CVE-2026-4793

Executive Summary

CVE-2026-4793 is an incorrect default permissions vulnerability affecting Synology Assistant versions prior to 7.0.7-50095. The flaw allows local users to read or write arbitrary files and conduct denial-of-service conditions during the installation process. With a CVSS score of 7.3, this vulnerability requires local access and user interaction but carries high confidentiality, integrity, and availability impact, making it a significant concern for organizations using Synology NAS management software on shared or multi-user systems.

Technical Analysis

CVE-2026-4793 is classified as CWE-276 (Incorrect Default Permissions), indicating that the software installs with overly permissive file or directory permissions by default. The NVD description specifically notes that the issue exists "during installation" of Synology Assistant, suggesting that the installation process creates files, directories, or registry entries (on Windows) with insecure access control lists (ACLs).

The vulnerability mechanism stems from the installation routine of Synology Assistant. When the installer executes, it likely creates temporary directories, configuration files, or application data folders with default permissions that are world-writable or readable by any local user on the system. This is a common flaw in installers that use generic temp directories or fail to specify restrictive ACLs on created resources.

The attack surface is the local file system during the installation window. Any local user with a standard (non-administrator) account can potentially:

1. Pre-create files or directories at predictable paths that the installer will later use
2. Modify files created by the installer before the installation completes
3. Read sensitive configuration data written during installation

The vulnerability is triggered during the time window when the installer is actively writing files. Because permissions are set incorrectly, a local attacker can intercept, modify, or replace files during this window. The write capability enables file tampering that could lead to arbitrary file write (potentially escalating to code execution in the installer's security context), while the read capability enables information disclosure of any data written during installation.

The CVSS vector emphasizes the local attack vector (AV:L), low attack complexity (AC:L), and required privileges (PR:L). The user interaction component (UI:R) indicates that the victim must perform some action — in this case, running the installer — for the attack window to exist. The confidentiality, integrity, and availability impacts are all rated High.

Exploitation Scenarios

A local attacker exploits CVE-2026-4793 through the following step-by-step chain:

1) Reconnaissance and Target Identification

• The attacker identifies a target system where the legitimate user plans to install or reinstall Synology Assistant. This could be observed through active directory inventory, previous installation logs, or social engineering (e.g., sending a legitimate-looking update notice).
• The attacker determines the file system layout and identifies the temp directory and the default installation path for Synology Assistant (e.g., the Program Files directory or the user's AppData temp folder).
• The attacker checks current permissions on directories where the installer will create files using commands like icacls or by inspecting ACLs via PowerShell.

2) Preparation and Weaponization

• Before the installation starts, the attacker pre-creates a directory structure at a predictable path that the installer will use. Because the installer does not enforce restrictive permissions, the attacker's pre-created directory inherits or retains its permissive ACLs.
• The attacker places a malicious DLL, executable, or configuration file in this pre-created directory. When the installer runs and attempts to write or load files from this location, the malicious file is used instead of a legitimate one.
• Alternatively, the attacker creates a symbolic link or junction pointing to a sensitive system location (e.g., System32 or a protected data directory) at a path the installer will write to.

3) Trigger and Execution

• The attacker waits for the legitimate user to execute the Synology Assistant installer (satisfying the UI:R requirement).
• As the installer writes files to the insecure directory, the attacker's malicious payload is executed on behalf of the installer. If the installer runs with elevated privileges, the payload inherits those privileges, enabling privilege escalation.
• For the denial-of-service scenario, the attacker pre-creates the target directory and fills it with files or sets permissions that cause the installer to fail, corrupting the installation or crashing the system.

4) Post-Exploitation Impact

• With arbitrary write capability, the attacker can plant files into sensitive locations, potentially achieving persistence by replacing system executables or adding autostart entries.
• With arbitrary read capability, the attacker harvests configuration data, credentials stored in installer-generated files, or network configuration details.
• Denial-of-service: the attacker prevents successful installation, forcing repeated failed installation attempts and consuming system resources.

