CVE-2025-59788 is a cross-site scripting (XSS) vulnerability identified in the Nextcloud PDF viewer component, specifically within a reachable example directory named files_pdfviewer. This vulnerability affects multiple Nextcloud versions prior to 22.2.10.33, 23.0.12.29, 24.0.12.28, 25.0.13.23, 26.0.13.20, 27.1.11.20, 28.0.14.11, 29.0.16.8, 30.0.17, 31.0.10, and 32.0.1. The flaw allows an attacker to craft a malicious PDF file that, when opened via the viewer.html interface, executes arbitrary JavaScript code in the context of the victim's browser session. This can lead to theft of session cookies, unauthorized actions on behalf of the user, or further exploitation such as phishing or malware delivery. The vulnerability stems from insufficient input sanitization or improper handling of PDF content in the example directory, which is publicly accessible. Although no active exploits have been reported, the similarity to the earlier CVE-2024-4367 suggests a persistent issue in the PDF viewer's security posture. The vulnerability does not require user authentication but does require user interaction to open the crafted PDF, making social engineering a likely attack vector. The absence of a CVSS score necessitates an assessment based on impact and exploitability factors. The vulnerability compromises confidentiality and integrity primarily, with a moderate impact on availability. The scope is limited to users accessing the vulnerable PDF viewer component, but given Nextcloud's widespread use in enterprise and public sectors, the potential reach is significant. The patch availability is implied but not explicitly linked, so timely updates are critical. Overall, this vulnerability represents a significant risk to organizations relying on Nextcloud for document management and collaboration.

For European organizations, the impact of CVE-2025-59788 can be substantial, especially for those using Nextcloud as a core platform for file sharing and collaboration. Successful exploitation could lead to unauthorized access to sensitive documents, session hijacking, and potential lateral movement within corporate networks. This could result in data breaches, loss of intellectual property, and reputational damage. Given the nature of XSS, attackers might also use this vulnerability as a foothold to deploy further attacks such as phishing campaigns or malware distribution within trusted environments. Organizations in regulated industries (e.g., finance, healthcare, government) face additional compliance risks due to potential exposure of personal or confidential data. The vulnerability's exploitation does not require authentication but does require user interaction, which means phishing or social engineering could be effective attack vectors. The impact on availability is limited but could be exacerbated if attackers leverage the vulnerability to disrupt user sessions or inject malicious scripts that degrade service performance. Overall, the threat undermines user trust in Nextcloud deployments and could lead to significant operational and security challenges.

To mitigate CVE-2025-59788, European organizations should prioritize the following actions: 1) Apply security updates and patches for Nextcloud immediately once they are released for the affected versions listed. 2) Restrict or disable access to example directories such as files_pdfviewer on production systems to reduce exposure. 3) Implement Content Security Policy (CSP) headers to limit the execution of unauthorized scripts within the browser context. 4) Educate users about the risks of opening PDF files from untrusted sources, emphasizing caution with files received via email or external channels. 5) Monitor web server logs and Nextcloud access logs for unusual activity related to the PDF viewer or attempts to access the example directory. 6) Employ web application firewalls (WAFs) with rules designed to detect and block XSS payloads targeting the PDF viewer. 7) Conduct regular security assessments and penetration testing focused on web application components, including third-party plugins and example directories. 8) Review and harden Nextcloud configuration to disable unnecessary features or example content that may introduce vulnerabilities. These measures, combined with prompt patching, will significantly reduce the risk posed by this vulnerability.