CVE-2025-13593

Executive Summary

CVE-2025-13593 is a local origin validation vulnerability in Synology ActiveProtect Agent prior to version 1.1.0-0439. The flaw allows a local attacker to write arbitrary files with restricted content during the installation process. This is a local privilege escalation and file write vulnerability that could be exploited to overwrite critical system files, inject malicious configuration, or plant persistence mechanisms. The CVSS score of 6.1 (Medium) reflects the need for local access but the high impact on integrity. No public exploit code or proof-of-concept is currently available, and the EPSS score is 0.0%, indicating low exploitation activity in the wild.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Local (L) – Requires local access to the system.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): Low (L) – Attacker needs low-privileged access.
• User Interaction (UI): None (N) – No user action needed beyond installation.
• Scope (S): Unchanged (U) – Vulnerability does not cross trust boundaries.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): High (H) – Arbitrary file write with restricted content.
• Availability (A): None (N) – No denial of service.

Threat Level: Medium. Exploitation requires local access, but the ability to write arbitrary files can lead to privilege escalation, persistence, or system compromise.

Deep Dive & Mechanics

The vulnerability resides in the origin validation logic of the Synology ActiveProtect Agent installer. During installation, the agent processes files or payloads from a source that is not properly validated against an expected origin. This allows a local attacker to supply a crafted payload that bypasses the origin check, enabling the writing of files to arbitrary locations on the filesystem.

Root Cause: The installer does not enforce strict origin validation when handling installation resources. An attacker with local access can manipulate the installation process to write files outside the intended installation directory.

Attack Scenario:

1. An attacker gains low-privileged access to a system running Synology ActiveProtect Agent < 1.1.0-0439.
2. The attacker triggers the installation process (or waits for a scheduled update).
3. By intercepting or modifying the installation payload, the attacker writes a malicious file (e.g., a cron job, SSH authorized key, or systemd service) to a restricted location.
4. The file content is restricted (e.g., limited to certain characters or size), but still sufficient for exploitation.

Vulnerable vs. Patched Code

Vulnerable Code (conceptual):

def install_package(source_path):
    # No origin validation
    for file in get_files(source_path):
        write_file(file, destination="/opt/synology/activeprotect/")

Patched Code (conceptual):

def install_package(source_path):
    # Validate origin
    if not is_valid_origin(source_path):
        raise Exception("Invalid origin")
    for file in get_files(source_path):
        write_file(file, destination="/opt/synology/activeprotect/")

The patch adds origin validation to ensure that only trusted sources can write files during installation.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to a system with Synology ActiveProtect Agent < 1.1.0-0439.
• Ability to trigger the installation process (e.g., via synoactiveprotect-agent install).

Exploitation Steps:

1. Create a malicious payload that writes a file to a target location (e.g., /etc/cron.d/malicious).
2. Use a symlink or race condition to redirect the installer's file write operation.
3. Trigger the installation process.

Example Payload (conceptual):

#!/bin/bash
# Malicious script to write a cron job
echo "* * * * * root /tmp/backdoor.sh" > /etc/cron.d/malicious

Note: No public exploit code is available. This is a conceptual demonstration.

Blue Team

Detection:

• Monitor file writes to sensitive directories during installation.
• Alert on unexpected file creations in /etc/cron.d/, /etc/systemd/system/, or /root/.ssh/.
• Use file integrity monitoring (FIM) to detect unauthorized changes.

Hunting:

• Search for files written by the synoactiveprotect-agent process outside its expected paths.
• Look for symlink attacks or race conditions in the installation directory.

Indicators of Compromise (IOCs)

• File Paths:
  • /etc/cron.d/* (unexpected cron jobs)
  • /etc/systemd/system/* (unexpected services)
  • /root/.ssh/authorized_keys (unauthorized SSH keys)
• Process: synoactiveprotect-agent writing to unexpected locations.
• Logs: Installation logs showing file writes to non-standard paths.

Detection Engineering

YARA Rule

rule CVE_2025_13593_FileWrite {
    meta:
        description = "Detects file writes by Synology ActiveProtect Agent to suspicious paths"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-13593"
    strings:
        $s1 = "/etc/cron.d/" nocase
        $s2 = "/etc/systemd/system/" nocase
        $s3 = "/root/.ssh/" nocase
    condition:
        any of ($s*)
}

Sigma Rule

title: Suspicious File Write by Synology ActiveProtect Agent
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects file writes by Synology ActiveProtect Agent to sensitive directories
author: Valters IT
date: 2025-04-10
logsource:
    category: file_event
    product: windows
detection:
    selection:
        Image|endswith: '\synoactiveprotect-agent.exe'
        TargetFilename|contains:
            - '\etc\cron.d\'
            - '\etc\systemd\system\'
            - '\root\.ssh\'
    condition: selection
falsepositives:
    - Legitimate updates to cron or systemd
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-13593 Synology ActiveProtect Agent File Write"; flow:to_server,established; content:"synoactiveprotect-agent"; nocase; content:"/etc/cron.d/"; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Synology ActiveProtect Agent to version 1.1.0-0439 or later.
2. Restrict local access to trusted users only.
3. Monitor installation logs for anomalous file writes.

Long-term Mitigations:

• Implement strict file integrity monitoring (FIM) on critical system directories.
• Use application whitelisting to prevent unauthorized executables.
• Apply the principle of least privilege to all local users.

Patch Details:

• Upgrade to version 1.1.0-0439 from the official Synology download center.

Valters IT
DevSecOps & Cyber Threat Intelligence