CVE-2025-66500 is a stored cross-site scripting (XSS) vulnerability identified in Foxit Software Inc.'s webplugins.foxit.com platform. The root cause is 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 to a script source element without sanitization or origin checks. As a result, an attacker can craft a malicious postMessage that injects arbitrary JavaScript code into the victim's browser context when the message is processed. This stored XSS can lead to the execution of unauthorized scripts, potentially compromising user confidentiality by stealing cookies, session tokens, or other sensitive data. The vulnerability affects all versions of the product before December 1, 2025. The CVSS 3.1 base score is 6.3 (medium severity), with vector AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:L/A:N, indicating network attack vector, low attack complexity, low privileges required, user interaction needed, unchanged scope, high confidentiality impact, low integrity impact, and no availability impact. No public exploits have been reported yet. The vulnerability highlights a common web security weakness classified under CWE-79, emphasizing the need for proper input validation and origin verification in web messaging APIs. Since Foxit web plugins are widely used in document management and PDF-related web applications, this vulnerability could affect many organizations relying on these tools for document processing and collaboration.

For European organizations, the impact of CVE-2025-66500 can be significant, especially for those integrating Foxit web plugins into their document management systems, intranet portals, or customer-facing web applications. Successful exploitation could allow attackers to execute arbitrary JavaScript in the context of the affected web application, leading to theft of sensitive information such as authentication tokens, personal data, or intellectual property. This could facilitate further attacks like session hijacking, phishing, or unauthorized access to internal resources. The confidentiality impact is high, while integrity and availability impacts are lower. Given the medium CVSS score and the requirement for some user interaction, the threat is moderate but should not be underestimated. Organizations handling sensitive or regulated data (e.g., GDPR-protected personal data) face compliance risks if such vulnerabilities are exploited. Additionally, the presence of this vulnerability could undermine trust in digital services and lead to reputational damage. The lack of known exploits in the wild currently reduces immediate risk but does not eliminate the potential for future attacks.

To mitigate CVE-2025-66500, European organizations should implement the following specific measures: 1) Immediately monitor for and apply any security patches or updates released by Foxit Software addressing this vulnerability. 2) Until patches are available, restrict or disable the use of the vulnerable webplugins.foxit.com components in web applications where feasible. 3) Implement strict validation of postMessage origins in custom code to ensure messages are only accepted from trusted domains. 4) Sanitize and validate any data used to dynamically assign script sources, preventing injection of malicious URLs or scripts. 5) Deploy Content Security Policies (CSP) that restrict the execution of inline scripts and limit script sources to trusted domains, reducing the impact of injected scripts. 6) Conduct security reviews and penetration testing focused on web messaging interfaces and script injection vectors. 7) Educate developers and administrators about secure handling of postMessage APIs and cross-origin communication. 8) Monitor web application logs and network traffic for suspicious postMessage activity or unexpected script loads. These targeted actions go beyond generic advice and address the specific mechanics of this vulnerability.