CVE-2025-66523 is a cross-site scripting (XSS) vulnerability categorized under CWE-79, discovered in the Foxit Software Inc. service na1.foxitesign.foxit.com. The root cause is the improper neutralization of input during web page generation, specifically the direct embedding of URL parameters into JavaScript code or HTML attributes without adequate encoding or sanitization. This flaw allows an attacker to craft a malicious URL containing executable script code. When an authenticated user clicks such a crafted link, the injected script executes in the user's browser context, potentially leading to session hijacking, theft of sensitive information, or unauthorized actions performed with the user's privileges. The vulnerability affects all versions of the service before the patch date of 2026-01-16. According to the CVSS v3.1 score of 6.1, the vulnerability is of medium severity, with an attack vector of network (remote), low attack complexity, no privileges required, but requiring user interaction. The scope is changed, indicating that the vulnerability can affect resources beyond the initially vulnerable component. No public exploits have been reported yet, but the vulnerability poses a risk especially in environments where users are likely to receive and click on links, such as corporate email or messaging platforms. The vulnerability highlights the importance of proper input validation and output encoding in web applications, particularly those handling sensitive operations like electronic signatures.

For European organizations, this XSS vulnerability could lead to unauthorized disclosure of sensitive information, such as session tokens or personal data, compromising confidentiality. Attackers could also manipulate the integrity of user interactions by performing actions on behalf of the user, such as altering documents or approvals within the Foxit e-signature platform. Although availability is not directly impacted, the trustworthiness of the service and user confidence could be undermined. Organizations relying on Foxit's e-signature services for legally binding document workflows may face compliance risks if attackers exploit this flaw to alter or intercept transactions. The requirement for user interaction means phishing or social engineering campaigns could be used to lure users into clicking malicious links. Given the widespread use of electronic signature solutions in sectors like finance, legal, and government across Europe, successful exploitation could disrupt critical business processes and lead to reputational damage and regulatory penalties under GDPR if personal data is compromised.

Organizations should prioritize applying the official patch from Foxit Software as soon as it becomes available (post-2026-01-16). Until then, they should implement strict input validation and output encoding on any user-controllable inputs, especially URL parameters, to prevent script injection. Employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts in browsers. Educate users to be cautious about clicking on unsolicited or suspicious links, particularly those purporting to come from trusted internal sources. Monitor logs for unusual URL access patterns that might indicate attempted exploitation. If feasible, restrict access to the affected service to trusted networks or via VPN to reduce exposure. Additionally, consider deploying web application firewalls (WAFs) with rules designed to detect and block XSS payloads targeting this vulnerability. Regularly review and update security awareness training to include phishing and XSS risks related to document signing platforms.