CVE-2026-25539 is a critical security vulnerability classified under CWE-22 (Improper Limitation of a Pathname to a Restricted Directory, commonly known as path traversal) affecting the siyuan-note personal knowledge management system. Specifically, the vulnerability exists in the /api/file/copyFile API endpoint in versions prior to 3.5.5. This endpoint accepts a 'dest' parameter that is intended to specify the destination path for copying files. However, the application fails to properly validate or sanitize this parameter, allowing an authenticated user to specify arbitrary filesystem paths outside the intended directory scope. By exploiting this flaw, an attacker can write files to sensitive locations on the server filesystem, such as cron job directories, SSH authorized_keys files, or shell configuration files. This capability enables remote code execution (RCE), as malicious payloads can be injected and executed with the privileges of the siyuan-note process or potentially higher privileges if privilege escalation is possible. The vulnerability requires the attacker to be authenticated but does not require additional user interaction, making it easier to exploit once credentials are obtained. The CVSS v3.1 base score is 9.1, reflecting its critical nature with network attack vector, low attack complexity, high privileges required, no user interaction, and complete impact on confidentiality, integrity, and availability. Although no known exploits have been reported in the wild, the potential impact is severe. The vendor has addressed this issue in version 3.5.5 by implementing proper validation of the 'dest' parameter to restrict file writes to authorized directories only.

For European organizations using siyuan-note versions prior to 3.5.5, this vulnerability poses a significant risk. Successful exploitation can lead to remote code execution on affected servers, potentially allowing attackers to take full control of the system, access sensitive data, disrupt services, or pivot to other internal systems. Confidentiality is compromised as attackers can read or overwrite critical files; integrity is impacted by unauthorized file modifications; availability can be disrupted by malicious payloads or system instability. Organizations relying on siyuan-note for knowledge management may face data breaches, operational disruptions, and reputational damage. Given the authentication requirement, insider threats or compromised credentials increase risk. The lack of known exploits in the wild suggests limited current exploitation but also highlights the importance of proactive patching to prevent future attacks. The critical severity and ease of exploitation once authenticated make this a high-priority issue for IT security teams.

European organizations should immediately upgrade all siyuan-note installations to version 3.5.5 or later, where the vulnerability is patched. Until upgrades are completed, restrict access to the /api/file/copyFile endpoint by implementing strict access controls and network segmentation to limit authenticated user privileges. Monitor logs for unusual file write activities or attempts to access sensitive filesystem locations. Employ multi-factor authentication (MFA) to reduce the risk of credential compromise. Conduct regular audits of user accounts and permissions within siyuan-note to ensure only trusted users have access. Consider deploying host-based intrusion detection systems (HIDS) to alert on unauthorized file modifications in critical directories. Additionally, review and harden server configurations to minimize the impact of potential RCE, such as running siyuan-note with least privilege and isolating it in containers or virtual machines. Establish incident response plans specific to web application compromises involving path traversal and RCE.