CVE-2025-51726

Executive Summary

CVE-2025-51726 identifies two critical security misconfigurations in the CyberGhost VPN Windows installer (CyberGhostVPNSetup.exe). First, the installer is signed using the SHA-1 cryptographic hash algorithm, which is known to be vulnerable to collision attacks. This allows an attacker to craft a malicious installer that carries a forged SHA-1 certificate, potentially bypassing Windows signature verification on systems without strict SmartScreen or trust policy enforcement. Second, the binary lacks High Entropy Address Space Layout Randomization (ASLR), as confirmed by BinSkim (rule BA2015) and repeated WinDbg analysis. This means the executable loads into predictable memory ranges, significantly increasing the success rate of memory corruption exploits. When combined, these weaknesses lower the bar for supply-chain attacks, privilege escalation, or malware distribution via a fake installer.

Supply Chain & Package Security

This vulnerability directly impacts the software supply chain of CyberGhost VPN. The installer is the primary distribution artifact for Windows users. An attacker who compromises the distribution channel (e.g., via a man-in-the-middle attack, compromised mirror, or social engineering) can replace the legitimate installer with a malicious one that still passes signature checks due to the weak SHA-1 signing. The lack of ASLR further enables exploitation of memory corruption vulnerabilities in the installer itself, potentially leading to code execution with the privileges of the user running the installer. No package manager (NPM, PyPI, Go, Ruby) is involved; this is a native Windows executable distribution.

CVSS Breakdown & Threat Level

CVSS Score: 8.5 (High)
Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

• Attack Vector (AV): Local (L) – The attacker must have local access to the system or trick the user into running the malicious installer.
• Attack Complexity (AC): Low (L) – SHA-1 collision attacks are well-documented and tooling is publicly available.
• Privileges Required (PR): None (N) – No prior authentication needed.
• User Interaction (UI): Required (R) – The victim must run the installer.
• Scope (S): Changed (C) – The vulnerability can affect resources beyond the installer’s initial security context (e.g., system-wide compromise).
• Confidentiality (C): High (H) – Full data access possible.
• Integrity (I): High (H) – Attacker can modify system files.
• Availability (A): High (H) – Attacker can cause denial of service or system crash.

Threat Level: High. The combination of weak signing and missing ASLR makes this a realistic vector for targeted attacks and malware distribution.

Deep Dive & Mechanics

SHA-1 Collision Attack

SHA-1 is a 160-bit hash function that has been proven to be vulnerable to collision attacks (e.g., SHAttered, SHAmbles). An attacker can generate two different inputs (one benign, one malicious) that produce the same SHA-1 hash. If the legitimate installer is signed with SHA-1, the attacker can create a malicious installer that has the same hash and thus the same digital signature. Windows will verify the signature as valid if the certificate chain is trusted and the system does not enforce SHA-1 deprecation policies (e.g., via SmartScreen or Group Policy).

Missing High Entropy ASLR

ASLR randomizes the base address of executable images to prevent predictable memory layouts. High Entropy ASLR (introduced in Windows 8) uses a 64-bit random offset, making it extremely difficult to predict. The CyberGhost installer does not have the DYNAMIC_BASE flag set (or uses a low-entropy variant), causing it to load at a fixed address. This allows an attacker to reliably place shellcode or ROP chains at known memory locations, bypassing ASLR-based mitigations.

Combined Impact

1. Supply-Chain Attack: Attacker replaces the installer on a download site or via MITM.
2. Signature Bypass: The fake installer has a valid SHA-1 signature.
3. Exploitation: The fake installer contains a memory corruption exploit (e.g., buffer overflow) that leverages the predictable memory layout to execute arbitrary code.
4. Privilege Escalation: The code runs with the user’s privileges, but can then exploit other vulnerabilities to gain SYSTEM.

Vulnerable vs. Patched Code

No patched version is available. The vulnerable code is the installer binary itself. The following pseudo-code illustrates the missing ASLR flag:

Vulnerable (current):

// Linker flags missing /DYNAMICBASE
// PE header does not have IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE

Patched (hypothetical):

// Linker flags: /DYNAMICBASE /HIGHENTROPYVA
// PE header includes IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE and IMAGE_DLLCHARACTERISTICS_HIGH_ENTROPY_VA

For SHA-1 signing, the fix would be to sign the installer with SHA-256 or SHA-384 and enforce timestamping.

