CVE-2021-47770: OpenPLC v3 RCE Vulnerability

CVE-2021-47770 Overview

CVE-2021-47770 is an authenticated remote code execution (RCE) vulnerability affecting OpenPLC v3, an open-source programmable logic controller platform. This vulnerability allows attackers with valid credentials to inject malicious code through the hardware configuration interface. By uploading a custom hardware layer containing embedded reverse shell code, attackers can establish a network connection to a specified IP address and port, enabling remote command execution on the underlying system.

Critical Impact

Authenticated attackers can achieve full remote code execution on OpenPLC v3 systems, potentially compromising industrial control systems and critical infrastructure.

Affected Products

• OpenPLC v3

Discovery Timeline

• 2026-01-21 - CVE CVE-2021-47770 published to NVD
• 2026-01-21 - Last updated in NVD database

Technical Details for CVE-2021-47770

Vulnerability Analysis

This vulnerability is classified as CWE-94 (Improper Control of Generation of Code, also known as Code Injection). The flaw exists within OpenPLC v3's hardware configuration interface, which fails to properly validate and sanitize user-supplied input when processing custom hardware layer uploads.

The vulnerability requires authenticated access to the OpenPLC web interface, meaning an attacker must first obtain valid credentials. Once authenticated, the attacker can exploit the hardware configuration functionality to upload a malicious hardware layer containing arbitrary code. This code is then executed by the OpenPLC runtime with the privileges of the underlying process.

The attack is network-accessible, can be executed with low complexity, and does not require user interaction beyond the initial authentication. However, the requirement for high privileges (administrator-level access) limits the attack surface somewhat. Despite this limitation, the impact is severe as successful exploitation grants complete control over the confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of this vulnerability is insufficient input validation in the hardware layer upload functionality. OpenPLC v3 allows administrators to customize hardware configurations by uploading custom hardware layer code. The application fails to properly sanitize or validate this uploaded code before compiling and executing it, creating a code injection vulnerability.

The hardware layer interface is designed to allow flexibility in configuring OpenPLC for various hardware platforms. However, this flexibility was implemented without adequate security controls, allowing attackers to embed malicious code such as reverse shells within the hardware layer definition.

Attack Vector

The attack vector for CVE-2021-47770 involves the following sequence:

1. Authentication: The attacker first authenticates to the OpenPLC v3 web interface using valid credentials (obtained through credential theft, brute force, or default credentials)

2. Hardware Layer Access: The attacker navigates to the hardware configuration section of the web interface

3. Malicious Upload: The attacker uploads a custom hardware layer containing embedded malicious code, typically a reverse shell payload

4. Code Execution: When the hardware layer is compiled and loaded, the embedded malicious code executes, establishing a connection back to the attacker's system

5. Remote Access: The attacker gains interactive shell access to the underlying operating system with the privileges of the OpenPLC process

A proof-of-concept exploit demonstrating this attack is documented in Exploit-DB #49803. Attackers typically embed reverse shell code that opens a network socket to an attacker-controlled server, providing persistent remote access to the compromised system.

Detection Methods for CVE-2021-47770

Indicators of Compromise

• Unexpected outbound network connections from OpenPLC processes to external IP addresses
• Modified hardware layer configuration files containing suspicious code patterns such as socket connections or shell commands
• Unusual process spawning from the OpenPLC runtime process
• Web server logs showing repeated access to hardware configuration endpoints

Detection Strategies

• Monitor OpenPLC web interface access logs for suspicious activity on hardware configuration endpoints
• Implement network segmentation and monitor for anomalous outbound connections from industrial control system networks
• Deploy file integrity monitoring on OpenPLC configuration directories to detect unauthorized modifications
• Review uploaded hardware layer code for suspicious patterns such as socket, exec, system, or reverse shell indicators

Monitoring Recommendations

• Establish baseline network behavior for OpenPLC systems and alert on deviations
• Configure SIEM rules to correlate authentication events with hardware configuration changes
• Implement endpoint detection and response (EDR) solutions on systems running OpenPLC to detect post-exploitation activity
• Monitor for process ancestry anomalies where OpenPLC spawns unexpected child processes

How to Mitigate CVE-2021-47770

Immediate Actions Required

• Restrict network access to OpenPLC web interfaces using firewall rules or network segmentation
• Review and audit all user accounts with access to OpenPLC systems, removing unnecessary accounts and enforcing strong passwords
• Implement multi-factor authentication for administrative access where possible
• Disable or restrict the hardware layer upload functionality if not required for operations

Patch Information

Organizations using OpenPLC v3 should monitor the GitHub OpenPLC Project for security updates and patches addressing this vulnerability. Review the VulnCheck OpenPLC Advisory for additional guidance and remediation information. The OpenPLC Project Homepage may also provide security announcements and recommended mitigations.

Workarounds

• Implement network access controls to limit who can reach the OpenPLC web interface
• Place OpenPLC systems on isolated network segments with strict egress filtering to prevent reverse shell connections
• Use web application firewalls (WAF) to filter potentially malicious uploads to the hardware configuration interface
• Monitor and restrict outbound network connections from systems running OpenPLC

# Example: Restrict outbound connections from OpenPLC using iptables
# Block all outbound connections except to known required destinations
iptables -A OUTPUT -m owner --uid-owner openplc -j DROP
# Allow only specific required outbound traffic
iptables -I OUTPUT -m owner --uid-owner openplc -d 192.168.1.0/24 -j ACCEPT