CVE-2016-20025: ZKTeco ZKAccess Privilege Escalation Flaw

CVE-2016-20025 Overview

CVE-2016-20025 is an insecure file permissions vulnerability affecting ZKTeco ZKAccess Professional 3.5.3. This physical access control software contains a flaw that allows authenticated users to escalate privileges by modifying executable files. The vulnerability stems from the Modify permission being granted to the Authenticated Users group on executable binaries, enabling attackers to replace these files with malicious code and achieve privilege escalation.

Critical Impact

Authenticated attackers can replace legitimate executable files with malicious code, leading to arbitrary code execution with elevated privileges on affected systems running ZKTeco ZKAccess Professional.

Affected Products

• ZKTeco ZKAccess Professional 3.5.3

Discovery Timeline

• 2026-03-16 - CVE CVE-2016-20025 published to NVD
• 2026-03-16 - Last updated in NVD database

Technical Details for CVE-2016-20025

Vulnerability Analysis

This vulnerability is classified under CWE-552 (Files or Directories Accessible to External Parties) and represents a classic insecure permissions configuration flaw. The root issue lies in how ZKTeco ZKAccess Professional 3.5.3 configures file system permissions during installation, specifically granting the Authenticated Users group excessive write access to program executable files.

When an application is installed with overly permissive file access controls, any authenticated user on the system can modify the executable files. This creates a direct path to privilege escalation, as services or scheduled tasks that run these executables with elevated privileges will execute the attacker's malicious code instead.

The vulnerability is exploitable over the network with low attack complexity and requires only low-level privileges to execute. No user interaction is required for successful exploitation, making this a particularly dangerous configuration flaw.

Root Cause

The root cause of this vulnerability is improper access control configuration during the installation of ZKTeco ZKAccess Professional 3.5.3. The installer grants the Authenticated Users group Modify permissions on the application's executable files and directories. This violates the principle of least privilege, as standard users should not have write access to system executables or program binaries that run with elevated privileges.

Attack Vector

The attack vector involves an authenticated user leveraging their file system permissions to replace legitimate ZKAccess Professional executables with malicious payloads. The attack follows this general pattern:

1. An attacker authenticates to a Windows system where ZKTeco ZKAccess Professional 3.5.3 is installed
2. The attacker identifies executable files within the ZKAccess installation directory that are writable by the Authenticated Users group
3. The attacker replaces or modifies these executables with malicious code
4. When the application service restarts or a privileged user launches the application, the malicious code executes with elevated privileges

Technical details and proof-of-concept information can be found in the Exploit-DB #40323 advisory and the ZeroScience Vulnerability ZSL-2016-5361 disclosure.

Detection Methods for CVE-2016-20025

Indicators of Compromise

• Unexpected modifications to executable files within the ZKTeco ZKAccess Professional installation directory
• Changes to file timestamps, sizes, or hashes of application binaries
• Unusual process execution originating from the ZKAccess installation path
• Creation of new executable files in the application directory by non-administrative users

Detection Strategies

• Monitor file integrity of the ZKAccess Professional installation directory using File Integrity Monitoring (FIM) solutions
• Audit Windows Security Event Logs for file modification events (Event ID 4663) targeting the application directory
• Deploy endpoint detection rules to alert on executable file replacements in program directories
• Review Access Control Lists (ACLs) on application directories to identify overly permissive configurations

Monitoring Recommendations

• Implement continuous file integrity monitoring on all ZKTeco ZKAccess Professional executable files and DLLs
• Configure Windows Advanced Audit Policy to log Object Access events for the application directory
• Set up alerts for any non-administrator modifications to files within %ProgramFiles%\ZKTeco\ZKAccess or equivalent installation paths
• Monitor process creation events for suspicious executables running from the application directory

How to Mitigate CVE-2016-20025

Immediate Actions Required

• Review and restrict file permissions on the ZKTeco ZKAccess Professional installation directory
• Remove Modify and Write permissions from the Authenticated Users group on all executable files
• Ensure only SYSTEM and Administrators have write access to application binaries
• Conduct a file integrity check to verify no executables have been modified

Patch Information

Contact ZKTeco support for updated versions of ZKAccess Professional that address the insecure default permissions. Review the VulnCheck ZKTeco Privilege Escalation Advisory for additional remediation guidance.

Workarounds

• Manually correct file permissions using icacls to remove Authenticated Users write access from the installation directory
• Implement application whitelisting to prevent unauthorized executable modifications from running
• Deploy a host-based intrusion prevention system (HIPS) to block unauthorized file modifications
• Consider running the application in an isolated environment with restricted network access

# Configuration example - Restrict permissions on ZKAccess installation directory
icacls "C:\Program Files\ZKTeco\ZKAccess" /inheritance:r
icacls "C:\Program Files\ZKTeco\ZKAccess" /grant:r "SYSTEM:(OI)(CI)F"
icacls "C:\Program Files\ZKTeco\ZKAccess" /grant:r "Administrators:(OI)(CI)F"
icacls "C:\Program Files\ZKTeco\ZKAccess" /grant:r "Users:(OI)(CI)RX"