Exploitation & Lab Setup

Red Team

Objective: Create a malicious installer that bypasses signature verification and exploits predictable memory layout.

Tools Required:

• sha1collisiondetection (for generating SHA-1 collisions)
• signtool (Windows SDK) for signing
• msfvenom or custom shellcode
• WinDbg for memory analysis

Steps:

1. Obtain the legitimate CyberGhost installer.
2. Use a SHA-1 collision tool to generate a malicious installer with the same SHA-1 hash.
3. Sign the malicious installer with the same certificate (if you have the private key) or use the collision to reuse the original signature.
4. Embed shellcode that exploits a buffer overflow in the installer (e.g., in a long command-line argument).
5. Since ASLR is disabled, calculate the fixed base address (e.g., 0x00400000) and place the shellcode there.
6. Deliver the fake installer via phishing or compromised download link.

Example (conceptual):

# Generate collision
sha1collisiondetection -i legit.exe -o malicious.exe -p payload.bin

# Sign (if private key available)
signtool sign /fd SHA256 /a /t http://timestamp.digicert.com malicious.exe

Blue Team

Detection:

• Monitor for installer downloads from untrusted sources.
• Check digital signature details: if the signature algorithm is SHA-1, flag as suspicious.
• Use BinSkim to scan for missing ASLR: binskim analyze CyberGhostVPNSetup.exe --output results.sarif
• Enable Windows SmartScreen and enforce SHA-1 deprecation via Group Policy.

Hunting:

• Search for processes with base address 0x00400000 (or other fixed addresses) that are not expected.
• Look for multiple files with the same SHA-1 hash but different content.

Indicators of Compromise (IOCs)

• File Hashes (SHA-1): (Legitimate installer hash – obtain from official source)
• File Name: CyberGhostVPNSetup.exe (any variant)
• Signature Algorithm: SHA-1 (in certificate details)
• PE Characteristics: Missing IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE (0x0040)
• Base Address: 0x00400000 (typical for non-ASLR executables)
• Network: Connections to known malicious domains hosting fake installers.

Detection Engineering

YARA Rule

rule CVE_2025_51726_CyberGhost_Installer {
    meta:
        description = "Detects CyberGhost VPN installer with SHA-1 signature and missing ASLR"
        author = "Valters IT"
        date = "2025-01-01"
        hash = "sha1: [insert known malicious hash]"
    strings:
        $s1 = "CyberGhostVPNSetup.exe" nocase
        $s2 = "CyberGhost" nocase
        $pe_magic = "MZ"
    condition:
        $pe_magic at 0 and
        $s1 and
        // Check for SHA-1 signature (OID 1.3.14.3.2.26)
        for any i in (0..pe.number_of_signatures - 1): (
            pe.signatures[i].algorithm == "sha1"
        ) and
        // Check for missing DYNAMIC_BASE
        not (pe.characteristics & 0x0040)
}

Sigma Rule

title: CyberGhost Installer SHA-1 Signature and Missing ASLR
id: cve-2025-51726
status: experimental
description: Detects execution of CyberGhost installer with weak SHA-1 signature and no ASLR
author: Valters IT
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\CyberGhostVPNSetup.exe'
    condition: selection
falsepositives:
    - Legitimate older versions (if any)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-51726 - CyberGhost Installer Download"; flow:to_server; content:"CyberGhostVPNSetup.exe"; http_uri; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: CyberGhost must re-sign the installer using SHA-256 or SHA-384 with a valid timestamp.
2. Enable High Entropy ASLR: Recompile the installer with /DYNAMICBASE /HIGHENTROPYVA linker flags.
3. User Guidance:
   • Verify the digital signature algorithm (should be SHA-256).
   • Enable SmartScreen and Windows Defender Application Guard.
   • Only download from the official CyberGhost website.
4. Enterprise:
   • Deploy Group Policy to block SHA-1 signed executables.
   • Use application whitelisting (e.g., AppLocker).
   • Monitor for installer execution with BinSkim or similar tools.
5. Long-term: CyberGhost should implement a secure build pipeline with code signing and ASLR enforcement.

Valters IT