CVE-2021-20739
📋 TL;DR
This vulnerability allows an unauthenticated attacker on the same network to execute arbitrary operating system commands on affected ELECOM wireless routers. Attackers can gain full control of the device without needing credentials. All users of the listed ELECOM router models are affected.
💻 Affected Systems
- WRC-300FEBK
- WRC-F300NF
- WRC-733FEBK
- WRH-300RD
- WRH-300BK
- WRH-300SV
- WRH-300WH
- WRH-H300WH
- WRH-H300BK
- WRH-300BK-S
- WRH-300WH-S
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of the router allowing attackers to intercept all network traffic, install persistent malware, pivot to internal systems, or brick the device.
Likely Case
Attackers gain administrative control of the router to modify DNS settings, intercept credentials, or use the device as a foothold for further attacks.
If Mitigated
With proper network segmentation and access controls, impact is limited to the router itself without lateral movement to other systems.
🎯 Exploit Status
Attack requires network adjacency but no authentication. Exploit vectors are unspecified in public disclosures.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check vendor advisory for specific firmware versions
Vendor Advisory: https://www.elecom.co.jp/news/security/20210706-01/
Restart Required: Yes
Instructions:
1. Visit ELECOM support website. 2. Download latest firmware for your specific model. 3. Log into router admin interface. 4. Navigate to firmware update section. 5. Upload and apply the firmware update. 6. Reboot the router.
🔧 Temporary Workarounds
Network Segmentation
allIsolate affected routers from critical internal networks using VLANs or separate physical networks
Access Control Lists
allImplement network ACLs to restrict access to router management interfaces
🧯 If You Can't Patch
- Replace affected routers with patched or different models
- Implement strict network monitoring for suspicious router access attempts
🔍 How to Verify
Check if Vulnerable:
Check router model number and compare with affected list. If model matches, assume vulnerable.Check Version:
Log into router admin interface and check firmware version in system statusVerify Fix Applied:
Check firmware version after update and compare with vendor's patched version list📡 Detection & Monitoring
Log Indicators:
- Unusual command execution in router logs
- Unexpected configuration changes
- Multiple failed login attempts followed by successful access
Network Indicators:
- Unusual outbound connections from router
- DNS queries to suspicious domains
- Unexpected traffic redirection
SIEM Query:
source="router_logs" AND (command_execution OR config_change OR admin_access)