Security assessments often start with a familiar pattern: throw payloads, observe failures, move on. During a recent VAPT engagement, the target application initially appeared exceptionally hardened. Input validation was strict, common injection vectors failed, and encryption mechanisms seemed correctly implemented.
Yet the most interesting vulnerabilities rarely announce themselves through obvious breakage.
Sometimes, they quietly hide behind "harmless" features.
What began as a routine test eventually uncovered a subtle but highly exploitable issue — one that transformed a benign profile field into a phishing vector delivered through a trusted channel.
🧭 Background
From an attacker's perspective, the application looked resilient:
- Standard XSS payloads → Blocked
- SQL injection attempts → Neutralized
- Malformed input → Sanitized
Repeated failures suggested robust server-side validation. With no immediate foothold through traditional injection techniques, deeper functional testing became the next logical step.
That's where the Profile section drew attention.
🔍 The Discovery
Inside the user profile settings was a simple text field intended for profile details. Nothing unusual — until testing with basic HTML markup:
<a href="https://attacker-control-site">Verify your account</a>
After saving, the application displayed the input as inert text. No rendering, no execution, no obvious vulnerability.
At first glance, everything behaved securely.
But security testing rewards curiosity and verification.
Later, while reviewing system-generated emails from the application, something unexpected appeared.
The HTML tag was rendering inside the email body.
📧 Stored HTML Injection via Email Templates
Although the web interface neutralized HTML content, the backend email template did not.
Result:
- User-supplied HTML persisted
- Email engine rendered markup
- Clickable attacker-controlled links delivered to recipients
This is a classic stored HTML injection, but with an important nuance: the payload executes not in the browser, but in the recipient's email client.
That distinction significantly alters the threat model.
Users inherently trust emails originating from legitimate services.
🧪 Escalation: The Critical Chaining Issue
Individually, HTML rendering inside emails is concerning.
Chained with flawed account controls, it becomes dangerous.
Testing the Change Email functionality revealed a second weakness:
- No effective ownership verification
- No confirmation workflow
- No validation of recipient identity
Any authenticated user could modify their account email address to an arbitrary target.
Which means…
An attacker could:
- Inject malicious HTML into their profile
- Change the account email to a victim's address
- Trigger application emails containing rendered payloads
Effectively converting the platform into a phishing delivery mechanism.
⚠️ Why This Matters
This vulnerability chain enables high-credibility social engineering:
✅ Trusted Delivery Channel
Emails originate from a legitimate domain, bypassing many suspicion triggers.
✅ Scalable Phishing Vector
Attackers can repeatedly target users without external infrastructure.
✅ Credential Harvesting Risk
Rendered links can redirect victims to cloned authentication portals.
✅ Reputational & Brand Damage
Users associate malicious activity with the legitimate service.
Unlike conventional phishing, this attack abuses the application itself as the delivery system.
🧠 Root Cause Analysis
Two independent design failures created the exploit path:
1. Inconsistent Output Handling User input sanitized in UI but rendered in email templates.
2. Broken Ownership Validation Sensitive account attribute (email) changeable without verification.
Security controls applied in isolation are insufficient. Attackers exploit gaps between components.
💣 Impact Summary
A realistic attacker could:
- Deliver convincing phishing emails
- Impersonate platform communications
- Trick users into credential disclosure
- Undermine user trust in the service
All without exploiting memory corruption, authentication bypass, or complex injection techniques.
🔐 Mitigation Strategies
Preventing this class of issue requires layered controls:
✅ Sanitize at Every Rendering Context
Validation must consider where input is displayed (UI vs email vs PDF vs API).
✅ Treat Emails as Active Content Surfaces
HTML rendering in emails should be tightly controlled or neutralized.
✅ Enforce Email Ownership Verification
Require confirmation workflows for email changes.
✅ Adopt Context-Aware Encoding
Avoid assuming sanitization in one component protects another.
✍️ Final Thoughts
This case highlights an important lesson in application security:
A feature that appears safe in one context may become exploitable in another.
Modern applications are ecosystems — web clients, APIs, email engines, background workers. Security assumptions rarely transfer cleanly between them.
Seemingly minor inconsistencies often produce major vulnerabilities.
#Cybersecurity #WebSecurity #ApplicationSecurity #Phishing #PenetrationTesting #Infosec #SecurityTesting #BugBounty