CVE-2025-39363 is a stored Cross-Site Scripting (XSS) vulnerability classified under CWE-79, affecting the AlphaEfficiencyTeam's Custom Login and Registration product. This vulnerability arises due to improper neutralization of input during web page generation, allowing malicious scripts to be stored and later executed in the context of users' browsers. Specifically, the flaw exists in versions up to 1.0.0 of the product, though exact affected versions are not fully enumerated. The vulnerability enables an attacker with at least limited privileges (PR:L) and requiring user interaction (UI:R) to inject malicious JavaScript code that is persistently stored and served to other users. The CVSS 3.1 base score is 6.5 (medium severity), reflecting network attack vector (AV:N), low attack complexity (AC:L), partial privileges required, and user interaction needed. The scope is changed (S:C), indicating that exploitation can affect resources beyond the vulnerable component. The impact includes partial confidentiality, integrity, and availability loss, as the injected scripts can steal session tokens, manipulate page content, or perform actions on behalf of users. No known exploits are reported in the wild yet, and no patches have been linked at the time of publication. The vulnerability was reserved in mid-April 2025 and published in early May 2025. Stored XSS vulnerabilities are particularly dangerous in login and registration modules because they can affect authentication workflows and user data entry points, potentially leading to account takeover or broader compromise within affected web applications.

For European organizations using AlphaEfficiencyTeam's Custom Login and Registration plugin, this vulnerability poses a significant risk to user data confidentiality and application integrity. Attackers exploiting this stored XSS could hijack user sessions, steal credentials, or perform unauthorized actions, potentially leading to data breaches or unauthorized access to sensitive systems. Given the vulnerability affects authentication-related components, the risk of account takeover is heightened. This can undermine trust in services, lead to regulatory non-compliance (e.g., GDPR violations due to data leakage), and cause reputational damage. Additionally, the scope change means that exploitation could impact other components or services integrated with the vulnerable module, amplifying the potential damage. European organizations with customer-facing web applications or internal portals using this product are at risk of targeted attacks, especially if they have users with elevated privileges or if the product is integrated into critical workflows.

1. Immediate mitigation should include disabling or restricting access to the vulnerable Custom Login and Registration module until a patch is available. 2. Implement strict input validation and output encoding on all user-supplied data fields within the login and registration workflows to neutralize malicious scripts. 3. Employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts in browsers. 4. Conduct thorough code reviews and penetration testing focusing on input handling in authentication modules. 5. Monitor web application logs for unusual input patterns or script injection attempts. 6. Educate developers and administrators about secure coding practices related to XSS prevention, especially in authentication components. 7. Once a vendor patch is released, prioritize immediate deployment after testing in staging environments. 8. Consider implementing multi-factor authentication (MFA) to reduce the impact of potential session hijacking. 9. Use web application firewalls (WAFs) with updated rules to detect and block XSS payloads targeting this product.