Impact Assessment

Business Impact: This vulnerability poses a moderate-to-high risk in multi-user environments such as corporate workstations, shared lab machines, or managed endpoints where multiple users share a single system. The high integrity impact means an attacker can achieve arbitrary file writes that could escalate to full system compromise if the installer runs with administrative privileges. The high availability impact enables deliberate disruption of legitimate NAS management operations.

Affected Systems: All installations of Synology Assistant prior to version 7.0.7-50095 on Windows (primarily), where the installer's default permissions are insufficiently restrictive.

Blast Radius: The CVSS subscores (C:H/I:H/A:H) indicate a worst-case scenario where confidentiality, integrity, and availability are all significantly compromised. While the attack requires local access, the XSS-style chaining with user interaction means a single rogue local account can potentially gain administrative control of the system or render the NAS management software unusable.

CISA KEV Status: Not listed on CISA's Known Exploited Vulnerabilities catalog.

EPSS Context: The EPSS score of 0.00134 (3.3rd percentile) indicates a low current probability of exploitation in the wild. However, the public availability of PoC material could drive this upward as enterprise scanning tools incorporate the detection logic.

Detection Guidance

Sigma Rule

title: Synology Assistant Installation Permission Anomaly
id: 5f6d0d4a-7c31-4b5e-abcf-1234567890ab
status: experimental
description: Detects anomalous file system activity during Synology Assistant installation indicating exploitation of CVE-2026-4793
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-4793
author: ValtersIT Threat Intelligence
date: 2026/11/02
logsource:
    category: file_event
    product: windows
detection:
    selection_synology_process:
        Image|contains:
            - 'SynologyAssistant'
            - 'synoinstall'
        EventID:
            - 11  # File created
            - 23  # File delete
    selection_suspicious_paths:
        TargetFilename|contains:
            - '\\Temp\\'
            - '\\AppData\\'
        TargetFilename|endswith:
            - '.dll'
            - '.exe'
            - '.sys'
    selection_low_integrity:
        IntegrityLevel:
            - 'Medium'
            - 'Low'
    condition: selection_synology_process and selection_suspicious_paths and selection_low_integrity
    timeframe: 5m
falsepositives:
    - Legitimate installer activities in non-restricted environments
    - Standard user-initiated installs in isolated test systems
level: high

YARA Rule

rule CVE_2026_4793_Synology_Assistant_Permission_Abuse
{
    meta:
        author = "ValtersIT Threat Intelligence"
        description = "Detects files commonly abused during Synology Assistant installation permission exploitation"
        cve = "CVE-2026-4793"
        date = "2026-11-02"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-4793"
        severity = "high"
    strings:
        $s1 = "Synology Assistant" ascii wide
        $s2 = "DSM" ascii wide
        $s3 = "install" ascii wide nocase
        $s4 = "{TEMP}" ascii wide
        $s5 = "C:\\Users" ascii
        $s6 = "setup" ascii wide nocase
        $s7 = "diskstation" ascii wide nocase
    condition:
        uint16(0) == 0x5A4D and  // MZ header
        filesize < 10MB and
        (2 of ($s1, $s2, $s7)) and
        (1 of ($s3, $s4, $s5, $s6)) and
        (metadata.windows_pe or metadata.elf)
}

Suricata IDS Rule

alert ip any any -> any any (msg:"CVE-2026-4793 - Suspicious Synology Assistant Download/Install Activity"; flow:to_client; content:"SynologyAssistant"; nocase; content:"setup"; nocase; distance:0; pcre:"/\.(exe|msi|dll)/i"; classtype:attempted-user; sid:10004793; rev:1; metadata:created_at 2026_11_02, cve CVE-2026-4793;)

Elastic Detection

sequence by host.name with maxspan=30s
  [file where process.name : "SynologyAssistant*.exe" or process.name : "synoinstall*.exe"
   and event.action : ("creation", "modification")
   and file.path : ("C:\\Users\\*\\AppData\\Local\\Temp\\*",
                    "C:\\Users\\*\\AppData\\Roaming\\*")] by file.path
  [file where event.action : "modification"
   and file.extension : ("dll", "sys", "exe")
   and not user.name : "*SYSTEM"] by file.path

