CVE-2025-66111 identifies a Stored Cross-Site Scripting (XSS) vulnerability in the Nelio Popups WordPress plugin developed by Nelio Software, affecting all versions up to 1.3.0. The vulnerability stems from improper neutralization of user-supplied input during the generation of web pages, allowing malicious scripts to be injected and stored within the plugin's data. When a victim accesses a page containing the malicious popup, the embedded script executes in their browser context, potentially enabling attackers to steal session cookies, perform actions on behalf of the user, or redirect users to malicious sites. This type of vulnerability is particularly dangerous because the malicious payload is persistent and does not require the attacker to trick the user into clicking a link or performing additional actions beyond visiting the affected page. No authentication is required to exploit this flaw, increasing its risk profile. Although no public exploits have been reported yet, the widespread use of WordPress and plugins like Nelio Popups in European organizations increases the likelihood of exploitation attempts. The lack of a CVSS score indicates that the vulnerability is newly published and pending detailed scoring. However, the nature of stored XSS vulnerabilities typically results in high severity due to their impact on confidentiality, integrity, and availability of user sessions and data. The vulnerability affects the plugin's core functionality of generating popups, which are often used for marketing, user engagement, or information collection, thereby increasing the attack surface. Organizations relying on Nelio Popups should monitor for patches and apply them promptly while employing additional security controls such as input validation, output encoding, and Content Security Policies to mitigate potential exploitation.

For European organizations, this vulnerability poses significant risks to web application security, particularly for those using Nelio Popups on WordPress sites. Exploitation could lead to unauthorized access to user sessions, theft of sensitive information such as credentials or personal data, and unauthorized actions performed on behalf of users, potentially damaging organizational reputation and violating data protection regulations like GDPR. The persistent nature of stored XSS increases the risk of widespread impact across multiple users. Organizations in sectors such as e-commerce, finance, healthcare, and public services, where user trust and data confidentiality are paramount, are especially vulnerable. Additionally, compromised websites could be used as vectors for further attacks or phishing campaigns targeting European users. The absence of known exploits currently provides a window for proactive mitigation, but the ease of exploitation and the common use of the affected plugin elevate the threat level. Failure to address this vulnerability promptly could result in regulatory penalties and loss of customer trust.

1. Monitor Nelio Software’s official channels for security patches addressing CVE-2025-66111 and apply updates immediately upon release. 2. Implement strict input validation and sanitization on all user-supplied data that the plugin processes to prevent injection of malicious scripts. 3. Employ robust output encoding techniques when rendering data in web pages to neutralize potentially harmful content. 4. Configure Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of XSS attacks. 5. Conduct regular security audits and penetration testing focused on web application vulnerabilities, including XSS. 6. Educate website administrators and developers about secure coding practices and the risks associated with third-party plugins. 7. Consider temporarily disabling Nelio Popups if immediate patching is not possible and the risk is deemed high. 8. Utilize web application firewalls (WAFs) with rules designed to detect and block XSS payloads targeting known vulnerable endpoints. 9. Review and limit user permissions within WordPress to minimize the potential damage from compromised accounts. 10. Maintain comprehensive logging and monitoring to detect suspicious activities that may indicate exploitation attempts.