CVE-2021-44827
📋 TL;DR
This vulnerability allows remote authenticated attackers to execute arbitrary operating system commands with root privileges on TP-Link Archer C20i routers. Attackers can exploit the X_TP_ExternalIPv6Address HTTP parameter to inject commands. Only users of specific TP-Link Archer C20i firmware versions are affected.
💻 Affected Systems
- TP-Link Archer C20i
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of the router with root access, allowing attackers to intercept all network traffic, install persistent malware, pivot to internal networks, and brick the device.
Likely Case
Attackers gain full control of the router to monitor network traffic, redirect DNS, or use the device as a foothold for further attacks on the internal network.
If Mitigated
If proper network segmentation and access controls are in place, impact is limited to the router itself, though attackers could still intercept traffic passing through it.
🎯 Exploit Status
Exploit requires valid credentials to the router's web interface. Public proof-of-concept demonstrates command injection via crafted HTTP requests.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check TP-Link for latest firmware updates
Vendor Advisory: https://www.tp-link.com/us/security
Restart Required: Yes
Instructions:
1. Log into TP-Link router web interface. 2. Navigate to System Tools > Firmware Upgrade. 3. Check for updates or download latest firmware from TP-Link website. 4. Upload and install the firmware update. 5. Router will reboot automatically.
🔧 Temporary Workarounds
Disable remote management
allPrevents external attackers from accessing the web interface
Change default credentials
allUse strong, unique passwords for router admin accounts
🧯 If You Can't Patch
- Isolate the router on a separate network segment to limit potential damage
- Implement strict firewall rules to limit access to the router's management interface
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in web interface under System Tools > Firmware UpgradeCheck Version:
Check via router web interface; no direct CLI command availableVerify Fix Applied:
Verify firmware version has been updated to a version later than the vulnerable build📡 Detection & Monitoring
Log Indicators:
- Unusual HTTP POST requests to router management interface containing shell metacharacters in parameters
- Multiple failed login attempts followed by successful authentication
Network Indicators:
- HTTP requests to router management interface with unusual parameter values containing shell commands
- Outbound connections from router to unexpected external IPs
SIEM Query:
source="router_logs" AND (http_method="POST" AND (uri="*X_TP_ExternalIPv6Address*" OR param="*X_TP_ExternalIPv6Address*")) AND (content="*;*" OR content="*|*" OR content="*`*" OR content="*$(*")🔗 References
- https://Full-Disclosure.eu
- https://full-disclosure.eu/reports/2022/CVE-2021-44827-tplink-authenticated-remote-code-execution.html
- https://www.tp-link.com/us/security
- https://Full-Disclosure.eu
- https://full-disclosure.eu/reports/2022/CVE-2021-44827-tplink-authenticated-remote-code-execution.html
- https://www.tp-link.com/us/security
