Prevent Phishing Click-Jacking in Google Workspace Email Templates

Ever opened a perfectly legit‑looking Google Workspace email, clicked a button, and then realized you just handed over your credentials to a stranger? I’ve been there. The feeling of “what the heck just happened?” sticks around longer than a coffee‑stained pull‑request comment. Click‑jack phishing sneaks into that exact moment right when you think you’re safe inside your org’s inbox.

Understanding Click‑Jack Phishing in Google Workspace

Look, click‑jack isn’t just a buzzword. It’s a technique where an attacker overlays an invisible button on top of a legitimate UI element, tricking the user into clicking something malicious. In Google Workspace, that usually means a crafted email template that hides a <iframe> or a malicious link behind a “Send” button that actually forwards your credentials to a rogue server.

What most people miss: Google’s own sandbox for Gmail doesn’t stop an attacker from embedding a malicious frame inside a custom email template. The sandbox only restricts script execution, not the visual trickery.

<!-- WRONG: Invisible iframe that covers the whole email body -->
<div style="position:relative;">
  <iframe src="https://evil.example.com/steal" style="opacity:0;position:absolute;top:0;left:0;width:100%;height:100%;z-index:999;"></iframe>
  <button style="position:relative;z-index:1000;">Approve Request</button>
</div>

Pro tip: That opacity:0; is the classic “hide‑and‑seek” move. Gmail will render it, and the user never sees a thing.

The fix? Never trust raw HTML in templates. Use Google’s Email Markup with strict CSP, or at the very least, enforce sandbox attributes on any embedded frames.

<!-- RIGHT: Sandbox the iframe, limit actions, and add a clear visual cue -->
<div style="position:relative;">
  <iframe src="https://trusted.example.com/info"
          sandbox="allow-same-origin allow-forms"
          style="border:1px solid #ddd;position:absolute;top:0;left:0;width:100%;height:100%;z-index:1;"></iframe>
  <button style="position:relative;z-index:2;background:#0066cc;color:#fff;padding:8px 12px;">
    Approve Request
  </button>
</div>

Key takeaway: Never embed an unrestricted iframe in a Gmail template. The sandbox attribute is your first line of defense.

Common Attack Vectors in Email Templates

• Hidden overlays – invisible <div>s that sit on top of legit buttons.

• Malicious mailto: links – crafted to auto‑fill credentials.

• target="_blank" without rel="noopener noreferrer" – opens a new tab that can hijack the original window via window.opener.

• Base64‑encoded payloads – sneaky data that decodes to a script when the email client renders it.

Here’s a “gotcha” I saw on a client’s internal announcement. The dev used target="_blank" to open a help article, but forgot the rel attribute. The attacker swapped the URL with a phishing page that stole the OAuth token.

<!-- WRONG: Opens a new tab that can control the original window -->
<a href="https://evil.example.com/phish" target="_blank">Read the guide</a>

And the corrected version?

<!-- RIGHT: Prevents the new tab from accessing the opener -->
<a href="https://trusted.example.com/guide" target="_blank" rel="noopener noreferrer">
  Read the guide
</a>

Another sneaky vector is a Base64‑encoded image that, when decoded, contains an SVG with embedded JavaScript. Gmail strips most scripts, but SVG can still execute in some clients.

<!-- WRONG: Base64‑encoded SVG that runs script -->
<img src="data:image/svg+xml;base64,PHN2ZyBvbmNsb3NlPSJhbGVydCgnc2NyaXB0Jyk...">

Fix it by disallowing data: URIs altogether in your template generator, or at least whitelist safe image types.

<!-- RIGHT: Use a hosted PNG instead of a data URI -->
<img src="https://cdn.example.com/logo.png" alt="Company Logo">

Bottom line: Validate every attribute that can change navigation or embed content. A simple regex check (see Regex Tester) can catch most of these.

Detection Techniques for IT Administrators

Think about it: you can’t stop what you don’t see. I set up a daily Apps Script that pulls every saved Gmail template and runs a static analysis looking for suspicious patterns. The script logs any matches to Stackdriver, where I get a Slack alert.

// WRONG: Scans only for <script> tags, misses hidden iframes
function scanTemplates() {
  var templates = GmailApp.getDraftMessages();
  templates.forEach(t => {
    if (t.getBody().includes('<script')) {
      Logger.log('Potential malicious script in template: ' + t.getId());
    }
  });
}

Warning: That approach gives a false sense of security. Attackers rarely use <script> tags in Gmail.

The improved version adds checks for iframe with no sandbox, target="_blank" without rel, and Base64 data URIs.

// RIGHT: Comprehensive pattern detection
function scanTemplates() {
  var templates = GmailApp.getDraftMessages();
  var suspicious = [];
  var patterns = [
    /<iframe[^>]*src=["'][^"']+["'][^>]*>/i,            // any iframe
    /target="_blank"(?![^>]*rel=["'].*noopener.*["'])/i, // missing rel
    /data:image\/svg\+xml;base64,/i                    // risky data URI
  ];
  templates.forEach(t => {
    var body = t.getBody();
    patterns.forEach(p => {
      if (p.test(body)) {
        suspicious.push({id: t.getId(), snippet: body.match(p)[0]});
      }
    });
  });
  if (suspicious.length) {
    suspicious.forEach(s => {
      Logger.log('Suspicious pattern in template %s: %s', s.id, s.snippet);
    });
    // Send to Slack (pseudo‑code)
    // sendToSlack(suspicious);
  }
}

Running this script caught a rogue template that used an invisible iframe to load https://phish.example.net. The alert saved the org from a potential credential dump.

