CVE-2025-66103 is a DOM-based Cross-site Scripting (XSS) vulnerability classified under CWE-79, found in the Revmakx WPCal.io plugin, a calendar and scheduling tool commonly used within WordPress environments. The vulnerability stems from improper neutralization of user-supplied input during the generation of web pages, allowing malicious JavaScript code to be injected and executed within the context of the victim's browser. This type of XSS is particularly dangerous because it manipulates the Document Object Model (DOM) directly, bypassing some traditional server-side input validation mechanisms. The affected versions include all releases up to 0.9.5.9, with no patch currently available as per the provided data. The CVSS 3.1 base score is 6.5 (medium severity), reflecting that the attack vector is network-based with low attack complexity but requires privileges and user interaction, and the impact affects confidentiality, integrity, and availability to a limited extent. While no known exploits have been reported in the wild, the vulnerability could be leveraged by attackers to perform actions such as session hijacking, defacement, or redirecting users to malicious sites. The scope is considered changed (S:C), indicating that the vulnerability can affect resources beyond the initially vulnerable component. The vulnerability was reserved in late November 2025 and published at the end of December 2025, indicating recent discovery and disclosure.

For European organizations, the impact of CVE-2025-66103 can be significant, particularly for those relying on WPCal.io for critical scheduling or calendar functionalities integrated into their WordPress sites. Successful exploitation could lead to unauthorized access to user sessions, theft of sensitive information, manipulation of displayed data, or disruption of service availability. This could result in reputational damage, loss of customer trust, and potential regulatory penalties under GDPR if personal data is compromised. The medium severity score reflects that while the vulnerability does not allow full system compromise, it can facilitate lateral attacks or data leakage. Organizations with public-facing websites or intranet portals using this plugin are at higher risk. Additionally, the requirement for user interaction means phishing or social engineering could be used to trigger the exploit, increasing the attack surface. The absence of known exploits in the wild currently reduces immediate risk but does not preclude future exploitation, especially as threat actors often weaponize such vulnerabilities post-disclosure.

To mitigate the risks posed by CVE-2025-66103, European organizations should: 1) Monitor Revmakx’s official channels for patches or updates addressing this vulnerability and apply them promptly once available. 2) Implement strict input validation and output encoding on all user-supplied data within the WPCal.io plugin context to prevent malicious script injection. 3) Deploy Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS attacks. 4) Conduct regular security audits and penetration testing focused on web application vulnerabilities, including DOM-based XSS. 5) Educate users and administrators about phishing and social engineering tactics that could trigger the vulnerability. 6) Consider isolating or sandboxing the plugin’s functionality where feasible to limit the scope of potential exploitation. 7) Use Web Application Firewalls (WAFs) with rules tailored to detect and block XSS payloads targeting WPCal.io. 8) Review and minimize plugin permissions and privileges to reduce the attack surface. These measures, combined, will strengthen defenses beyond generic advice and help contain the threat effectively.