CVE-2025-63709 is a stored Cross-Site Scripting (XSS) vulnerability identified in SourceCodester Simple To-Do List System version 1.0. The flaw exists in the 'Add Tasks' text input field, where an authenticated user can submit HTML or JavaScript code that is not properly sanitized or encoded upon output. This malicious code is stored persistently in the application’s database and later rendered in the browsers of any users who view the affected task entries. Because the injected script executes in the context of the victim's browser, it can perform actions such as stealing session cookies, performing actions on behalf of the user, or redirecting users to malicious sites. The vulnerability requires the attacker to have authenticated access to the system and requires victim users to view the malicious task entry for exploitation. The CVSS v3.1 base score is 5.4, indicating medium severity, with the vector indicating network attack vector, low attack complexity, privileges required, user interaction required, and a scope change. There are no known public exploits or patches at the time of publication, which means organizations must rely on mitigations until a patch is available. The vulnerability is classified under CWE-79, a common XSS weakness category.

For European organizations, this vulnerability poses a risk primarily to confidentiality and integrity of user sessions and data within the affected application. Attackers with authenticated access can inject scripts that execute in other users’ browsers, potentially leading to session hijacking, unauthorized actions, or data theft. While availability is not directly impacted, the trustworthiness of the affected system is compromised. Organizations using SourceCodester Simple To-Do List System 1.0, especially in collaborative environments, face risks of internal attacks or lateral movement if attackers leverage this vulnerability. The requirement for authentication and user interaction limits the attack surface but does not eliminate risk, particularly in environments with many users or weak access controls. Given the lack of patches, the threat persists until remediation is implemented. The impact is heightened in sectors where task management tools are critical for daily operations and where sensitive information may be handled within task descriptions.

Organizations should immediately review and restrict user privileges to minimize the number of users who can add or modify tasks. Implement strict input validation and output encoding on the 'Add Tasks' field to prevent injection of malicious scripts. Employ Content Security Policy (CSP) headers to reduce the impact of potential XSS by restricting script execution sources. Monitor logs for unusual task creation or modification activities that could indicate exploitation attempts. Educate users about the risks of interacting with untrusted content within the application. If possible, isolate the affected application from critical systems and sensitive data until a patch or update is available. Consider deploying web application firewalls (WAFs) with rules to detect and block XSS payloads targeting this application. Engage with the vendor or community for updates or patches and apply them promptly once released.