CVE-2026-3089 is a path traversal vulnerability identified in Actual Sync Server, specifically affecting versions prior to 26.3.0, including 26.2.1. The vulnerability arises from improper validation of the user-controlled HTTP header 'x-actual-file-id' used during file uploads via the POST /sync/upload-user-file endpoint. Attackers with valid authentication credentials can exploit this flaw by including directory traversal sequences (../) in the header value, allowing them to escape the designated userFiles directory. This improper limitation enables arbitrary file writes to locations outside the intended directory structure. The vulnerability does not require user interaction or elevated privileges beyond authentication, making it easier to exploit within compromised or legitimate user contexts. The CVSS 4.0 base score is 5.3 (medium severity), reflecting network attack vector, low attack complexity, no privileges required beyond authentication, and limited impact on confidentiality and integrity. No known exploits have been reported in the wild as of the publication date. The root cause is a failure to sanitize and validate the pathname input, which is a classic CWE-22 (Improper Limitation of a Pathname to a Restricted Directory) issue. This can lead to unauthorized file overwrites or creation, potentially allowing attackers to manipulate server behavior or persist malicious payloads.

The primary impact of this vulnerability is unauthorized file write capability outside the intended directory, which can compromise the integrity of the server and potentially its confidentiality if sensitive files are overwritten or replaced. Attackers could use this to implant malicious files, modify configuration files, or disrupt service availability by overwriting critical files. Since exploitation requires authentication, the threat is elevated in environments where user credentials are weak, shared, or compromised. The vulnerability could facilitate further attacks such as privilege escalation, data tampering, or persistent backdoors. Organizations relying on Actual Sync Server for file synchronization and sharing may face data integrity risks and operational disruptions. The scope is limited to authenticated users but can be significant in multi-tenant or collaborative environments where users have upload permissions.

1. Upgrade Actual Sync Server to version 26.3.0 or later, where this vulnerability is fixed. 2. Implement strict input validation and sanitization on the 'x-actual-file-id' header to reject any path traversal sequences such as '../' or absolute paths. 3. Employ server-side checks to enforce that file writes remain strictly within the designated userFiles directory, using canonicalization techniques to resolve and validate paths before writing. 4. Restrict file upload permissions to the minimum necessary set of authenticated users and monitor upload activity for anomalies. 5. Conduct regular security audits and penetration tests focusing on file upload functionalities. 6. Use application-layer firewalls or runtime application self-protection (RASP) solutions to detect and block path traversal attempts in real-time. 7. Educate users on secure credential management to reduce risk of compromised accounts that could exploit this vulnerability.