CVE-2026-25539 in SiYuan
Summary
by MITRE • 02/05/2026
SiYuan is a personal knowledge management system. Prior to version 3.5.5, the /api/file/copyFile endpoint does not validate the dest parameter, allowing authenticated users to write files to arbitrary locations on the filesystem. This can lead to Remote Code Execution (RCE) by writing to sensitive locations such as cron jobs, SSH authorized_keys, or shell configuration files. This issue has been patched in version 3.5.5.
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Analysis
by VulDB Data Team • 02/12/2026
The vulnerability identified as CVE-2026-25539 affects SiYuan, a personal knowledge management system that has gained popularity among users seeking organized digital note-taking and information storage solutions. This particular flaw resides within the application's file handling mechanisms and represents a critical security oversight that could enable attackers to escalate privileges and potentially compromise entire systems. The vulnerability specifically impacts versions prior to 3.5.5, indicating that the developers have acknowledged and addressed this issue in their subsequent releases. The affected endpoint /api/file/copyFile demonstrates a fundamental lack of input validation that creates an exploitable condition for authenticated users.
The technical flaw manifests through insufficient validation of the dest parameter within the copyFile API endpoint, which allows attackers to manipulate file destination paths during file operations. This parameter validation failure creates a path traversal vulnerability that enables authenticated users to write files to arbitrary locations on the filesystem. The root cause of this vulnerability aligns with CWE-22, which describes improper limitation of a pathname to a restricted directory, commonly known as path traversal or directory traversal attacks. The vulnerability operates under the principle that unvalidated user input can be leveraged to manipulate system operations, particularly in file system interactions where path manipulation can lead to unauthorized file creation or modification.
The operational impact of this vulnerability extends far beyond simple file manipulation, as it opens the door to potential remote code execution through strategic file placement. An attacker with valid credentials can leverage this vulnerability to write malicious payloads to critical system locations such as cron jobs directories, SSH authorized_keys files, or shell configuration files like .bashrc or .profile. This capability enables persistent access and privilege escalation, as the attacker can establish backdoors, modify system behavior, or execute arbitrary code with the privileges of the SiYuan process. The vulnerability's exploitation potential aligns with ATT&CK technique T1059, which covers command and scripting interpreter, and T1074, which addresses data staging, as the attacker can use the compromised system to execute malicious commands and move data.
Security professionals should recognize this vulnerability as a prime example of how seemingly minor input validation gaps can create significant security risks in web applications. The issue demonstrates the importance of implementing proper access controls and file system validation mechanisms, particularly in applications that handle user-generated content or file operations. Organizations using SiYuan should prioritize immediate patching to version 3.5.5 or later, as this update addresses the underlying validation flaw. Additionally, system administrators should implement monitoring for unusual file creation patterns and verify that the application runs with minimal necessary privileges to limit potential damage from exploitation attempts. The vulnerability serves as a reminder that personal knowledge management systems, while designed for individual use, can become attractive targets for attackers seeking persistent access to potentially sensitive information and system resources.