How Hackers Find Vulnerabilities: A Bug Bounty Hunter's Methodology

People imagine hacking as a burst of clever code. In practice, finding vulnerabilities is mostly patient, methodical work. After years of bug bounty hunting, reaching HackerOne's global top 100 and being named a Bugcrowd MVP, with vulnerabilities reported to organisations including Google, Microsoft, Tesla, PayPal and the U.S. Department of Defense, I can say the methodology matters more than any single trick.

Here is how it works, and what it means for defenders.

1. Recon is most of the job

The best bugs are often on assets nobody else looked at. Before touching the main application, map everything:

• Subdomains from certificate transparency, DNS datasets and permutations.
• Open ports and services.
• Old endpoints from web archives.
• API routes and hostnames extracted from JavaScript bundles.
• Acquired companies and secondary brands.

For defenders: you cannot protect what you do not know you have. Run the same recon on yourself.

2. Understand the application before attacking it

Use the product like a real customer. Sign up, create objects, invite teammates, change settings, upgrade plans. Map the roles, the data model and the workflows. The questions that lead to bugs come from understanding: "What stops a member from doing what an admin can do?" "What ties this invoice to this account?"

For defenders: threat modelling is the same exercise. Ask those questions during design.

3. Test access control relentlessly

With two accounts, ideally in two organisations, replay every request from one account using the other's identifiers. Try admin functions as a normal user. Add fields like role to update requests. This is where many high-severity bounties come from: IDOR, broken object level authorisation and privilege escalation.

For defenders: centralise authorisation and write tests that use two users.

4. Follow the data

Track where input goes. A name you entered appears in an admin dashboard, an email, a PDF and an export. Each is a different rendering context, and one may not escape it. A URL you supplied is fetched by the server, which raises SSRF questions. A file you uploaded is processed by a library.

For defenders: consider every place data is consumed, not just where it is entered.

5. Look at the edges

• Old API versions that missed recent fixes.
• Mobile APIs that are less tested than the web app.
• Password reset, email change and account linking flows.
• Import, export and integration features.
• Features launched recently.

For defenders: your newest and oldest code deserve extra testing.

6. Chain small issues

An open redirect alone may be low severity. Combined with an OAuth flow, it becomes account takeover. An information leak reveals IDs that enable an IDOR. Hunters think in chains.

For defenders: do not dismiss low-severity findings without asking what they could combine with.

7. Verify and report clearly

A good report shows exact steps, evidence and impact. Without verification, a "possible" bug is just noise.

For defenders: prefer testing that verifies findings with evidence.

Turning methodology into automation

This methodology is what SecurityInfinity's agents are built on: recon first, understanding the application, multi-identity access control testing, following data into every context, chaining findings, and verifying everything before it reaches your report. It runs in minutes, on every release. Start free with 100 credits.

Frequently asked questions

Do hackers mostly use automated tools?

Tools handle recon and repetitive checks, but the best findings come from understanding the application and testing its logic.

What is the most common high-severity bug in bug bounties?

Access control issues, such as IDOR and privilege escalation, are consistently among the most common serious findings.

How can a company learn from bug bounty hunters?

Run the same recon on yourself, test with multiple accounts, review the edges of your application and take chained low-severity findings seriously.