CVE-2021-45989
📋 TL;DR
This vulnerability is a stack overflow in Tenda G1 and G3 routers that allows attackers to cause a Denial of Service (DoS) by sending specially crafted requests to the guestWifiRuleRefresh function. Attackers can exploit this remotely via the qosGuestUpstream and qosGuestDownstream parameters. Users of affected Tenda router models with vulnerable firmware are at risk.
💻 Affected Systems
- Tenda G1 Router
- Tenda G3 Router
📦 What is this software?
G1 Firmware by Tendacn
G3 Firmware by Tendacn
⚠️ Risk & Real-World Impact
Worst Case
Complete router crash requiring physical reboot, potentially disrupting all network connectivity for connected devices.
Likely Case
Router becomes unresponsive, requiring manual reboot to restore functionality, causing temporary network outage.
If Mitigated
If properly segmented or behind firewalls, impact limited to isolated guest network functionality.
🎯 Exploit Status
Proof of concept code is publicly available on GitHub. Exploitation requires sending crafted HTTP requests to the router's web interface.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown - Check Tenda for updated firmware
Vendor Advisory: Not publicly available
Restart Required: Yes
Instructions:
1. Log into Tenda router admin interface. 2. Navigate to System Tools > Firmware Upgrade. 3. Download latest firmware from Tenda website. 4. Upload and install firmware update. 5. Reboot router after installation.
🔧 Temporary Workarounds
Disable Guest WiFi
allDisable the guest WiFi feature to prevent exploitation via vulnerable parameters.
Restrict Management Access
allConfigure firewall rules to restrict access to router management interface to trusted IPs only.
🧯 If You Can't Patch
- Segment router on isolated network segment with strict firewall rules
- Implement network monitoring for unusual traffic patterns to router management interface
🔍 How to Verify
Check if Vulnerable:
Check router firmware version via admin interface. If version is v15.11.0.17(9502)_CN and guest WiFi is enabled, device is vulnerable.Check Version:
Log into router web interface and check System Status or Firmware Version page.Verify Fix Applied:
Verify firmware version has been updated to a version later than v15.11.0.17(9502)_CN.📡 Detection & Monitoring
Log Indicators:
- Multiple failed login attempts
- Unusual HTTP POST requests to guestWifiRuleRefresh endpoint
- Router reboot logs without user action
Network Indicators:
- Unusual HTTP traffic to router port 80/443 with qosGuest parameters
- Sudden drop in router responsiveness
SIEM Query:
source="router_logs" AND (uri="*guestWifiRuleRefresh*" OR message="*reboot*" OR message="*crash*")