Executive Summary

The CVE-2026-6138 vulnerability affects the TOTOLINK A7100RU router, version 7.4cu.2313_b20191024, and is a command injection flaw located in the cstecgi.cgi CGI script.

Specifically, the vulnerability arises in the function that processes a user-supplied parameter named "mac." This parameter's value is concatenated and passed through several functions until it is executed as an operating system command.

An attacker can send a crafted HTTP POST request containing a malicious "mac" value to the /cgi-bin/cstecgi.cgi endpoint, causing the router to execute arbitrary OS commands remotely.

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Impact Analysis

This vulnerability allows remote attackers to execute arbitrary operating system commands on the affected TOTOLINK A7100RU router without any authentication.

Exploitation can lead to full compromise of the device, enabling attackers to control the router, intercept or manipulate network traffic, deploy malware, or use the device as a pivot point for further attacks within the network.

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Detection Guidance

This vulnerability can be detected by monitoring for suspicious HTTP POST requests sent to the /cgi-bin/cstecgi.cgi endpoint containing a JSON payload with a "mac" parameter that includes command injection patterns.

A practical detection method is to capture and analyze network traffic for POST requests targeting /cgi-bin/cstecgi.cgi with unusual or suspicious "mac" parameter values that attempt to execute commands such as wget or other shell commands.

For example, using a tool like curl or wget to test the endpoint with a benign payload can help verify if the endpoint is vulnerable.

• Use tcpdump or Wireshark to capture HTTP POST requests to /cgi-bin/cstecgi.cgi and inspect the payload for suspicious "mac" parameter values.
• Example command to monitor network traffic: tcpdump -i <interface> -A 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
• Use curl to send a test POST request: curl -X POST http://<router-ip>/cgi-bin/cstecgi.cgi -d '{"mac":"test"}' -H 'Content-Type: application/json'
• Check router logs or system logs for evidence of executed commands or unusual activity triggered by the "mac" parameter.

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Mitigation Strategies

Immediate mitigation steps include restricting access to the vulnerable /cgi-bin/cstecgi.cgi endpoint to trusted networks or IP addresses only.

If possible, disable or block HTTP POST requests to the /cgi-bin/cstecgi.cgi script until a patch or update is applied.

Monitor network traffic for suspicious requests targeting this endpoint and block any malicious IP addresses.

Apply any available firmware updates or patches from the vendor that address this vulnerability.

Consider isolating the affected device from the internet or untrusted networks to prevent remote exploitation.

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Compliance Impact

The vulnerability allows remote attackers to execute arbitrary operating system commands on the affected TOTOLINK A7100RU router by exploiting a command injection flaw. This can lead to unauthorized access, data breaches, and potential manipulation or exfiltration of sensitive information.

Such unauthorized access and potential data compromise can negatively impact compliance with common standards and regulations like GDPR and HIPAA, which require protection of personal and sensitive data against unauthorized access and breaches.

Therefore, if exploited, this vulnerability could result in violations of these regulations due to failure to adequately secure the device and protect data confidentiality and integrity.

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