CVE-2026-4793: Synology Assistant Auth Bypass Vulnerability

CVE-2026-4793 Overview

CVE-2026-4793 is an incorrect default permissions vulnerability [CWE-276] affecting Synology Assistant versions before 7.0.7-50095. The flaw allows local users to read or write arbitrary files and cause denial-of-service conditions during installation of the application. Exploitation requires local access with low privileges and some user interaction, but successful attacks compromise the confidentiality, integrity, and availability of the affected system.

Critical Impact
Local users can read or write arbitrary files and trigger denial-of-service conditions on systems where Synology Assistant is being installed, potentially enabling privilege escalation on the host.

Affected Products

Synology Assistant versions before 7.0.7-50095
Windows, macOS, and Linux desktop installers distributed by Synology
Environments where Synology Assistant is deployed to manage Synology NAS devices

Discovery Timeline

2026-08-03 - CVE-2026-4793 published to the National Vulnerability Database (NVD)
2026-08-03 - Last updated in NVD database

Technical Details for CVE-2026-4793

Vulnerability Analysis

The vulnerability stems from incorrect default permissions applied by Synology Assistant during installation. The installer creates files or directories with permissions that grant broader access than required. Local users who are not administrators can leverage these permissions to read sensitive files or write to protected locations. The flaw also enables denial-of-service scenarios, disrupting the installation flow or the resulting application state.

Because the attack vector is local and requires user interaction, exploitation typically occurs when an authenticated user runs the installer on a shared or multi-user system. An attacker with existing low-privilege access on the target host can race or manipulate the installer to redirect file operations executed with higher privileges.

Root Cause

The root cause is a failure to enforce restrictive access control lists (ACLs) on installer-created resources. Files, directories, or registry entries provisioned by the installer inherit permissive defaults rather than being locked to trusted principals. This condition maps directly to CWE-276: Incorrect Default Permissions.

Attack Vector

A local attacker with low privileges waits for or triggers the installation of Synology Assistant. During the installation window, the attacker abuses the loose permissions on installer-controlled paths to read privileged data, overwrite arbitrary files, or corrupt state to induce denial of service. No network access is required, but user interaction from the installing account is a prerequisite. See the Synology Security Advisory SA-26-12 for vendor details.

Detection Methods for CVE-2026-4793

Indicators of Compromise

Unexpected files created in Synology Assistant installation directories by non-administrative user accounts
Modifications to files inside the installer working directory during an active installation session
Anomalous file writes to sensitive system paths originating from installer child processes

Detection Strategies

Audit installed versions of Synology Assistant across managed endpoints and flag any build older than 7.0.7-50095
Monitor process creation events for Synology Assistant installer executables followed by writes from unrelated user contexts
Correlate file integrity monitoring alerts with installer activity to identify suspicious concurrent access

Monitoring Recommendations

Enable file access auditing on the Synology Assistant install directory and log all permission changes
Track privilege escalation attempts on hosts where Synology Assistant is deployed or being upgraded
Alert on non-standard child processes spawned by the Synology Assistant installer during runtime

How to Mitigate CVE-2026-4793

Immediate Actions Required

Upgrade Synology Assistant to version 7.0.7-50095 or later on all affected endpoints
Restrict installation of Synology Assistant to administrators on multi-user systems
Remove older installer binaries from shared file locations to prevent reuse of vulnerable builds

Patch Information

Synology addressed the issue in Synology Assistant 7.0.7-50095. Refer to the Synology Security Advisory SA-26-12 for the official fix, download links, and version guidance.

Workarounds

Perform installations only from accounts without untrusted local users active on the system
Verify and harden permissions on the Synology Assistant installation directory after setup completes
Remove local logon rights for untrusted users on hosts that manage Synology NAS devices

bash

# Example: verify installed version on Windows via PowerShell
Get-ItemProperty "HKLM:\Software\Microsoft\Windows\CurrentVersion\Uninstall\*" |
  Where-Object { $_.DisplayName -like "Synology Assistant*" } |
  Select-Object DisplayName, DisplayVersion