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The vulnerability, identified as CVE-2026-3089, resides in the Actual Sync Server, a budgeting synchronization backend. It affects all versions prior to 26.3.0. The core issue is an improper validation of user-supplied input during the file upload process. Specifically, an authenticated user can make a POST request to the `/sync/upload-user-file` endpoint. The server relies on the `x-actual-file-id` HTTP header to determine the destination filename and path for the uploaded file. Due to the lack of proper sanitization, an attacker can include path traversal sequences (such as ../) within this header value. This allows the malicious actor to break out of the intended `userFiles` directory and write arbitrary files to any location on the server’s filesystem that the application process has write permissions for . This vulnerability is classified under CWE-22, which pertains to Improper Limitation of a Pathname to a Restricted Directory (‘Path Traversal’) .
dailycve form:
Platform: Actual Sync Server
Version: prior to 26.3.0
Vulnerability: Authenticated Path Traversal
Severity: Moderate
Date: Mar 9, 2026
Prediction: Patched 2026-03-07
What Undercode Say:
Analytics:
The vulnerability can be identified by checking the server version. A system administrator can use the following command to examine the package version if installed via npm:
npm list actual-sync-server
To simulate a check for the vulnerability, a security tester might use `curl` to attempt a malicious upload and observe the server’s response or file system for changes. This command attempts to write a file named `test.txt` one directory above the intended root:
curl -X POST -H "x-actual-file-id: ../../test.txt" -F "file=@local_test.txt" http://vulnerable-server.com/sync/upload-user-file
Server logs should be monitored for unusual file paths or errors related to file writes outside the standard `userFiles` directory. A command to grep for such attempts might look like:
grep -i "upload-user-file" /var/log/actual-sync/access.log | grep ".."
Exploit:
To exploit this vulnerability, an attacker must first have a valid set of credentials for the Actual Sync Server. The exploitation does not require any additional privileges beyond a standard authenticated user. The attacker crafts a malicious HTTP POST request to the `/sync/upload-user-file` endpoint. The critical part of the request is the `x-actual-file-id` header, which is set to a value containing path traversal sequences, for example, ../../../var/www/html/webshell.php. The body of the request contains the malicious file payload. Upon processing, the server concatenates this traversed path with the base upload directory, resulting in a full path like /app/userFiles/../../../var/www/html/webshell.php, which resolves to /var/www/html/webshell.php. The server then writes the file content to this location, effectively allowing the attacker to place a web shell or overwrite critical system files.
Protection from this CVE:
The primary and most effective protection is to upgrade the Actual Sync Server to version 26.3.0 or later, where the input validation for the `x-actual-file-id` header has been fixed to properly neutralize path traversal sequences. For systems that cannot be immediately upgraded, a critical mitigation is to run the sync server within a filesystem sandbox or a container with strict filesystem restrictions. This would limit the impact of any path traversal attempt, confining it to the sandboxed environment. Additionally, implementing strict input validation on the server-side, such as denying any file ID that contains `../` or ..\, can serve as a temporary virtual patch. Web Application Firewall (WAF) rules can also be configured to block requests containing path traversal patterns in the `x-actual-file-id` header.
Impact:
Successful exploitation of this path traversal vulnerability allows an authenticated attacker to write files to arbitrary locations on the server. This can lead to severe consequences, including Remote Code Execution (RCE) if the attacker can upload a script to a web-accessible directory. It could also result in the defacement of websites, overwriting of critical application or system configuration files, and potential privilege escalation. The integrity and availability of the server are at risk, as malicious files can be placed anywhere, potentially leading to a full system compromise .
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Sources:
Reported By: github.com
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