CVE-2026-7747: Critical Buffer Overflow in Totolink N300RH Router loginauth Function

Executive Summary

A critical stack-based buffer overflow vulnerability (CVE-2026-7747) has been disclosed in the Totolink N300RH wireless router, affecting firmware version 3.2.4-B20220812. The vulnerability carries a CVSS score of 9.8 (Critical) and enables unauthenticated remote code execution by exploiting the loginauth function in the /cgi-bin/cstecgi.cgi handler when processing an oversized Password parameter.

CVSS Score: 9.8 (Critical) Attack Vector: Network (remotely exploitable) Authentication Required: None

The Totolink N300RH is a consumer and small-business wireless router. Successful exploitation allows an attacker to overwrite the stack return address and redirect execution flow to attacker-controlled code — effectively taking full control of the router, the network it serves, and all traffic passing through it.

Vulnerability Overview

Affected Products

The Totolink N300RH is a 300 Mbps wireless N router marketed to home and small office users. Totolink devices have a history of security vulnerabilities and are frequently targeted in IoT botnet recruitment campaigns.

Technical Details

Vulnerability Mechanism

The loginauth function in /cgi-bin/cstecgi.cgi is responsible for processing login authentication requests from the router's web management interface. It reads the Password parameter from the HTTP request body and copies it into a fixed-size stack buffer without performing bounds checking.

When an attacker supplies a Password value that exceeds the buffer's allocated size, the excess data overwrites adjacent stack memory including the saved return address. Upon function return, the CPU jumps to the attacker-controlled address — typically pointing to shellcode placed within the overflowing payload or into memory regions containing existing executable code (return-oriented programming).

Attack Chain

1. Attacker sends HTTP POST request to router's web interface:
   POST /cgi-bin/cstecgi.cgi HTTP/1.1
   Host: [router-ip]
   Content-Type: application/x-www-form-urlencoded
 
   action=login&username=admin&Password=[AAAA...overlong_payload...SHELLCODE]
 
2. loginauth() copies Password into fixed-size stack buffer — no bounds check
 
3. Stack frame corrupted: saved return address overwritten with attacker value
 
4. Function returns; CPU jumps to attacker-controlled address
 
5. Arbitrary code executes with the privileges of the CGI process (typically root on embedded Linux)
 
6. Attacker achieves persistent root access to the router

Why This Is Dangerous on Routers

Routers are uniquely high-value compromise targets:

• Traffic interception: All network traffic can be monitored and manipulated
• DNS hijacking: Redirect legitimate domains to malicious servers
• NAT traversal: Use the router as a pivot into the internal network
• Botnet recruitment: Enroll the device in a Mirai-style DDoS botnet
• Persistent backdoor: Router firmware rarely gets user attention after initial setup

Impact Assessment

Recommendations

Immediate Mitigations

1. Check for vendor firmware update — visit Totolink's support site for patches addressing CVE-2026-7747
2. Disable remote management — ensure the router's web interface is not accessible from the WAN interface:

   Router Admin → Advanced → Remote Management → Disable

   Copy

3. Change default admin credentials — ensure non-default username and password are set, reducing attack surface for credential-based exploitation
4. Restrict LAN-side access — if possible, limit which internal IP addresses can access the admin interface

Network-Level Controls

If behind a perimeter firewall:
- Block inbound access to port 80/443 of the router's WAN IP
- Enable logging on all administrative access attempts
- Alert on connections from unexpected internal IPs to router management port

Consider Device Replacement

Given the vulnerability history of Totolink devices and the lack of consistent security patch support, organizations and individuals relying on Totolink N300RH devices in security-sensitive environments should evaluate migrating to devices from vendors with established security response programs.

Detection Indicators

Post-Remediation Checklist

1. Confirm firmware patched or device replaced
2. Factory reset the router to evict any implants before applying new firmware
3. Change all passwords on devices that were connected to the router during the exposure window
4. Review DHCP/DNS settings to confirm they have not been tampered with
5. Monitor internal network for signs of lateral movement from the compromise period
6. Audit connected devices for unusual outbound connections made during the exposure window

References

• NIST NVD — CVE-2026-7747
• Totolink Support
• CWE-120: Buffer Copy without Checking Size of Input
• CISA ICS Advisory — IoT Router Vulnerabilities