CVE-2021-43931
📋 TL;DR
CVE-2021-43931 is an authentication bypass vulnerability in WebHMI portal software that allows attackers to circumvent authentication mechanisms and gain unauthorized access. This affects industrial control systems using vulnerable versions of WebHMI. The vulnerability stems from a separate weakness that undermines the otherwise sound authentication algorithm.
💻 Affected Systems
- WebHMI
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of industrial control systems, allowing attackers to manipulate HMI interfaces, disrupt operations, or cause physical damage to industrial processes.
Likely Case
Unauthorized access to HMI interfaces enabling monitoring of industrial processes, data exfiltration, or preparation for further attacks.
If Mitigated
Limited impact with proper network segmentation and access controls preventing exploitation attempts from reaching vulnerable systems.
🎯 Exploit Status
The authentication bypass is straightforward to exploit once the separate weakness is identified, requiring minimal technical skill.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.1.1
Vendor Advisory: https://us-cert.cisa.gov/ics/advisories/icsa-21-336-03
Restart Required: Yes
Instructions:
1. Download WebHMI version 4.1.1 or later from the vendor. 2. Backup current configuration. 3. Install the update following vendor instructions. 4. Restart the WebHMI service. 5. Verify the update was successful.
🔧 Temporary Workarounds
Network Segmentation
allIsolate WebHMI systems from untrusted networks and restrict access to authorized IP addresses only.
Access Control Lists
allImplement firewall rules to restrict access to WebHMI ports (typically 80/443) to authorized management networks.
🧯 If You Can't Patch
- Implement strict network segmentation to isolate WebHMI systems from untrusted networks
- Deploy web application firewall (WAF) rules to detect and block authentication bypass attempts
🔍 How to Verify
Check if Vulnerable:
Check WebHMI version via the web interface or configuration files. Versions below 4.1.1 are vulnerable.Check Version:
Check WebHMI web interface or consult configuration files for version informationVerify Fix Applied:
Verify WebHMI version is 4.1.1 or higher and test authentication functionality to ensure proper access controls are enforced.📡 Detection & Monitoring
Log Indicators:
- Failed authentication attempts followed by successful access
- Access from unexpected IP addresses
- Authentication bypass patterns in web logs
Network Indicators:
- Unauthorized access to WebHMI web ports
- Traffic patterns indicating authentication bypass attempts
SIEM Query:
source="webhmi" AND (event="authentication_failure" OR event="authentication_bypass")