CVE-2020-37037: Avast SecureLine Privilege Escalation Flaw

CVE-2020-37037 Overview

CVE-2020-37037 is an unquoted service path vulnerability in Avast SecureLine 5.5.522.0 that allows local users to potentially execute code with elevated system privileges. Attackers can exploit the unquoted path in the service configuration to inject malicious code that would execute with LocalSystem account permissions during service startup.

Critical Impact

Local attackers with limited privileges can achieve privilege escalation to LocalSystem by exploiting the unquoted service path during service startup, potentially gaining full control of the affected system.

Affected Products

• Avast SecureLine VPN 5.5.522.0
• Avast SecureLine VPN versions prior to patched releases
• Windows systems running affected Avast SecureLine installations

Discovery Timeline

• 2026-02-01 - CVE CVE-2020-37037 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2020-37037

Vulnerability Analysis

This vulnerability is classified as CWE-428 (Unquoted Search Path or Element), a common configuration flaw in Windows services. When a Windows service executable path contains spaces and is not enclosed in quotation marks, the Windows Service Control Manager (SCM) may interpret the path incorrectly, creating an opportunity for privilege escalation.

The Avast SecureLine VPN service runs with LocalSystem privileges, which represents the highest privilege level available on a Windows system. When the service starts, if the path contains spaces and is unquoted, Windows attempts to execute binaries at each space boundary in the path sequentially until it finds a valid executable.

This local attack vector requires no user interaction and can be exploited by attackers who have already gained limited access to the system. The vulnerability allows for complete compromise of confidentiality, integrity, and availability of the affected system once exploited.

Root Cause

The root cause of CVE-2020-37037 is the improper configuration of the Windows service executable path. When the Avast SecureLine service is installed, the service path is registered in the Windows Registry without proper quotation marks around the full path string.

For example, a path like C:\Program Files\Avast Software\SecureLine\vpnsvc.exe would be vulnerable because Windows interprets spaces as delimiters. The SCM would first attempt to execute C:\Program.exe, then C:\Program Files\Avast.exe, and so on until finding a valid executable or the complete path.

Attack Vector

The attack leverages local system access to place a malicious executable in a location that Windows will execute before reaching the legitimate service binary. An attacker with write access to vulnerable path locations (such as C:\ or C:\Program Files\Avast) can plant a malicious Program.exe or similarly named executable.

When the Avast SecureLine service starts or restarts, the SCM will execute the attacker's malicious binary with LocalSystem privileges instead of the legitimate service executable. This grants the attacker the highest possible privilege level on the system, enabling complete system compromise.

The attack requires local access to the target machine and the ability to write files to specific directory locations in the unquoted service path. Exploitation is deterministic and does not require user interaction beyond triggering a service restart.

Detection Methods for CVE-2020-37037

Indicators of Compromise

• Presence of unexpected executables named Program.exe, Avast.exe, or similar in root directories or C:\Program Files
• Unusual processes spawned by the Avast SecureLine service with unexpected parent-child relationships
• Registry modifications to the Avast SecureLine service ImagePath value
• Unexpected service restarts or failures for the Avast SecureLine VPN service

Detection Strategies

• Monitor for file creation events in C:\ and C:\Program Files\ directories for executables matching path segment names
• Query Windows services for unquoted paths using PowerShell: Get-WmiObject win32_service | Where-Object {$_.PathName -match " " -and $_.PathName -notmatch '"'}
• Implement application whitelisting to prevent unauthorized executables from running with elevated privileges
• Use endpoint detection tools to identify privilege escalation attempts through service manipulation

Monitoring Recommendations

• Enable Windows Security Event logging for service creation and modification events (Event IDs 7045, 4697)
• Monitor file system activity in common unquoted path locations for newly created executables
• Implement baseline monitoring for the Avast SecureLine service process to detect anomalous behavior
• Configure alerts for any modification to service registry keys under HKLM\SYSTEM\CurrentControlSet\Services

How to Mitigate CVE-2020-37037

Immediate Actions Required

• Audit all installed Windows services for unquoted service paths and remediate immediately
• Restrict write permissions on directories within the service path to prevent malicious executable placement
• Update Avast SecureLine VPN to the latest available version from the vendor
• Implement application control policies to prevent execution of unauthorized binaries

Patch Information

Organizations should update Avast SecureLine VPN to the latest version available from Avast Security Solutions. Review the VulnCheck Advisory for Avast for additional vendor guidance and patch availability information.

Additional technical details about this vulnerability can be found at Exploit-DB #48249.

Workarounds

• Manually quote the service path in the Windows Registry by navigating to HKLM\SYSTEM\CurrentControlSet\Services\<ServiceName> and adding quotation marks around the ImagePath value
• Remove write permissions for non-administrative users on all directories in the service path
• Implement file integrity monitoring on vulnerable path locations to detect malicious file placement
• Consider temporarily disabling the vulnerable service until a patch can be applied

# Configuration example - Quote the service path in Windows Registry
# Run PowerShell as Administrator:

# Check for unquoted service paths
Get-WmiObject win32_service | Where-Object {$_.PathName -match " " -and $_.PathName -notmatch '"'} | Select-Object Name, PathName

# To manually fix the Avast SecureLine service path:
# 1. Open Registry Editor (regedit.exe) as Administrator
# 2. Navigate to: HKLM\SYSTEM\CurrentControlSet\Services\<AvastSecureLineServiceName>
# 3. Double-click ImagePath and add quotes around the full path
# Example: "C:\Program Files\Avast Software\SecureLine\vpnsvc.exe"