CVE-2021-27245
📋 TL;DR
This vulnerability allows attackers to bypass firewall protections on TP-Link Archer A7 and C7 routers by exploiting improper IPv6 SSH connection filtering. Attackers can combine this with other vulnerabilities to execute arbitrary code with root privileges. Affected users are those running vulnerable firmware versions on these specific router models.
💻 Affected Systems
- TP-Link Archer A7 AC1750
- TP-Link Archer C7 AC1750
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete router compromise allowing root-level remote code execution, enabling attackers to intercept traffic, modify router settings, pivot to internal networks, or install persistent malware.
Likely Case
Firewall bypass allowing unauthorized network access, potentially leading to internal network reconnaissance and lateral movement opportunities.
If Mitigated
Limited impact if IPv6 is disabled or proper network segmentation isolates the router from critical systems.
🎯 Exploit Status
Requires combination with other vulnerabilities for full RCE; firewall bypass alone provides network access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Archer A7(US)_V5_200220 or later, Archer C7(US)_V5_210125 or later
Vendor Advisory: https://www.tp-link.com/us/support/download/
Restart Required: Yes
Instructions:
1. Log into router admin interface. 2. Navigate to System Tools > Firmware Upgrade. 3. Download latest firmware from TP-Link support site. 4. Upload and install firmware. 5. Reboot router.
🔧 Temporary Workarounds
Disable IPv6
allCompletely disable IPv6 functionality on the router to prevent exploitation
Restrict SSH Access
allConfigure firewall rules to restrict SSH access to trusted IP addresses only
🧯 If You Can't Patch
- Isolate router on separate VLAN with strict firewall rules
- Implement network monitoring for unusual IPv6 SSH traffic patterns
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in admin interface under System Tools > Firmware UpgradeCheck Version:
Login to router web interface and check firmware version in System Tools sectionVerify Fix Applied:
Confirm firmware version is Archer A7(US)_V5_200220 or later for A7, or Archer C7(US)_V5_210125 or later for C7📡 Detection & Monitoring
Log Indicators:
- Unusual IPv6 SSH connection attempts
- Firewall rule bypass events
- Unexpected configuration changes
Network Indicators:
- IPv6 SSH traffic to router on non-standard ports
- Unusual outbound connections from router
SIEM Query:
source_ip=router_ip AND (protocol=ssh OR port=22) AND ip_version=6