Cross-Site Scripting (XSS) remains one of the most consistently rewarded and impactful vulnerabilities in modern bug bounty programs. Even today — with frameworks, security headers, and filtering libraries — one overlooked input field is all it takes for an attacker to gain access to user sessions, perform unauthorized actions, or run malicious scripts inside a victim’s browser.
Recently, a security team participating in a private bug bounty program uncovered a $3,000 XSS vulnerability inside a popular SaaS platform. Although the reward was impressive, the journey to uncover it was even more valuable: it revealed how vulnerabilities can survive despite strong security processes — and why attackers still succeed.
Here’s a breakdown of how the team approached the target, the scenario they uncovered, and the lessons your engineering team should take away.
Step 1: The Recon Phase — Understanding How the Application Thinks
The SaaS platform had a well-protected authentication flow, CSP headers on major pages, and input validation on most user-facing features. At first glance, it looked bulletproof.
But experienced pentesters know:
XSS rarely hides in obvious places.
It hides in:
- forgotten legacy modules
- new features developed quickly
- admin-only pages
- user-generated content previews
- search filters
- mobile endpoints repurposed for web
During recon, the team focused on understanding how data moved through the system, not just where users typed text. That meant checking:
- What content gets rendered back to users?
- Which parts of the UI pull data using dynamic JavaScript?
- Are there pages that accept HTML-like characters?
- Are there “preview” or “draft” features?
A key find emerged: a feature that allowed users to customize their profile bio with formatting options.
Step 2: The Discovery — A Formatting Feature With a Flaw
The profile bio editor allowed bold, italics, and colored text. The platform used a filtering library intended to remove unsafe tags — but a specific formatting option slipped through the cracks.
Real-World Style Scenario (Safe Example)
Imagine this flow:
- A user writes a profile bio and adds special formatting.
- The system stores the formatted content.
- Other users viewing the profile see that content rendered dynamically.
Most formatting tags were sanitized, but one lesser-used attribute — intended for text styling — allowed unexpected characters inside it. When rendered by the browser, it could be interpreted in ways the developers never intended.
This isn’t unusual. XSS often appears in:
- SVG attributes
- data-* attributes
- markdown preview features
- custom-made rich text editors
- helpdesk ticket replies
- product review fields
The team noticed something odd:
When previewing the bio, the application inserted user content inside a JavaScript template string, which is notoriously tricky to secure if even one character is improperly escaped.
This was the opening.
Step 3: Triggering the Bug — Without Breaking Anything
Even during a bounty test, responsible security teams avoid sending disruptive or malicious scripts. Instead, they use safe, harmless payloads that simply confirm whether code executes, such as:
- harmless pop-up test
- benign console message
- safe DOM event
Once the team confirmed that their harmless test executed, they documented:
- the exact field
- how the data traveled
- which browser context executed it
- whether authentication was needed
- whether other users could be affected
The key detail:
Any visitor who viewed the malicious profile would unknowingly execute the script.
That made it a stored XSS, which is more severe (and higher paying) than reflected XSS.
Step 4: Real-World Impact — Why the Severity Was High Enough for $3,000
The platform hosted thousands of business accounts. If exploited by a malicious actor, the vulnerability could have allowed:
- session token theft
- full account takeover
- impersonation of users
- fraud or unauthorized billing actions
- phishing inside the platform
- spreading malware across user profiles
Because it was stored XSS, the exploit could spread passively: one compromised profile could trigger hundreds of cascading attacks as other users viewed it.
That level of systemic risk is exactly why bug bounty programs reward XSS so highly.
Step 5: The Report — Clear Evidence, Zero Noise
A strong report is often the difference between $300 and $3,000.
The team provided:
- a clean explanation of the flaw
- steps to reproduce (non-destructive)
- safe screenshots of the executed test
- affected endpoints
- recommended fix (server-side sanitization + CSP strengthening)
- impact analysis showing real business risk
The response from the platform’s security engineers was immediate. They acknowledged:
- XSS slipped through during a UI redesign
- automated scanning hadn’t caught the flaw
- their filter library was outdated in this module
The team received a $3,000 reward within 48 hours.
Real-World Lessons for Developers and Companies
This story highlights critical points every engineering and security team should understand:
1. New features often introduce old vulnerabilities.
A UI redesign, new editor, or new form field is enough to reintroduce XSS.
2. Client-side filters are not security.
Server-side validation and consistent escaping rules are non-negotiable.
3. CSP helps, but it cannot replace good coding practices.
4. Stored XSS is especially dangerous.
It spreads silently to every viewer.
5. Bug bounty hunters succeed because they think like attackers.
They go beyond automated scanners. They look at context, not just input boxes.
Final Thoughts
This $3,000 XSS bug wasn’t discovered with fancy zero-days or elite tricks. It was found through methodical analysis, attention to detail, and a deep understanding of how web applications render user data.
For companies, it’s a reminder:
One small oversight can lead to large-scale compromise.
For pentesters and bounty hunters, it shows:
Understanding the application is your greatest weapon — not the payload.
