CVE-2026-4255 Thermalright TR-VISION HOME DLL Side-Loading Privilege Escalation

Severity: HIGH (CVSS 8.4)
CWE: CWE-829 Inclusion of Functionality from Untrusted Control Sphere
Published: 16/03/2026
CNA: Toreon
Affected versions: Thermalright TR-VISION HOME ≤ 2.0.5 (Windows 64-bit)
Discovered by: Ard33

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Description

Thermalright TR-VISION HOME loads Ftd2xx.dll (FTDI USB-to-serial library) using the default Windows DLL search order. This order includes user-writable directories before protected system locations. Since TR-VISION HOME always executes with administrative privileges and performs no integrity or signature verification on loaded libraries, an attacker who can write to any directory on the PATH can plant a crafted Ftd2xx.dll and have it loaded with elevated privileges the next time the application launches.

No administrative rights are required to plant the DLL. A low-privileged user can exploit this against any user who subsequently launches TR-VISION HOME with elevation.

CVSS 4.0 Vector

CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

Metric	Value
Attack Vector	Local
Attack Complexity	Low
Attack Requirements	None
Privileges Required	None
User Interaction	Active (victim launches app)

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Root Cause

The application calls LoadLibrary("Ftd2xx.dll") (or imports it implicitly at load time) without specifying a fully qualified path. Windows resolves the library by walking the search order defined in MSDN Dynamic-Link Library Search Order:

1. The directory from which the application was loaded
2. The system directory (C:\Windows\System32)
3. The 16-bit system directory
4. The Windows directory
5. The current working directory
6. Directories listed in the PATH environment variable ← exploitable

An attacker plants Ftd2xx.dll in a user-writable directory that appears on PATH before System32. C:\Users\<User>\.local\bin) is one such directory and is present on PATH by default, making it a reliable drop location.

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Affected Software

Software	Version	Platform
Thermalright TR-VISION HOME	≤ 2.0.5	Windows x64

Tested on TR-VISION HOME 2.0.5 on Windows 11.

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Proof of Concept

Prerequisites

• Any standard Windows user account (no administrator rights required to plant the DLL)
• A user-writable directory present on %PATH%
• Target user must launch TR-VISION HOME (it requests elevation via UAC on start)

Steps

1. Identify a writable PATH directory

$env:PATH -split ';' | Where-Object {
    if (-not $_ -or -not (Test-Path $_)) { return $false }
    try {
        $t = Join-Path $_ '.wrtest'
        [System.IO.File]::Create($t).Close()
        Remove-Item $t -Force
        $true
    } catch { $false }
}

A reliable default: C:\Users\<User>\.local\bin

2. Compile the PoC DLL

MinGW (cross or native):

gcc -shared -o Ftd2xx.dll poc/Ftd2xx.c -Wl,--out-implib,Ftd2xx.lib

MSVC:

cl /LD poc\Ftd2xx.c /Fe:Ftd2xx.dll

3. Plant the DLL

Choose any writable directory from step 1. Two common defaults:

:: uv / pipx default scripts directory
copy Ftd2xx.dll "C:\Users\<victim>\.local\bin\Ftd2xx.dll"

:: Python Launcher (standard Python install)
copy Ftd2xx.dll "C:\Users\<victim>\AppData\Local\Programs\Python\Launcher\Ftd2xx.dll"

No elevation required for this step.

4. Trigger

The victim launches TR-VISION HOME (e.g. from Start Menu or system tray). The UAC prompt runs the application with administrative privileges. During initialization, Ftd2xx.dll is resolved via the search order, the planted copy is found before System32, and the attacker's DllMain executes inside the elevated process.

5. Expected result

• A popup appears confirming the DLL was loaded inside the elevated process
• C:\Users\Public\CVE-2026-4255_pwned.txt is created, containing:
  • Process ID of TR-VISION HOME
  • Confirmation that TokenIsElevated = YES

Screenshot evidence

Procmon showing TR-VISION HOME (PID 78940) searching for Ftd2xx.dll across PATH directories — all returning NAME NOT FOUND until the planted copy is resolved:

Ftd2xx.dll planted in AppData\Local\Programs\Python\Launcher\ (date 26/12/2025 — discovery day):

Elevated cmd shell spawned by the loaded DLL — title bar confirms Administrator context:

TR-VISION HOME version 2.0.5:

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Mitigation

For Thermalright:

1. Load Ftd2xx.dll using a fully qualified path at runtime:

   LoadLibraryExA("C:\\Windows\\System32\\Ftd2xx.dll", NULL, LOAD_LIBRARY_SEARCH_SYSTEM32);

2. Use LOAD_LIBRARY_SEARCH_SYSTEM32 or LOAD_LIBRARY_SEARCH_APPLICATION_DIR flags with LoadLibraryEx.
3. Verify the digital signature of loaded libraries via WinVerifyTrust.
4. Enable Safe DLL Search Mode (HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\SafeDllSearchMode = 1), though this does not fully mitigate PATH-based planting.

For users (temporary):

• Place the legitimate Ftd2xx.dll (from the FTDI D2XX driver package) directly in the TR-VISION HOME installation directory (e.g. C:\Program Files\TR-VISION HOME\). The application directory is the first location searched, so a legitimate copy there takes priority over any PATH-based plant.
• Do not run TR-VISION HOME until a patched version is available.

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Timeline

Date	Event
26/12/2025	Vulnerability discovered
27/12/2025	Vendor (Thermalright) contacted via support channels
16/03/2026	Public disclosure CVE-2026-4255 assigned by Toreon CNA

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References

• NVD CVE-2026-4255
• CWE-829 Inclusion of Functionality from Untrusted Control Sphere
• MSDN DLL Search Order
• FTDI Ftd2xx Driver
• Thermalright TR-VISION HOME

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Disclaimer

This proof-of-concept is provided for educational and research purposes only. The PoC payload is intentionally benign it writes a text file and shows a message box. Use only against systems you own or have explicit written permission to test. The author bears no responsibility for misuse.