Splunk SPL Query

index=windows source="WinEventLog:Security" OR source="WinEventLog:Microsoft-Windows-Sysmon/Operational"
| search (EventCode=4663 OR EventCode=11 OR EventCode=23)
    AND (Image="*SynologyAssistant*" OR Image="*synoinstall*")
    AND (ObjectName="*\\Temp\\*" OR ObjectName="*\\AppData\\*")
    AND (ObjectName="*.dll" OR ObjectName="*.exe" OR ObjectName="*.sys")
| eval suspicious=true
| stats count by host, Image, ObjectName, AccountName
| where count >= 3
| table _time, host, Image, ObjectName, AccountName, count
| sort - _time

Wazuh OSSEC Rule

<group name="synology_assistant,">
  <rule id="103539" level="12">
    <if_sid>5000000</if_sid>
    <field name="win.event.system.eventID">^11$|^23$</field>
    <field name="win.event.data.image">\.*SynologyAssistant\.*|\.*synoinstall\.*</field>
    <field name="win.event.data.targetFilename">\.*Temp\.*|\.*AppData\.*</field>
    <field name="win.event.data.targetFilename">\.*(dll|exe|sys)$</field>
    <description>Potential CVE-2026-4793 exploitation - Synology Assistant writing to insecure paths during installation</description>
    <group>cve_2026_4793,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED — no patch evidence from any tracked source.

Despite the NVD description stating the issue exists "before 7.0.7-50095," the current patch confidence assessment indicates that no patch evidence has been tracked from official sources. This means the fix may be unreleased, incomplete, or unverified as of this report. The following guidance assumes a patched version will be or has been made available, but organizations should verify directly with Synology.

1) Immediate Mitigations:

• Restrict local user accounts on machines where Synology Assistant is installed or will be installed. Ensure only trusted users have interactive login rights.
• Perform all Synology Assistant installations in a controlled environment with no untrusted local users present.
• After installation, manually audit and tighten ACLs on all directories created by Synology Assistant:
  • Navigate to installation directories (typically in Program Files or AppData).
  • Use icacls <path> /inheritance:r to remove inherited permissive ACLs.
  • Grant only SYSTEM and Administrators full control; grant standard users read/execute only.
• Use AppLocker or Windows Defender Application Control to restrict execution of files in Temp directories.
• Deploy software deployment tools (SCCM, GPO) to install Synology Assistant centrally with administrative privileges, rather than permitting end-user self-installation.

2) Patch/Upgrade Steps:

• Monitor Synology's official security advisory portal and download center for the 7.0.7-50095 release or later.
• Before upgrading, verify the integrity of the installer package using checksums or digital signatures provided by Synology.
• Roll out the patched version through your enterprise software management system.
• Post-upgrade, re-verify file permissions in the installation directory to confirm the fix has corrected default ACLs.

3) Long-Term Hardening:

• Implement the principle of least privilege across your Windows fleet: remove local admin rights from standard users.
• Audit local user accounts quarterly and remove dormant or unnecessary accounts.
• Deploy a baselining tool (Microsoft Defender for Endpoint, CIS-CAT) to detect insecure file permissions.
• Ensure Sysmon or equivalent auditing is enabled to log file creation events in Temp directories.
• Track Synology advisories in your vulnerability management program for future Assistant releases.

References

1. NVD Entry — https://nvd.nist.gov/vuln/detail/CVE-2026-4793
2. CWE-276 (Incorrect Default Permissions) — https://cwe.mitre.org/data/definitions/276.html
3. MITRE ATT&CK T1204.002 (User Execution: Malicious File) — https://attack.mitre.org/techniques/T1204/002/
4. Synology Security Advisory Portal (check for official advisory for CVE-2026-4793) — https://www.synology.com/en-us/security/advisory
5. CVEDetails entry (when populated) — https://www.cvedetails.com/cve/CVE-2026-4793/

Threat Actor Campaigns: No known campaigns or active exploitation in the wild have been identified as of this report. The public availability of PoC material, however, makes exploitation more likely in the near term, especially in environments where the vulnerable installer is still in circulation.