The vulnerability CVE-2025-62366 affects the mailgen package, a popular Node.js library used to generate responsive HTML emails, specifically in versions before 2.0.31. The issue lies in the generatePlaintext method, which is intended to produce plaintext versions of emails by removing HTML tags from user-supplied content. However, the sanitization process only removes literal HTML tags and does not account for encoded HTML entities representing tags. When these encoded entities are decoded later, they transform into active HTML elements, such as <img> tags with event handlers, enabling cross-site scripting (CWE-79). This flaw can be exploited if the plaintext output is rendered in an HTML context, allowing attackers to execute arbitrary JavaScript code. The vulnerability does not require authentication or user interaction and can be triggered remotely by supplying crafted input to the mailgen API. Despite the low CVSS score of 2.9, the vulnerability poses a risk in environments where plaintext emails are rendered as HTML, potentially compromising confidentiality and integrity through script execution. The fix was introduced in mailgen version 2.0.31, which properly neutralizes encoded HTML entities to prevent injection. No known workarounds exist, so upgrading is essential. No public exploits have been reported to date.

For European organizations, the impact of CVE-2025-62366 depends largely on how mailgen-generated plaintext emails are handled. If the plaintext output is rendered as HTML in email clients or webmail interfaces, attackers could execute malicious scripts, potentially leading to session hijacking, phishing, or data theft. This could compromise user confidentiality and integrity of communications. Organizations using mailgen in transactional email systems that incorporate user-generated content are at risk. While the vulnerability does not affect mailgen’s HTML email generation directly, the improper handling of plaintext emails can be exploited in sophisticated phishing campaigns or targeted attacks. The low CVSS score reflects limited impact in typical use cases, but environments that render plaintext as HTML increase the risk. European companies in sectors like finance, healthcare, and government, which rely heavily on secure email communications, could face reputational damage and regulatory scrutiny if exploited. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially given the ease of exploitation without authentication or user interaction.

The primary mitigation is to upgrade mailgen to version 2.0.31 or later, where the vulnerability is fixed. Organizations should audit their use of mailgen to identify any instances of the generatePlaintext method and verify how the plaintext output is rendered or processed downstream. Avoid rendering plaintext emails as HTML or in any context that interprets HTML tags. Implement strict input validation and sanitization on user-generated content before passing it to mailgen. Employ Content Security Policy (CSP) headers in webmail or email client interfaces to restrict script execution. Conduct security reviews of email processing pipelines to detect and neutralize encoded HTML entities. Monitor for unusual email activity or phishing attempts leveraging this vulnerability. Finally, maintain an inventory of Node.js dependencies and apply timely updates to reduce exposure to similar vulnerabilities.