CVE-2021-20776
📋 TL;DR
CVE-2021-20776 is an authentication bypass vulnerability in specific Sharp and Aterm routers that allows attackers to execute arbitrary commands via telnet without proper credentials. This affects SCT-40CM01SR and AT-40CM01SR router models. Attackers can gain unauthorized access to the device's operating system.
💻 Affected Systems
- Sharp SCT-40CM01SR
- Aterm AT-40CM01SR
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of router allowing persistent backdoor installation, network traffic interception, lateral movement to connected devices, and device bricking.
Likely Case
Unauthorized administrative access leading to configuration changes, network disruption, credential harvesting, and potential data exfiltration.
If Mitigated
Limited impact if telnet is disabled and strong network segmentation prevents access to management interfaces.
🎯 Exploit Status
Exploitation requires only telnet access to the device; no authentication or special tools needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check vendor for latest firmware updates
Vendor Advisory: https://jvn.jp/en/jp/JVN21636825/index.html
Restart Required: Yes
Instructions:
1. Check vendor website for firmware updates. 2. Download appropriate firmware. 3. Access router admin interface. 4. Upload and apply firmware update. 5. Reboot router.
🔧 Temporary Workarounds
Disable Telnet Service
allCompletely disable telnet access to prevent exploitation
Access router admin interface and disable telnet service in network/management settings
Network Access Control
allRestrict telnet access to trusted management networks only
Configure firewall rules to block telnet (port 23) from untrusted networks
🧯 If You Can't Patch
- Isolate affected routers in separate VLAN with strict access controls
- Implement network monitoring for telnet connection attempts and unusual router behavior
🔍 How to Verify
Check if Vulnerable:
Attempt telnet connection to router on port 23 and check if authentication is bypassed or weakCheck Version:
Check router admin interface or use vendor-specific CLI commands to display firmware versionVerify Fix Applied:
Verify telnet service is disabled or requires proper authentication; check firmware version against patched versions📡 Detection & Monitoring
Log Indicators:
- Failed telnet authentication attempts
- Successful telnet connections from unusual IPs
- Router configuration changes
Network Indicators:
- Telnet traffic to router management interfaces
- Unusual outbound connections from router
SIEM Query:
source_port=23 OR destination_port=23 AND (device_type="router" OR device_vendor="Sharp" OR device_vendor="Aterm")