CVE-2025-58596 is a medium-severity vulnerability classified under CWE-79, which refers to Improper Neutralization of Input During Web Page Generation, commonly known as Cross-site Scripting (XSS). This vulnerability affects the MailOptin plugin developed by properfraction, specifically versions up to 1.2.75.0. The flaw allows an attacker to inject malicious scripts that are stored and later executed in the context of users' browsers when they access affected web pages generated by the vulnerable MailOptin plugin. The vulnerability is a Stored XSS, meaning the malicious payload is saved on the server side and served to users, increasing the potential impact compared to reflected XSS. The CVSS 3.1 score of 5.9 indicates a medium severity, with the vector string CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:L showing that the attack can be performed remotely over the network with low attack complexity but requires high privileges and user interaction. The scope is changed (S:C), meaning the vulnerability affects resources beyond the initially vulnerable component. The impact on confidentiality, integrity, and availability is low to medium, as the attacker can execute scripts that may steal user data, manipulate displayed content, or cause minor service disruptions. No known exploits are currently in the wild, and no patches have been linked yet, indicating that mitigation may rely on vendor updates or configuration changes. The vulnerability arises from insufficient input sanitization or output encoding during web page generation, allowing malicious input to be embedded in web pages served to users.

For European organizations using the MailOptin plugin, this vulnerability poses a risk of client-side attacks that can lead to theft of sensitive information such as session cookies, personal data, or credentials, potentially enabling further compromise of user accounts or systems. The stored nature of the XSS increases the risk as multiple users can be affected once the malicious payload is stored on the server. This can damage organizational reputation, lead to regulatory non-compliance (e.g., GDPR violations due to data leakage), and disrupt marketing or communication workflows dependent on MailOptin. Since the vulnerability requires high privileges to exploit, the risk is higher for organizations with multiple administrators or users with elevated rights who might inadvertently introduce malicious content. The changed scope indicates that the impact could extend beyond the plugin itself, potentially affecting other components or users interacting with the compromised pages. European organizations with public-facing websites or intranets using MailOptin are particularly at risk, especially if they do not have robust input validation or Content Security Policies (CSP) in place.

Organizations should immediately audit their use of the MailOptin plugin and restrict administrative privileges to trusted personnel only, minimizing the risk of malicious input submission. Implement strict input validation and output encoding on all user-supplied data fields within the plugin configuration or content areas to prevent injection of malicious scripts. Deploy Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS attacks. Monitor web application logs and user activity for unusual or suspicious input patterns that may indicate exploitation attempts. Until an official patch is released, consider disabling or limiting the use of vulnerable MailOptin features that accept user input or temporarily replacing the plugin with alternative solutions. Regularly update the plugin once a patch becomes available and verify that the fix properly neutralizes the input. Conduct security awareness training for administrators and users with elevated privileges to recognize and avoid introducing malicious content. Additionally, perform periodic security assessments and penetration tests focusing on web application vulnerabilities including XSS.