CVE-2025-63709 identifies a stored Cross-Site Scripting (XSS) vulnerability in the SourceCodester Simple To-Do List System version 1.0. The flaw exists in the 'Add Tasks' text input field, where authenticated users can submit HTML or JavaScript code that is not properly sanitized or encoded upon output. This malicious code is stored persistently in the system and executed in the browsers of any users who subsequently view the affected task entries. The vulnerability enables attackers to execute arbitrary scripts within the security context of the victim's browser, potentially leading to session hijacking, credential theft, unauthorized actions, or further malware delivery. Exploitation requires the attacker to have valid authentication credentials to submit the malicious input, but no additional user interaction is required beyond viewing the infected content. Although no public exploits have been reported, the vulnerability poses a significant risk due to the persistent nature of stored XSS and the broad impact on any user accessing the task list. The lack of a CVSS score indicates that the vulnerability has not yet been fully assessed or scored, but the technical details confirm the presence of a classic stored XSS attack vector. The vulnerability highlights insufficient input validation and output encoding controls within the affected application, common weaknesses in web application security.

For European organizations, this vulnerability threatens the confidentiality and integrity of user data and sessions within the affected application. Attackers exploiting this flaw could hijack user sessions, steal sensitive information such as authentication tokens, or perform unauthorized actions on behalf of legitimate users. This could lead to data breaches, unauthorized task modifications, or lateral movement within the organization's network if the application is integrated with other systems. The stored nature of the XSS increases the risk as multiple users may be affected over time. Although exploitation requires authentication, many organizations use such task management systems internally, where users have legitimate access, increasing the attack surface. The absence of known exploits suggests limited current risk, but the vulnerability could be weaponized in targeted attacks or insider threat scenarios. The impact on availability is limited, but the potential for reputational damage and compliance violations under GDPR due to unauthorized data access is significant.

To mitigate this vulnerability, organizations should implement strict input validation and output encoding on all user-supplied data, especially in the 'Add Tasks' input field. Employ a whitelist approach to allow only safe characters and disallow HTML or script tags. Use established libraries or frameworks that automatically encode output to prevent script execution. Deploy Content Security Policy (CSP) headers to restrict the execution of inline scripts and reduce the impact of any injected code. Conduct regular security code reviews and penetration testing focused on input handling. If possible, update or patch the affected software version once a fix is released by the vendor or community. In the interim, restrict access to the application to trusted users and monitor logs for suspicious activity. Educate users about the risks of stored XSS and encourage reporting of unusual behavior. Consider implementing multi-factor authentication to reduce the risk of compromised credentials being used to exploit this vulnerability.