I Found 15 Critical Vulnerabilities in One Afternoon

It was supposed to be a routine Wednesday. I had a few hours between meetings, my coffee was still warm, and I decided to poke around a new bug bounty program on HackerOne. The target was a mid-sized SaaS platform — nothing massive, but with a complex API that looked promising.

I expected to spend an hour or two doing some basic reconnaissance before moving on to other things. I didn't expect to break the application in fifteen distinct ways before the sun went down.

This isn't a post to brag. I'm not a wizard, and the target wasn't a poorly coded toy. This post is about pattern recognition, methodology, and the compounding nature of vulnerabilities. When you find one flaw, it often exposes the next.

If you are an aspiring bug bounty hunter or a developer looking to secure your own applications, this breakdown of my afternoon might offer some valuable insights into how critical flaws hide in plain sight.

The Setup and Methodology

Before I even sent a single request to the target, I spent twenty minutes mapping the application. Many hunters dive straight into automated scanners or start throwing payloads at endpoints. I prefer a more architectural approach.

My methodology for this session was simple but rigorous:

1.Subdomain Enumeration & Content Discovery: I used tools like subfinder and ffuf to map the entire attack surface. I wasn't just looking for the main domain; I wanted the dev, staging, and internal API endpoints.

2.Proxy Traffic Capture: I fired up Burp Suite and navigated the application as a standard user, creating multiple accounts with different roles (Admin, Standard User, Read-Only).

3.API Endpoint Analysis: I focused heavily on the GraphQL and REST APIs, exporting the traffic history to analyze parameter structures and access controls.

By 2:00 PM, I had a comprehensive map of the application's architecture and a clear understanding of its user roles. The real hunting began.

The Avalanche: Finding the First 5 Vulnerabilities

The first vulnerability I found was the catalyst for the rest. It was a classic Insecure Direct Object Reference (IDOR). The application allowed users to view their own profile via an API call to /api/v1/users/{user_id}. The user_id was a predictable sequential integer.

I requested /api/v1/users/1 and immediately received the profile data of an Administrator.

But it didn't stop there. Because I had the Admin's user ID, I looked for functions that accepted a user ID parameter. I found a password reset endpoint: /api/v1/users/{user_id}/reset-password. I submitted a POST request to reset the Administrator's password. The server responded with a 200 OK and a new token.

That was two critical vulnerabilities in one chain.

I immediately halted my testing on that specific endpoint to avoid actually compromising the Admin account, but the IDOR had opened the floodgates. I began testing other endpoints using the Admin's ID as the target.

1.IDOR in User Profile: Accessing other users' PII.

2.IDOR in Password Reset: Ability to reset any user's password.

3.Privilege Escalation (BOLA): By intercepting a request to update user roles, I changed the role parameter from "user" to "admin". The server accepted it.

4.Stored XSS in User Bio: The Admin bio field lacked output encoding. I injected , and it executed on every profile page.

5.Unrestricted File Upload: The profile picture upload endpoint accepted PHP files and didn't enforce MIME type validation.

By 3:30 PM, my coffee was cold, and I had five critical findings. But the momentum was building.

Expanding the Attack Surface

Once I established a foothold with these initial findings, I shifted my focus to the underlying infrastructure and how the application handled data. The complexity of the system meant that fixing one issue wouldn't magically secure the rest.

I began looking for vulnerabilities related to data injection and parsing. I discovered that the application's search functionality used Elasticsearch under the hood. The search query parameter was directly concatenated into the Elasticsearch JSON payload.

By injecting specific JSON syntax into the search bar, I triggered an Insecure Deserialization vulnerability, which allowed me to execute arbitrary commands on the backend. This was a Remote Code Execution (RCE) flaw.

With RCE secured, I pivoted back to the web layer to look for lower-hanging fruit that automated scanners often miss.

Vulnerability

Endpoint / Location

Impact

SQL Injection

/api/v1/search?query=

Extracting the entire user database via UNION-based injection.

