CVE-2025-66548 is a vulnerability classified under CWE-116, which relates to improper encoding or escaping of output data. Specifically, in Nextcloud Deck versions prior to 1.12.7, 1.14.4, and 1.15.1, an attacker can exploit the handling of Right-to-Left Override (RTLO) Unicode characters to spoof file extensions. RTLO is a Unicode control character that reverses the display order of text, which attackers can use to mask the true file extension by making it appear as a benign type (e.g., .txt) while the actual file is an executable or other potentially harmful format. This spoofing can deceive users into downloading and possibly executing malicious files under false pretenses. The vulnerability does not require user interaction (UI:N) or elevated privileges (PR:L), but the attacker must have local access (AV:L) to the system or environment where Nextcloud Deck is deployed. The flaw impacts the integrity of the system by misleading users about file types, but it does not directly compromise confidentiality or availability. The issue was addressed in the specified patched versions by properly encoding or escaping output to prevent RTLO-based spoofing. No known exploits have been reported in the wild as of the publication date, but the risk remains for social engineering attacks leveraging this spoofing technique.

For European organizations, the primary impact of this vulnerability is the potential for social engineering attacks that exploit file extension spoofing to trick users into executing malicious files. This can lead to localized integrity issues, such as unauthorized code execution or malware infection, especially if users are not vigilant about verifying file types. While the vulnerability itself does not directly expose sensitive data or disrupt service availability, it can serve as an initial vector for more severe attacks if combined with other vulnerabilities or poor security practices. Organizations relying on Nextcloud Deck for project management and file sharing may face increased risk of user-targeted attacks, potentially affecting operational security and trust in collaboration tools. The low CVSS score reflects limited direct impact, but the indirect consequences through user deception warrant attention, particularly in sectors with high collaboration demands such as finance, government, and critical infrastructure within Europe.

European organizations should immediately update Nextcloud Deck installations to versions 1.12.7, 1.14.4, or 1.15.1 or later, where the vulnerability is patched. Beyond patching, administrators should implement strict file upload and download policies, including filtering or blocking files with suspicious Unicode characters such as RTLO. User awareness training should emphasize verifying file extensions and exercising caution with unexpected downloads, especially from internal collaboration platforms. Employing endpoint security solutions that detect and block execution of files with mismatched or suspicious extensions can further reduce risk. Additionally, monitoring logs for unusual file download patterns or user reports of suspicious files can help detect exploitation attempts. Organizations should also review and harden their Nextcloud configurations to limit local access privileges and restrict file sharing to trusted users to minimize the attack surface.