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An origin validation error (CWE-346) in the Synology Active Backup for Business Agent, tracked as CVE-2025-66592, affects versions prior to 3.1.0-4967. The flaw exists within the agent’s Installation Handler component on the Windows platform, allowing a local user to write arbitrary files with restricted content during the installation process. The root cause is insufficient validation of file sources during the installation phase. During a typical installation, the agent creates files and directories in specific locations. However, a local attacker can manipulate the installation workflow, using techniques that may involve symbolic links or path traversal. The “restricted content” constraint means that while the attacker can place a file of their choosing, the content is likely sanitized or filtered, which reduces the potential payload complexity. Despite this limitation, the impact is significant in enterprise environments where backup agents run with elevated privileges. The core problem is the agent’s inability to verify the origin of the files it is writing, trusting local file operations that should be validated. The attack occurs before the system is fully operational or during configuration, giving the attacker a window to inject malicious files that could survive a reboot. The CVSS 3.1 score is 6.1, with the vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:H. This indicates the attack is local, requires no privileges but does require user interaction, has a low impact on integrity, and a high impact on availability.
DailyCVE Form:
Platform: Windows
Version: Before 3.1.0-4967
Vulnerability: Origin validation error
Severity: Moderate
date: 27/05/2026
Prediction: 08/12/2025
What Undercode Say:
Check installed version on Windows dir "C:\Program Files\Synology\Active Backup for Business Agent\" Check registry for version reg query "HKLM\SOFTWARE\Synology\Active Backup for Business Agent" /v Version Check agent logs for suspicious writes findstr /s /i "write install" "C:\ProgramData\Synology\Active Backup for Business Agent\Logs.log" Monitor file system for writes to critical paths during installation using Sysinternals ProcMon procmon.exe /AcceptEula /Quiet /Minimized /BackingFile C:\logs\pmc.pml Check for unexpected files in program directory dir "C:\Program Files\Synology\Active Backup for Business Agent\" /s /b > expected.txt After baseline, compare during install dir "C:\Program Files\Synology\Active Backup for Business Agent\" /s /b > post_install.txt fc expected.txt post_install.txt
How Exploit:
The attacker gains local access to the Windows system. They then place specially crafted files or symbolic links in directories that the installation routine accesses. When the Active Backup for Business Agent installs (or upgrades), the vulnerable Installation Handler fails to validate the source of these files. Consequently, the agent writes the attacker’s payload as a system file with restricted content. The “restricted content” constraint suggests the attacker cannot embed arbitrary binaries but can craft data to trigger a secondary flaw, such as a configuration parser bug, or cause a denial-of-service condition by corrupting system files.
Protection:
- Upgrade: Immediately update the Synology Active Backup for Business Agent to version 3.1.0-4967 or later, as this release fixes the origin validation error.
- Access Control: Restrict local user account permissions to prevent unauthorized installations. Only authorized administrators should perform installations.
- File Integrity Monitoring (FIM): Deploy FIM solutions (e.g., Sysmon, Wazuh) to monitor the agent’s installation directory and critical system paths for unexpected file changes during installs.
- Network Segmentation: Keep backup agents in a separate network segment to limit local attack surface.
Impact:
Successful exploitation allows a local user to write arbitrary files with restricted content to the system during installation. The impact on integrity is low, as the attacker can only write data, not arbitrary binaries. However, the availability impact is high because writing to critical system files can cause a denial-of-service condition, system crashes, or prevent the backup agent from functioning correctly. In a worst-case scenario, combined with a second vulnerability, this could lead to privilege escalation or code execution. Enterprise backup operations are particularly at risk, as the compromise could extend to backup repositories and disaster recovery plans.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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