Server-Side Request Forgery (SSRF)

/api/v1/fetch?url=

Forcing the server to make requests to internal metadata services (e.g., AWS EC2).

XML External Entity (XXE)

/api/v1/xml-import

Reading local files (/etc/passwd) via malicious XML payloads.

Path Traversal

/api/v1/docs?file=

Accessing sensitive configuration files outside the web root.

Open Redirect

/auth/redirect?url=

Bypassing OAuth authentication flows.

The Final Push: 15 Before Sunset

As the afternoon wore on, I found myself falling into a flow state. Every time the application failed to validate input in one area, I tested it in another.

I noticed that the application used WebSockets for real-time notifications. The WebSocket authentication relied on a JWT token passed in the URL query string. However, the token validation logic was flawed — it accepted any token signed with an empty secret key (""). This allowed me to forge administrative JWT tokens and gain instant access to internal admin panels.

The final four vulnerabilities I found before packing up for the day were deeply intertwined with the application's logic:

1.Race Condition in Coupon Redemption: By sending multiple parallel requests to redeem a single-use coupon, I bypassed the database lock and redeemed it multiple times.

2.Broken Access Control on Admin API: The /admin endpoints were protected only by frontend route guards, not backend authentication checks.

3.Information Disclosure via Source Map: The application leaked its entire minified React source code and API keys via exposed .js.map files.

4.Prototype Pollution: By injecting proto properties into a JSON payload submitted to the user settings API, I modified the base object prototype of the Node.js server.

The Anatomy of a "Lucky" Afternoon

It is easy to look at a list of 15 critical vulnerabilities and assume the target was incredibly insecure or that I got lucky. The truth is a mix of both, but heavily skewed toward systematic testing.

The "luck" was the initial IDOR. The rest was pure methodology. Once I bypassed the access controls, the application's poor input validation across the board became the obvious next step.

Here are the key takeaways from this experience:

1. Context is Everything

Automated scanners are great for finding known CVEs in outdated libraries, but they miss logic flaws. By understanding how the application was supposed to work, I could test where it broke those rules.

2. The Chain Reaction

Vulnerabilities rarely exist in isolation. An IDOR often leads to a privilege escalation, which then opens the door to stored XSS or RCE. Finding one bug should always prompt you to ask, "What else can I do with this access?"

3. Look Beyond the Main App

Don't just focus on the primary user interface. Dig into the APIs, the WebSockets, the background workers, and the third-party integrations. The edges of an application are often its weakest points.

Responsible Disclosure and The Aftermath

Finding 15 critical bugs in an afternoon is exciting, but the responsibility of disclosing them is paramount. I immediately compiled a comprehensive, well-structured report detailing each vulnerability, complete with Proof of Concept (PoC) scripts and reproduction steps.

I submitted the report to the HackerOne program. Within two hours, the security team acknowledged the submission.

What followed was a tense but productive 48 hours of collaboration. The development team was initially overwhelmed, but the clear, actionable reports helped them triage and begin patching the most severe issues — starting with the IDOR and the authentication bypass.

For my efforts, I was awarded a significant bounty, but the real reward was the satisfaction of knowing that a major security overhaul was underway because of the work done in a single afternoon.

Lessons for Developers and Hunters Alike

Whether you are a bug bounty hunter looking for your next payout or a developer tasked with securing your company's platform, this scenario highlights a critical reality of modern web security.

For Hunters: Do not abandon a target just because you haven't found a bug in the first hour. Deepen your understanding of the architecture. Map the endpoints. Test the edge cases. The biggest bounties are often hidden behind complex business logic.

For Developers: Defense in depth is not just a buzzword. Relying solely on frontend validation or basic WAF rules is a recipe for disaster. Implement strict access controls on every API endpoint, enforce the principle of least privilege, and regularly conduct thorough penetration testing.

The internet is built on code, and where there is code, there are bugs. The goal isn't to eliminate them entirely, but to find them before the malicious actors do.

Have you ever found a string of vulnerabilities in a single session? Share your methodology and experiences in the comments below.