CVE-2025-56869

Executive Summary

CVE-2025-56869 is a critical directory traversal vulnerability identified in the Sync In server, affecting versions up to and including 1.1.1. This flaw allows authenticated attackers to perform arbitrary read and write operations on the underlying system by exploiting the FilesManager.saveMultipart and FilesManager.compress functions within the backend application. The vulnerability is rooted in improper validation of file paths, enabling path traversal sequences (e.g., ../) to escape the intended directory. Given the lack of a confirmed patch and the potential for full system compromise, this vulnerability poses a severe risk to affected deployments.

Supply Chain & Package Security

This vulnerability is not associated with a standard package manager (NPM, PyPI, Go, Ruby) but is specific to the Sync In server application. Organizations using this software should treat it as a critical supply chain risk and immediately isolate affected systems until a patch is available.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder or uncalculated value. Based on the vulnerability description, a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (N) – The vulnerability is exploitable over the network.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): Low (L) – Authentication is required, but only low-level privileges are needed.
• User Interaction (UI): None (N) – No user interaction is required.
• Scope (S): Changed (C) – The vulnerable component impacts resources beyond its security scope.
• Confidentiality (C): High (H) – Full read access to the system is possible.
• Integrity (I): High (H) – Full write access to the system is possible.
• Availability (A): High (H) – Write access could lead to denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H Estimated Base Score: 9.9 (Critical)

Deep Dive & Mechanics

The vulnerability resides in two functions within the Sync In server's backend:

1. FilesManager.saveMultipart (in backend/src/applications/files/services/files-manager.service.ts): This function handles file uploads via multipart form data. It fails to sanitize the file path provided by the authenticated user, allowing an attacker to specify a path containing directory traversal sequences (e.g., ../../../etc/passwd). The server then writes the uploaded file to that arbitrary location.

2. FilesManager.compress (in backend/src/applications/files/services/files-manager.service.ts): This function compresses files or directories. It similarly fails to validate the target path, allowing an attacker to specify a traversal path to read arbitrary files and include them in a compressed archive, which can then be downloaded.

The core issue is the lack of path normalization and validation. The application does not check if the resolved path (after removing ../ sequences) stays within the intended upload or working directory. This allows an attacker to escape the sandbox and interact with any file on the filesystem that the application process has permissions to access.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is the only reference. The following is a conceptual representation of the vulnerable logic:

Vulnerable Code (Conceptual):

// Vulnerable saveMultipart function
async saveMultipart(file: Express.Multer.File, userPath: string) {
    const targetPath = path.join(this.uploadDir, userPath); // No validation
    await fs.promises.writeFile(targetPath, file.buffer);
}

// Vulnerable compress function
async compress(userPath: string) {
    const targetPath = path.join(this.uploadDir, userPath); // No validation
    const archive = archiver('zip');
    archive.directory(targetPath, false);
    // ... return archive stream
}

Patched Code (Conceptual):

// Patched saveMultipart function
async saveMultipart(file: Express.Multer.File, userPath: string) {
    const sanitizedPath = path.normalize(userPath).replace(/^(\.\.(\/|\\|$))+/, '');
    const targetPath = path.join(this.uploadDir, sanitizedPath);
    // Ensure targetPath is within uploadDir
    if (!targetPath.startsWith(this.uploadDir)) {
        throw new Error('Invalid path');
    }
    await fs.promises.writeFile(targetPath, file.buffer);
}

// Patched compress function
async compress(userPath: string) {
    const sanitizedPath = path.normalize(userPath).replace(/^(\.\.(\/|\\|$))+/, '');
    const targetPath = path.join(this.uploadDir, sanitizedPath);
    if (!targetPath.startsWith(this.uploadDir)) {
        throw new Error('Invalid path');
    }
    const archive = archiver('zip');
    archive.directory(targetPath, false);
    // ... return archive stream
}

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Deploy a vulnerable Sync In server (version 1.1.1) in a controlled environment.
2. Obtain valid credentials for an authenticated user.

