Stack-Based Buffer Overflow in TOTOLINK N600R

A critical security vulnerability (CVE-2026-79911) affects the TOTOLINK N600R router, specifically firmware version 4.3.0cu.7647_B20210106. The vulnerability resides within the setSystemConfig function of the /cgi-bin/cstecgi.cgi component, which serves as the router's CGI handler. Attackers can remotely trigger a stack-based buffer overflow by sending a crafted request that manipulates the 'Hostname' argument. Due to the lack of proper bounds checking in the underlying memory operation, the payload can overwrite stack memory, potentially leading to arbitrary code execution or a denial of service (DoS) state. Publicly available exploit material exists, making this a high-risk exposure for any internet-facing N600R devices.

Attack Chain

1. Attacker performs reconnaissance to identify internet-exposed TOTOLINK N600R devices.
2. Attacker crafts a malicious HTTP request targeting the /cgi-bin/cstecgi.cgi endpoint.
3. The request includes a specially crafted, oversized value for the 'Hostname' parameter.
4. The router receives the request and passes the input to the setSystemConfig function.
5. The function fails to perform adequate bounds checking on the 'Hostname' argument before copying it to a stack buffer.
6. The buffer overflow occurs, overwriting adjacent memory on the stack with the attacker's payload.
7. The system's execution flow is diverted to the attacker's instructions.
8. Final objective is achieved, such as establishing persistent access or executing remote commands on the device.

Impact

Successful exploitation of CVE-2026-79911 results in full system compromise, allowing an unauthenticated remote attacker to execute arbitrary code with the privileges of the web service. This could lead to total control over the network traffic passing through the device, unauthorized access to the internal network, or permanent denial of service. The vulnerability has a CVSS base score of 10.0, reflecting the maximum severity for remote exploitation.

Recommendation

Prioritized actions for security and network teams:

• Immediately identify all TOTOLINK N600R devices exposed to the internet.
• Apply the latest firmware patches provided by TOTOLINK if available.
• If patching is not possible, restrict access to the web management interface to known, trusted internal IP addresses only.
• Implement network-level blocking of inbound traffic to /cgi-bin/cstecgi.cgi on edge firewalls.
• Monitor logs for unusual HTTP POST requests containing exceptionally long 'Hostname' values in the query or body.