CVE-2026-7747: Totolink N300RH Buffer Overflow Vulnerability

CVE-2026-7747 Overview

CVE-2026-7747 is a buffer overflow vulnerability in the Totolink N300RH router running firmware version 3.2.4-B20220812. The flaw resides in the loginauth function within the /cgi-bin/cstecgi.cgi binary, part of the device's Parameter Handler component. Attackers can trigger the overflow by manipulating the Password argument during authentication. The vulnerability is exploitable remotely without authentication or user interaction. A public exploit has been released, increasing the risk of opportunistic attacks against exposed devices.

Critical Impact

Unauthenticated remote attackers can corrupt memory on the device, potentially leading to arbitrary code execution, full device compromise, or denial of service.

Affected Products

• Totolink N300RH router
• Firmware version 3.2.4-B20220812
• /cgi-bin/cstecgi.cgi Parameter Handler component

Discovery Timeline

• 2026-05-04 - CVE-2026-7747 published to NVD
• 2026-05-04 - Last updated in NVD database

Technical Details for CVE-2026-7747

Vulnerability Analysis

The vulnerability is classified as a buffer overflow [CWE-119] affecting the loginauth handler in /cgi-bin/cstecgi.cgi. This CGI binary processes HTTP requests submitted to the router's web management interface. When a client submits login data, the loginauth function reads the Password parameter from the incoming request and copies it into a fixed-size stack or heap buffer without validating length. Supplying an oversized Password value overruns the buffer boundary and corrupts adjacent memory.

Because the affected endpoint is reachable without prior authentication, any attacker who can connect to the router's HTTP service can send the malicious request. Successful exploitation can overwrite saved return addresses, function pointers, or control structures, enabling arbitrary code execution as the CGI process. At minimum, the corruption causes the service to crash, producing denial of service on the device's management plane.

Root Cause

The root cause is missing length validation on the Password argument before it is copied into a fixed-size buffer. The loginauth function in /cgi-bin/cstecgi.cgi trusts attacker-controlled input from the HTTP request body. This pattern matches the broader CWE-119 category of improper restriction of operations within memory buffer bounds. Embedded vendor binaries on consumer routers commonly rely on unsafe C string operations such as strcpy or sprintf, which do not enforce destination size.

Attack Vector

The attack vector is network-based. An attacker sends a crafted HTTP POST request to /cgi-bin/cstecgi.cgi with a Password parameter exceeding the expected size. No credentials or user interaction are required. Public exploit details are referenced in the Notion writeup on the TOTOLINK loginauth Password issue and tracked in VulDB Vulnerability #360922. No verified exploit code is reproduced here.

Detection Methods for CVE-2026-7747

Indicators of Compromise

• Unexpected crashes or restarts of the cstecgi.cgi process or the router's HTTP daemon
• HTTP requests to /cgi-bin/cstecgi.cgi containing abnormally long Password field values
• Inbound traffic to the router's web management port from untrusted external IP addresses
• New or unexplained outbound connections from the router following login attempts

Detection Strategies

• Inspect HTTP request bodies destined for /cgi-bin/cstecgi.cgi and flag Password parameters exceeding reasonable lengths such as 128 bytes
• Deploy network intrusion detection signatures targeting oversized POST payloads to the loginauth endpoint
• Correlate router crash logs with preceding HTTP traffic to identify exploitation attempts

Monitoring Recommendations

• Forward router syslog data to a centralized logging or SIEM platform for anomaly review
• Alert on management interface access from outside the trusted administrative network
• Track repeated failed authentication attempts followed by service unavailability on Totolink devices

How to Mitigate CVE-2026-7747

Immediate Actions Required

• Disable WAN-side access to the router's web management interface and restrict it to trusted internal hosts
• Place affected Totolink N300RH devices behind a network segmentation boundary that blocks untrusted HTTP traffic
• Audit device inventories for firmware version 3.2.4-B20220812 and prioritize remediation

Patch Information

No vendor patch has been published in the referenced advisories at the time of NVD publication. Monitor the TOTOLINK official website for firmware updates addressing the loginauth buffer overflow. Until a fixed firmware release is available, compensating network controls are required.

Workarounds

• Block external access to TCP ports used by the router's HTTP management service at the upstream firewall
• Replace end-of-life or unpatched Totolink N300RH units with supported devices that receive active security updates
• Apply ACLs to restrict /cgi-bin/cstecgi.cgi access to a fixed administrative IP range

# Example: restrict router management access at an upstream firewall (iptables)
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -s <admin_subnet> -j ACCEPT
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 -j DROP
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -s <admin_subnet> -j ACCEPT
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -j DROP