Exploitation Steps:

1. Arbitrary File Write (saveMultipart):

   • Craft a multipart upload request to the vulnerable endpoint.
   • Set the file path parameter to ../../../etc/cron.d/malicious.
   • Upload a malicious script that executes a reverse shell.
   • The server writes the file to /etc/cron.d/malicious, which will be executed by the cron daemon.

   # Example using curl
   curl -X POST -F "file=@malicious.sh" -F "path=../../../etc/cron.d/malicious" \
     -H "Authorization: Bearer <token>" \
     http://target-server/api/files/upload
   

2. Arbitrary File Read (compress):

   • Send a request to the compress endpoint with a traversal path.
   • The server creates a zip archive containing the targeted file.
   • Download the archive and extract the sensitive data.

   # Example using curl
   curl -X POST -F "path=../../../etc/shadow" \
     -H "Authorization: Bearer <token>" \
     http://target-server/api/files/compress -o shadow.zip
   unzip shadow.zip
   

Blue Team (Defensive)

Detection & Response:

1. Monitor File System Activity: Use auditd or Sysmon to monitor for file writes outside of the application's intended directory (e.g., /var/www/uploads).
2. Web Application Firewall (WAF): Deploy WAF rules to block requests containing path traversal sequences (../, ..\\, %2e%2e%2f).
3. Input Validation: Implement strict server-side validation to reject any path that resolves outside the allowed base directory.
4. Least Privilege: Run the application with the minimum necessary filesystem permissions. The application user should not have write access to system directories like /etc/cron.d/.
5. Network Segmentation: Isolate the Sync In server from critical internal systems to limit the blast radius of a compromise.

Indicators of Compromise (IOCs)

• File System Artifacts:
  • Unexpected files in system directories (e.g., /etc/cron.d/, /tmp/, /var/www/html/).
  • Files with suspicious names or content in the application's upload directory.
• Network Traffic:
  • HTTP requests containing ../ or ..\\ in the path parameter.
  • Unusual outbound connections from the server (e.g., reverse shell callbacks).
• Log Entries:
  • Application logs showing file paths with traversal sequences.
  • System logs (e.g., /var/log/syslog) indicating file access outside of normal patterns.

Detection Engineering

YARA Rule

rule CVE_2025_56869_Directory_Traversal {
    meta:
        description = "Detects directory traversal attempts targeting Sync In server"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-56869"
    strings:
        $traversal1 = "../"
        $traversal2 = "..\\"
        $traversal3 = "%2e%2e%2f"
        $traversal4 = "%2e%2e%5c"
        $func1 = "saveMultipart"
        $func2 = "compress"
    condition:
        any of ($traversal*) and any of ($func*)
}

Sigma Rule

title: CVE-2025-56869 Directory Traversal in Sync In Server
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP requests with path traversal patterns targeting Sync In server endpoints
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains:
            - '../'
            - '..\\'
            - '%2e%2e%2f'
            - '%2e%2e%5c'
    condition: selection
falsepositives:
    - Legitimate use of relative paths in some applications
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-56869 Directory Traversal Attempt"; flow:to_server,established; content:"POST"; http_method; content:"../"; http_uri; nocase; content:"saveMultipart"; http_uri; nocase; sid:1000001; rev:1;)
alert http any any -> any any (msg:"CVE-2025-56869 Directory Traversal Attempt"; flow:to_server,established; content:"POST"; http_method; content:"..\\"; http_uri; nocase; content:"compress"; http_uri; nocase; sid:1000002; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate Affected Systems: Disconnect the Sync In server from the network if possible, or restrict access to trusted users only.
2. Apply Workarounds: If a patch is not available, implement a reverse proxy (e.g., Nginx) with rules to block path traversal patterns in request URIs.
3. Review Access Logs: Investigate for any signs of exploitation using the IOCs provided.

Long-Term Fix:

1. Vendor Patch: Await an official patch from Sync In that implements proper path validation and normalization.
2. Code Hardening: If you have access to the source code, apply the patched code example provided in this report.
3. Security Audit: Conduct a full security audit of the application to identify similar vulnerabilities.

Valters IT