Takeaway: Automate pattern scanning, and include all known vectors, not just the obvious ones.

Preventive Measures for Template Designers

When I was building a quarterly newsletter for my team, I thought “just paste the HTML and go.” Bad move. The real safeguard is to sanitize any user‑generated content before it hits the template engine. I used the DOMPurify library in a Node.js build step, but I initially configured it to allow iframe tags big oops.

// WRONG: Allowing iframes defeats the purpose
const DOMPurify = require('dompurify')(window);
let clean = DOMPurify.sanitize(dirtyHtml, { ALLOWED_TAGS: ['b','i','iframe'] });

The corrected config strips out any potentially dangerous elements and only permits a safe whitelist.

// RIGHT: Strict whitelist, no iframes
const DOMPurify = require('dompurify')(window);
let clean = DOMPurify.sanitize(dirtyHtml, {
  ALLOWED_TAGS: ['b','i','u','a','img','p','br','ul','li','ol'],
  ALLOWED_ATTR: ['href','src','alt','title','style']
});

Even better, enforce a Content‑Security‑Policy header on any hosted assets referenced by the template. For Gmail add‑ons, you can specify CSP in the manifest:

{
  "name": "Secure Email Template",
  "description": "Template with strict CSP",
  "manifest_version": 2,
  "csp": "script-src 'self'; object-src 'none';"
}

Pro tip: If you need to embed an iframe (e.g., a video), host it on a domain you control and add sandbox attributes both in the HTML and CSP.

Another practical tip: always validate URLs before they go into the template. A quick call to the URL Encoder & Decoder can reveal hidden characters that attackers use to obscure malicious domains.

Bottom line: Sanitize, whitelist, and never assume a URL is safe just because it looks nice.

User Education & Incident Response

Let’s be real: tech controls only go so far. The moment a user clicks a malicious button, you need a clear incident response plan. I once rolled out a “Phishing Alert” banner that was too subtle—people ignored it. The revised version is loud, red, and includes a single “Report” button that auto‑generates a ticket.

<!-- WRONG: Faint banner that blends with the email -->
<div style="background:#f9f9f9;padding:5px;">
  <p>Possible phishing detected.</p>
</div>

<!-- RIGHT: Bold banner with actionable link -->
<div style="background:#ffdddd;border:1px solid #ff0000;padding:10px;">
  <strong>⚠️ Phishing warning:</strong> This email may be fraudulent.
  <a href="https://security.example.com/report?msgId={{MESSAGE_ID}}" 
     style="margin-left:10px;color:#d00;font-weight:bold;">Report it</a>
</div>

When the user clicks “Report it,” a Google Workspace Apps Script logs the message ID, notifies the security team via Chat, and revokes any OAuth tokens associated with the sender.

// Simple incident response script (pseudo‑code)
function onReport(e) {
  var msgId = e.parameter.msgId;
  // Log to Cloud Logging
  console.log('User reported phishing: ' + msgId);
  // Notify security channel
  var payload = {text: 'Phishing reported for message: ' + msgId};
  UrlFetchApp.fetch('https://chat.googleapis.com/v1/spaces/.../messages', {
    method: 'post',
    contentType: 'application/json',
    payload: JSON.stringify(payload)
  });
}

Key takeaway: Make the reporting flow obvious and automate the backend cleanup. The faster you cut off the attacker's foothold, the less damage you do.

Quick Summary

I’ve lived through a night‑mare where an invisible iframe stole my admin credentials. The fix? Sanitize everything, sandbox any frame, scan templates with a robust script, and train users to hit the big red “Report” button. If you ignore any of those layers, you’re leaving a door open for a click‑jack thief. My advice: treat every email template like a public‑facing web page apply the same hardening you’d use for a React app.

FAQs

Q1. How can I programmatically detect hidden iframes in a Gmail draft using Apps Script?
A: Use a regular expression that looks for <iframe tags without a sandbox attribute, as shown in the detection script above. Combine it with GmailApp.getDraftMessages() to iterate over drafts.

Q2. Does Google Workspace’s built‑in sanitizer strip out data: URIs?
A: No. Gmail’s sanitizer removes most script tags, but it allows data: URIs for images. You need to add an extra validation step (e.g., reject any src that starts with data:) before saving the template.

Q3. Can I enforce a CSP for Gmail add‑on HTML content?
A: Yes. Define the csp field in the add‑on’s manifest JSON. Keep it restrictive script-src 'self' and object-src 'none' are good defaults.

Q4. What’s the safest way to let users insert external links in a template?
A: Encode the URL with the URL Encoder & Decoder, validate it against an allowlist of domains, and always add rel="noopener noreferrer" when using target="_blank".

Q5. How do I automatically create a ticket when a user reports a phishing email?
A: Expose a simple HTTP endpoint via Apps Script (doGet(e)) that receives the msgId, then use the Google Cloud Logging API and your ticketing system’s webhook to create the incident. The sample incident response script demonstrates the basics.