CVE-2025-66103 identifies a DOM-based Cross-site Scripting (XSS) vulnerability in the revmakx WPCal.io plugin for WordPress, affecting all versions up to and including 0.9.5.9. This vulnerability stems from improper neutralization of user-supplied input during the generation of web pages, allowing malicious scripts to be injected and executed within the context of the victim's browser. The attack vector is remote (network accessible), requires low attack complexity, and privileges at the level of a logged-in user (PR:L), with user interaction necessary to trigger the exploit (UI:R). The vulnerability impacts confidentiality, integrity, and availability, as attackers can steal session tokens, manipulate page content, or perform actions on behalf of the user. The scope is changed (S:C), indicating that the vulnerability affects resources beyond the initially vulnerable component, potentially impacting the entire user session or site. Although no known exploits have been reported in the wild, the vulnerability poses a significant risk to websites using this plugin. The lack of available patches at the time of publication increases urgency for mitigation. The vulnerability is categorized under improper input neutralization during web page generation, a common cause of DOM-based XSS, which is particularly dangerous because it exploits client-side script execution and can bypass some server-side defenses.

The impact of CVE-2025-66103 is substantial for organizations running WordPress sites with the revmakx WPCal.io plugin. Successful exploitation can lead to theft of sensitive information such as authentication cookies, session tokens, or personal data, enabling account takeover or unauthorized actions. It can also facilitate phishing attacks by injecting deceptive content or redirecting users to malicious sites. The integrity of website content can be compromised, damaging organizational reputation and user trust. Availability may be affected if attackers use the vulnerability to execute denial-of-service attacks or disrupt normal site operations. Given the plugin’s role in calendar and event management, disruption could impact business operations relying on scheduling and customer engagement. The vulnerability’s requirement for user interaction limits automated exploitation but does not eliminate risk, especially in environments with high user traffic. Organizations lacking timely patching or compensating controls face increased exposure to targeted attacks and broader exploitation campaigns once public exploits emerge.

Organizations should prioritize updating the revmakx WPCal.io plugin to the latest patched version as soon as it becomes available. Until a patch is released, implement strict input validation and output encoding on all user-supplied data processed by the plugin to prevent script injection. Deploy Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts and reduce the impact of potential XSS attacks. Review and minimize user privileges to reduce the attack surface, ensuring users have only necessary access rights. Employ web application firewalls (WAFs) with rules tailored to detect and block XSS payloads targeting this plugin. Conduct security audits and penetration testing focusing on client-side script handling within the plugin. Educate users about the risks of interacting with suspicious links or content, as user interaction is required for exploitation. Monitor web server and application logs for unusual activity indicative of attempted exploitation. Finally, maintain an incident response plan that includes procedures for handling XSS incidents to minimize damage and recovery time.