CVE-2025-66500 is a stored cross-site scripting (XSS) vulnerability identified in Foxit Software Inc.'s webplugins.foxit.com platform. The vulnerability stems from improper input neutralization during web page generation, specifically within a postMessage handler that fails to validate the origin of incoming messages. This handler directly assigns the 'externalPath' parameter from the received postMessage to a script source element without sanitization or origin checks. Consequently, an attacker capable of sending a crafted postMessage can inject and execute arbitrary JavaScript in the context of the affected web plugin. This can lead to unauthorized actions such as session hijacking, data theft, or manipulation of the web application's behavior. The vulnerability affects all versions prior to December 1, 2025, and has been assigned a CVSS 3.1 base score of 6.3, reflecting medium severity. The attack vector is network-based with low attack complexity, requiring low privileges but user interaction to trigger the malicious postMessage. The impact on confidentiality is high due to potential data exposure, while integrity impact is low and availability is unaffected. No public exploits have been reported yet, but the vulnerability's nature makes it a candidate for targeted attacks, especially in environments where Foxit web plugins are integrated into enterprise web applications or document workflows.

For European organizations, this vulnerability poses a significant risk to the confidentiality of sensitive information processed or displayed through Foxit web plugins. Exploitation could lead to unauthorized access to user sessions, theft of confidential documents, or injection of malicious scripts that compromise internal systems. Organizations in sectors such as finance, healthcare, legal, and government, which often rely on Foxit products for document management and secure viewing, are particularly vulnerable. The attack requires user interaction but can be executed remotely, increasing the attack surface especially in web-facing applications. The medium severity rating suggests that while the vulnerability is not critical, it can facilitate further attacks or data breaches if combined with other weaknesses. The absence of known exploits currently provides a window for proactive mitigation before widespread exploitation occurs.

European organizations should immediately verify their use of Foxit webplugins.foxit.com and identify affected versions prior to December 1, 2025. Since no official patch links are provided yet, organizations should implement strict validation of postMessage origins within their web environments to prevent unauthorized script injection. This includes enforcing a whitelist of trusted origins and sanitizing all inputs assigned to script sources. Additionally, applying Content Security Policy (CSP) headers can help restrict script execution to trusted domains, mitigating the impact of injected scripts. Monitoring web traffic for unusual postMessage activity and educating users about the risks of interacting with untrusted web content can further reduce exposure. Organizations should stay alert for official patches or updates from Foxit and apply them promptly once available. Finally, conducting regular security assessments and penetration testing focused on web plugin integrations will help identify and remediate similar vulnerabilities.