

puredns bruteforce 2m-subdomains.txt — resolvers resolvers.txt x.com — trusted-only — rate-limit-trusted 100

Thank you for reading :)

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