CVE-2020-36938: WinAVR Privilege Escalation Vulnerability

CVE-2020-36938 Overview

CVE-2020-36938 is an insecure permissions vulnerability affecting WinAVR version 20100110, an open-source software development toolchain for Atmel AVR microcontrollers on Windows platforms. The vulnerability stems from overly permissive access controls on the installation directory, which allows authenticated local users to modify system files, DLLs, and executable files within the WinAVR installation path.

This vulnerability falls under CWE-732 (Incorrect Permission Assignment for Critical Resource) and enables attackers with local access to potentially escalate privileges by replacing legitimate executables or DLLs with malicious versions. When these modified files are subsequently executed by higher-privileged users or system processes, the attacker's code runs with elevated permissions.

Critical Impact

Authenticated local users can modify critical system files and executables in the WinAVR installation directory, potentially leading to privilege escalation and arbitrary code execution.

Affected Products

• WinAVR version 20100110
• Windows systems with WinAVR installed with default permissions

Discovery Timeline

• 2026-01-27 - CVE CVE-2020-36938 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2020-36938

Vulnerability Analysis

The insecure permissions vulnerability in WinAVR version 20100110 occurs due to improper access control configuration during installation. The WinAVR toolchain installs its files with overly permissive folder and file permissions, allowing any authenticated user on the system to read, write, and modify files within the installation directory.

This configuration issue creates a significant attack surface for privilege escalation attacks. An attacker with low-privilege local access can identify the WinAVR installation directory and exploit the weak permissions to replace legitimate DLL files or executables with malicious payloads. When a higher-privileged user or automated system process subsequently invokes any WinAVR tool, the malicious code executes with that user's privileges.

The attack requires user interaction—specifically, a privileged user must execute a WinAVR tool after the attacker has planted malicious files. However, in environments where WinAVR is used as part of automated build processes or development workflows, this condition may be easily met.

Root Cause

The root cause of this vulnerability is the failure to properly restrict file system permissions during WinAVR installation. The installer sets permissions that allow the Users group or Authenticated Users to have write access to the installation directory and its contents. This violates the principle of least privilege, as standard users should not have the ability to modify executable files and libraries used by software development tools.

Proper installation should restrict write permissions to the installation directory to administrators only, while allowing read and execute permissions for standard users. The permissive configuration likely stems from legacy installation practices or oversight in the packaging process.

Attack Vector

The attack vector is local, requiring an authenticated attacker to have access to the target system. The exploitation follows a DLL hijacking or binary replacement attack pattern:

1. The attacker identifies the WinAVR installation directory (typically C:\WinAVR-20100110\)
2. Due to insecure permissions, the attacker can write to this directory
3. The attacker replaces a legitimate DLL or executable with a malicious version
4. When a privileged user executes a WinAVR tool, the malicious code runs with their permissions

The vulnerability mechanism involves file permission misconfiguration that allows unauthorized modification of executable content. For detailed technical analysis, see the VulnCheck Advisory for WinAVR and Exploit-DB #49379 for proof-of-concept details.

Detection Methods for CVE-2020-36938

Indicators of Compromise

• Unexpected modifications to files within the WinAVR installation directory (C:\WinAVR-20100110\ or custom installation paths)
• Changes to DLL files or executables with recent modification timestamps that don't correspond to legitimate updates
• Unusual processes spawning from WinAVR executable paths
• File integrity monitoring alerts for WinAVR binaries

Detection Strategies

• Implement file integrity monitoring (FIM) on the WinAVR installation directory to detect unauthorized modifications to executables and DLLs
• Monitor Windows Security Event Log for file modification events (Event ID 4663) targeting the WinAVR installation path
• Use endpoint detection and response (EDR) solutions to identify suspicious child processes spawned from WinAVR tools
• Audit file system permissions regularly to identify directories with overly permissive access controls

Monitoring Recommendations

• Configure alerts for any write operations to the WinAVR installation directory by non-administrator accounts
• Implement baseline comparisons of file hashes for WinAVR executables and DLLs
• Monitor for privilege escalation patterns following execution of development tools
• Review scheduled tasks and automated build processes that invoke WinAVR components

How to Mitigate CVE-2020-36938

Immediate Actions Required

• Audit current file system permissions on the WinAVR installation directory and all subdirectories
• Restrict write permissions to administrators only while maintaining read/execute for standard users
• Consider relocating WinAVR to a protected directory with proper access controls
• Verify file integrity of existing WinAVR binaries against known-good hashes before continuing use

Patch Information

WinAVR version 20100110 is the last officially released version (from 2010), and the project is no longer actively maintained. There is no vendor patch available for this vulnerability. Organizations should consider migrating to alternative, actively maintained toolchains such as the official Microchip AVR toolchain or using containerized development environments that isolate the toolchain from the broader system.

For more information about the WinAVR project, see the SourceForge WinAVR Project page.

Workarounds

• Manually correct file system permissions on the WinAVR installation directory using icacls or Windows Explorer's security properties
• Remove write permissions for the Users and Authenticated Users groups from the installation directory
• Consider running WinAVR tools within a containerized or virtualized environment to limit the impact of potential exploitation
• Implement application whitelisting to prevent execution of unauthorized binaries from the WinAVR directory

# Windows command to restrict permissions on WinAVR installation directory
# Run as Administrator in Command Prompt
icacls "C:\WinAVR-20100110" /inheritance:r /grant:r "Administrators:(OI)(CI)F" /grant:r "Users:(OI)(CI)RX"