CVE-2021-27177
📋 TL;DR
This vulnerability allows attackers to bypass authentication on FiberHome HG6245D devices by sending a specific decoded string to the telnet server. It affects FiberHome HG6245D optical network terminal devices, potentially exposing them to unauthorized access and control.
💻 Affected Systems
- FiberHome HG6245D
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise leading to network infiltration, data interception, or use as a pivot point for attacking other network devices.
Likely Case
Unauthorized access to device configuration, potential for service disruption, or credential harvesting from the compromised device.
If Mitigated
Limited impact if telnet is disabled or network segmentation prevents access to vulnerable devices.
🎯 Exploit Status
The exploit involves sending the decoded value of GgpoZWxwCmxpc3QKd2hvCg== (which decodes to 'help\nlist\nwho\n') to bypass authentication. This is documented in public research.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: Not publicly available
Restart Required: No
Instructions:
No official patch available. Check with FiberHome for firmware updates beyond RP2613.
🔧 Temporary Workarounds
Disable Telnet Service
allDisable the telnet service on affected devices to prevent exploitation.
telnet disable
no telnet server enable
Network Access Control
linuxRestrict telnet access to trusted management networks only using firewall rules.
iptables -A INPUT -p tcp --dport 23 -s TRUSTED_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 23 -j DROP
🧯 If You Can't Patch
- Segment vulnerable devices on isolated network segments
- Implement strict network monitoring for telnet traffic to/from affected devices
🔍 How to Verify
Check if Vulnerable:
Attempt to connect via telnet and send the decoded string 'help\nlist\nwho\n' to see if authentication is bypassed.Check Version:
show version or equivalent command on device CLIVerify Fix Applied:
Verify telnet service is disabled or that the authentication bypass no longer works with the decoded string.📡 Detection & Monitoring
Log Indicators:
- Failed authentication attempts followed by successful telnet sessions
- Telnet connections from unexpected sources
Network Indicators:
- Telnet traffic to port 23 on affected devices
- Network traffic patterns showing the specific decoded string being sent
SIEM Query:
source_port:23 AND (payload_contains:"help\nlist\nwho\n" OR auth_result:"bypass")