CVE-2021-22699
📋 TL;DR
An improper input validation vulnerability in Schneider Electric Modicon M241/M251 logic controllers allows attackers to cause denial of service via specially crafted HTTP requests. This affects organizations using these industrial controllers with firmware versions before V5.1.9.1 in their operational technology environments.
💻 Affected Systems
- Schneider Electric Modicon M241 logic controller
- Schneider Electric Modicon M251 logic controller
📦 What is this software?
Modicon M241 Firmware by Schneider Electric
Modicon M251 Firmware by Schneider Electric
⚠️ Risk & Real-World Impact
Worst Case
Complete controller unavailability leading to production stoppage, safety system disruption, or process control failure in industrial environments.
Likely Case
Temporary controller unavailability requiring manual reboot, causing production delays and potential equipment damage.
If Mitigated
Minimal impact with proper network segmentation and monitoring detecting attack attempts before successful exploitation.
🎯 Exploit Status
Exploitation requires sending crafted HTTP requests to the controller's web interface. No authentication is required, making exploitation straightforward for attackers with network access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: V5.1.9.1 or later
Vendor Advisory: https://download.schneider-electric.com/files?p_Doc_Ref=SEVD-2021-130-05
Restart Required: Yes
Instructions:
1. Download firmware V5.1.9.1 or later from Schneider Electric website. 2. Backup current controller configuration. 3. Upload new firmware using EcoStruxure Machine Expert software. 4. Restart controller. 5. Verify firmware version and restore configuration if needed.
🔧 Temporary Workarounds
Disable HTTP Interface
allDisable the HTTP web server interface if not required for operations
Configure via EcoStruxure Machine Expert: Disable HTTP server in controller settings
Network Segmentation
allIsolate controllers in separate VLAN with strict firewall rules
Add firewall rule: deny all HTTP traffic to controller IP except from authorized engineering stations
🧯 If You Can't Patch
- Implement strict network access controls allowing only authorized engineering stations to communicate with controllers
- Deploy network intrusion detection systems monitoring for anomalous HTTP traffic patterns to controller IPs
🔍 How to Verify
Check if Vulnerable:
Check firmware version via EcoStruxure Machine Expert or web interface. If version is below V5.1.9.1, system is vulnerable.Check Version:
Using EcoStruxure Machine Expert: Connect to controller → Controller → Properties → Firmware versionVerify Fix Applied:
Confirm firmware version shows V5.1.9.1 or higher in controller properties. Test HTTP interface functionality remains operational.📡 Detection & Monitoring
Log Indicators:
- Multiple HTTP 400/500 errors from controller
- Controller reboot events without scheduled maintenance
- Unusual HTTP request patterns to controller IP
Network Indicators:
- HTTP requests with malformed headers or unusual payloads to controller port 80/8080
- Sudden increase in HTTP traffic to industrial controllers
SIEM Query:
source="controller_logs" AND (http_status>=400 OR event_type="reboot") | stats count by src_ip, dest_ip