CVE-2024-56408 is a high-severity cross-site scripting (XSS) vulnerability affecting the PHPOffice PhpSpreadsheet library, a widely used PHP library for reading and writing spreadsheet files. The vulnerability exists in versions prior to 3.7.0, 2.3.5, 2.1.6, and 1.29.7, specifically in the /vendor/phpoffice/phpspreadsheet/samples/Engineering/Convert-Online.php file. The issue arises due to improper neutralization of input during web page generation, meaning that user-supplied input is not properly sanitized before being embedded in web pages. This allows an attacker to inject malicious scripts that execute in the context of a victim's browser when they access the affected web page. The vulnerability is classified under CWE-79, which covers improper input sanitization leading to XSS attacks. The CVSS 4.0 base score is 8.3, indicating a high severity with the following characteristics: network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), user interaction required (UI:A), high impact on integrity (VI:H), low scope (S:C), and low impact on confidentiality and availability. Although no known exploits are currently reported in the wild, the vulnerability poses a significant risk due to the ease of exploitation and the potential impact on data integrity and user trust. The vulnerability affects multiple version branches of PhpSpreadsheet, emphasizing the need for patching or upgrading to the fixed versions (3.7.0, 2.3.5, 2.1.6, and 1.29.7).

For European organizations, the impact of this vulnerability can be considerable, especially for those relying on PhpSpreadsheet in web applications that process or display spreadsheet data online. Successful exploitation could allow attackers to execute arbitrary JavaScript in the context of users' browsers, potentially leading to session hijacking, credential theft, or unauthorized actions performed on behalf of users. This can compromise the integrity of data processing workflows and erode user trust. Organizations in sectors such as finance, government, healthcare, and education—where spreadsheet data is commonly used and sensitive—are particularly at risk. The vulnerability could also be leveraged as an initial vector for more complex attacks, including lateral movement or data exfiltration. Given the cross-site scripting nature, the availability impact is limited, but the confidentiality and integrity impacts are significant. The requirement for user interaction (e.g., visiting a maliciously crafted page) means that phishing or social engineering could facilitate exploitation. The widespread use of PhpSpreadsheet in PHP-based web applications across Europe means that many organizations could be exposed if they have not updated to patched versions.

European organizations should take immediate steps to mitigate this vulnerability by upgrading PhpSpreadsheet to the patched versions: 3.7.0, 2.3.5, 2.1.6, or 1.29.7, depending on their version branch. If upgrading is not immediately feasible, organizations should audit their web applications to identify any usage of the vulnerable Convert-Online.php sample or similar code paths that handle user input without proper sanitization. Implementing strict input validation and output encoding for any user-supplied data rendered in web pages is critical. Employing Content Security Policy (CSP) headers can help mitigate the impact of XSS by restricting the execution of unauthorized scripts. Additionally, organizations should conduct security awareness training to reduce the risk of successful phishing attacks that could trigger user interaction. Regular code reviews and penetration testing focused on XSS vulnerabilities should be integrated into the development lifecycle. Monitoring web application logs for suspicious activity and anomalous inputs can help detect attempted exploitation. Finally, organizations should maintain an up-to-date inventory of third-party libraries like PhpSpreadsheet to ensure timely patch management.