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Introduction:
In the ever-evolving landscape of web security, Stored HTML Injection remains a deceptively simple yet critically dangerous vulnerability. As highlighted by a recent bug bounty discovery, an attacker can embed malicious HTML or JavaScript into a platform’s database—often through a comment section—which then executes for every subsequent visitor. This turns a trusted website into an unwitting accomplice for phishing, credential theft, and user redirection attacks.
Learning Objectives:
- Understand the mechanics and real-world impact of Stored HTML Injection vulnerabilities.
- Learn to identify, test for, and responsibly disclose this class of web application flaw.
- Implement robust defensive coding practices to sanitize user input and secure application output.
You Should Know:
1. Deconstructing the Stored HTML Injection Vulnerability
A Stored HTML Injection flaw occurs when a web application fails to properly validate or sanitize user-supplied input before permanently storing it in a database, file, or other storage mechanism. Unlike reflected attacks, the injected payload is “stored” and served to all users who view the compromised page, amplifying its impact exponentially. The core failure is the absence of proper context-aware output encoding.
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Identify Input Vectors. Look for any user-controlled data that is persisted and displayed: comment boxes, user profiles, support tickets, product reviews, or forum posts.
Step 2: Craft a Basic Probe. Submit a simple HTML tag to see if it’s rendered. For example, in a comment field, try: `test` or <i>injection</i>. If the text appears bold or italicized upon reload, the site is vulnerable.
Step 3: Escalate the Payload. If basic tags work, an attacker can escalate to steal cookies or redirect users. A classic proof-of-concept payload is: <img src="x" onerror="alert(document.cookie)">. This attempts to load a broken image, triggering the JavaScript `onerror` event handler.
2. From Proof-of-Concept to Full-Scale Exploitation
A simple alert box proves vulnerability, but real-world attacks are more sinister. The goal is to craft payloads that perform malicious actions silently.
Step‑by‑step guide explaining what this does and how to use it.
Step 1: Craft a Credential-Harvesting Payload. An attacker might inject a fake login overlay. The payload could be an `