CVE-2021-27691 – This CVE describes a critical command injection... (How to Fix)

Q: What is the severity of CVE-2021-27691?

CVE-2021-27691 has a CVSS score of 9.8 (CRITICAL). This CVE describes a critical command injection vulnerability in Tenda G0, G1, and G3 routers that allows remote attackers to execute arbitrary operating system commands. Attackers can exploit this by sending a specially crafted request to the router's web interface, potentially taking full control of the device. Users with affected router models and firmware versions are at risk.

📋 TL;DR

This CVE describes a critical command injection vulnerability in Tenda G0, G1, and G3 routers that allows remote attackers to execute arbitrary operating system commands. Attackers can exploit this by sending a specially crafted request to the router's web interface, potentially taking full control of the device. Users with affected router models and firmware versions are at risk.

💻 Affected Systems

Products:

Tenda G0 router
Tenda G1 router
Tenda G3 router

Versions: G0: v15.11.0.6(9039)_CN and v15.11.0.5(5876)_CN; G1/G3: v15.11.0.17(9502)_CN or v15.11.0.16(9024)_CN

Operating Systems: Embedded Linux (router firmware)

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects Chinese firmware versions (_CN suffix). The web management interface must be accessible for exploitation.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete compromise of the router allowing attackers to intercept all network traffic, install persistent malware, pivot to internal networks, and brick the device.

🟠

Likely Case

Attackers gain remote shell access to the router, enabling them to modify DNS settings, capture credentials, and use the router as a foothold for further attacks.

🟢

If Mitigated

With proper network segmentation and firewall rules, impact is limited to the router itself without allowing lateral movement to other systems.

🌐 Internet-Facing: HIGH - Routers are typically internet-facing devices, and the exploit requires no authentication, making them easily accessible to attackers.

🏢 Internal Only: MEDIUM - If the router's management interface is exposed internally, attackers on the local network could exploit this vulnerability.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploit requires sending a single HTTP POST request to the vulnerable endpoint with command injection payload. No authentication required.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Check Tenda website for latest firmware updates

Vendor Advisory: Not publicly available

Restart Required: Yes

Instructions:

1. Visit Tenda official website. 2. Download latest firmware for your router model. 3. Access router web interface. 4. Navigate to System Tools > Firmware Upgrade. 5. Upload and install new firmware. 6. Reboot router.

🔧 Temporary Workarounds

Disable Remote Management

all

Prevent external access to router web interface

Restrict Management Interface Access

all

Configure firewall to only allow management from trusted IP addresses

🧯 If You Can't Patch

Replace affected routers with non-vulnerable models
Place router behind a separate firewall that blocks access to management interface

🔍 How to Verify

Check if Vulnerable:

Check router firmware version via web interface at System Status > Firmware Version

Check Version:

Not applicable - check via web interface

Verify Fix Applied:

Verify firmware version has been updated to a version not listed in affected versions

📡 Detection & Monitoring

Log Indicators:

HTTP POST requests to /goform/setDebugCfg with suspicious parameters
Unusual command execution in router logs

Network Indicators:

Unexpected outbound connections from router
DNS hijacking or unusual DNS queries

SIEM Query:

http.method:POST AND http.uri:"/goform/setDebugCfg" AND (http.param:*|* OR http.param:*;*)

📊 Metadata

CVE ID: CVE-2021-27691

CVSS v3 Score: 9.8 (CRITICAL)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CWE: CWE-78

Published: April 16, 2021

Last Updated: November 21, 2024

🔗 References

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