CVE-2025-59526 is a security vulnerability classified as CWE-79, which corresponds to Cross-site Scripting (XSS) due to improper neutralization of input during web page generation. The affected product is 'mailgen', a Node.js package used to generate responsive HTML emails for transactional purposes. Specifically, versions of mailgen prior to 2.0.30 are vulnerable. The vulnerability arises when the method Mailgen.generatePlaintext(email) is used with user-generated content that contains HTML. This method does not properly sanitize or neutralize HTML tags in the input, leading to an HTML injection vulnerability. An attacker could potentially inject malicious HTML or script content into the plaintext email output, which could be rendered by email clients that support HTML or rich text, potentially leading to XSS attacks. The vulnerability has been patched in version 2.0.30. As a workaround, users can strip all HTML tags from any user-generated content before passing it to the Mailgen.generatePlaintext(email) method. The CVSS 4.0 score is 2.7, indicating a low severity vulnerability. The vector indicates that the attack vector is network-based, with low complexity, no privileges or user interaction required, and no impact on confidentiality, integrity, or availability, but with limited scope and impact on system integrity or availability. There are no known exploits in the wild at this time.

For European organizations, the impact of this vulnerability is generally low but should not be ignored. Organizations using mailgen in their email generation workflows, especially those that incorporate user-generated content into transactional emails, could be at risk of delivering emails containing malicious HTML or scripts. This could lead to phishing attacks, social engineering, or the execution of malicious code in the context of the email client, potentially compromising user data or credentials. While the vulnerability does not directly affect system confidentiality, integrity, or availability, it could be leveraged as part of a broader attack chain targeting employees or customers. Given the widespread use of Node.js and email-based communications in Europe, organizations in sectors such as finance, healthcare, and government that rely on transactional emails should be particularly cautious. The low CVSS score reflects the limited direct impact and ease of exploitation, but the risk increases if attackers combine this vulnerability with other weaknesses or social engineering tactics.

To mitigate this vulnerability, European organizations should upgrade mailgen to version 2.0.30 or later immediately. If upgrading is not feasible in the short term, implement a strict input sanitization process that strips all HTML tags and potentially dangerous characters from any user-generated content before passing it to the Mailgen.generatePlaintext(email) method. Additionally, review email generation workflows to ensure that no untrusted content is embedded without proper sanitization. Employ Content Security Policy (CSP) headers where applicable in webmail clients and educate users about the risks of interacting with unexpected or suspicious email content. Regularly audit dependencies for vulnerabilities and maintain an up-to-date software inventory to quickly identify and remediate such issues. Monitoring email logs for unusual content or patterns can also help detect exploitation attempts early.