OS Command Injection in TRENDnet TEW-821DAP Firmware

Executive Summary

This vulnerability is a command injection flaw found in the firmware update process of the Trendnet TEW-821DAP router (firmware version 1.11B03). It occurs in the function sub_42026C of the /goform/tools_ddns component, where user inputs such as hostname, username, and password are not properly validated.

These inputs are directly concatenated into DDNS configuration strings and written to a config file. When the DDNS service restarts, any malicious commands injected into these inputs are executed on the system.

The attack can be launched remotely via an HTTP POST request to /goform/tools_ddns, exploiting the vulnerable sprintf function in the DDNS handler.

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Impact Analysis

This vulnerability allows an attacker to execute arbitrary operating system commands remotely on the affected device by injecting malicious input into the hostname, username, or password fields.

Successful exploitation could lead to unauthorized control over the device, potentially allowing the attacker to manipulate device settings, disrupt network services, or use the device as a foothold for further attacks within the network.

However, this vulnerability only affects devices that are no longer supported by the vendor, which may limit exposure if the device is no longer in use or has been replaced.

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Detection Guidance

This vulnerability can be detected by monitoring for HTTP POST requests to the endpoint /goform/tools_ddns on the Trendnet TEW-821DAP router firmware version 1.11B03. Specifically, look for suspicious payloads in the hostname, username, or password parameters that may contain command injection attempts.

You can use network traffic inspection tools like tcpdump or Wireshark to capture and analyze HTTP POST requests targeting this endpoint.

Example command to capture relevant HTTP POST requests using tcpdump:

• tcpdump -i <interface> -A 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' | grep '/goform/tools_ddns'

Additionally, inspecting the router's logs for unusual DDNS restart commands or unexpected configuration changes may help detect exploitation attempts.

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Mitigation Strategies

Immediate mitigation steps include disabling remote access to the /goform/tools_ddns endpoint to prevent attackers from sending malicious POST requests.

Restrict network access to the affected device by implementing firewall rules that block unauthorized external traffic to the router's management interface.

Since the affected products are no longer supported by the vendor, consider replacing the device with a supported model that receives security updates.

Monitor the device for any signs of compromise, such as unexpected DDNS restarts or configuration changes.

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