CVE-2026-7749 Overview
CVE-2026-7749 is a buffer overflow vulnerability in the Totolink N300RH router running firmware version 3.2.4-B20220812. The flaw resides in the setWanConfig function within /cgi-bin/cstecgi.cgi, which is exposed through the device's POST request handler. Attackers manipulate the priDns argument to overflow a fixed-size buffer in memory. The vulnerability is reachable over the network and the exploit has been disclosed publicly. Successful exploitation can corrupt memory and may lead to arbitrary code execution on the affected device. The issue is tracked under CWE-119, covering improper restriction of operations within memory buffer bounds.
Critical Impact
Remote attackers with low privileges can trigger a buffer overflow in the Totolink N300RH web management interface, threatening confidentiality, integrity, and availability of the router.
Affected Products
- Totolink N300RH router
- Firmware version 3.2.4-B20220812
- Component: /cgi-bin/cstecgi.cgi POST request handler (setWanConfig function)
Discovery Timeline
- 2026-05-04 - CVE-2026-7749 published to NVD
- 2026-05-04 - Last updated in NVD database
Technical Details for CVE-2026-7749
Vulnerability Analysis
The vulnerability exists in the setWanConfig handler implemented inside the cstecgi.cgi binary on the Totolink N300RH. This handler processes WAN configuration parameters submitted through HTTP POST requests to the router's web interface. One of the parameters, priDns, is copied into a fixed-length stack or heap buffer without sufficient length validation. When an attacker supplies an oversized priDns value, the copy operation writes past the buffer boundary and corrupts adjacent memory.
Because cstecgi.cgi typically runs with elevated privileges on embedded routers, memory corruption in this process can directly compromise the device. The CWE-119 classification confirms the root cause as improper bounds checking on a memory buffer operation. Public disclosure of the exploit increases the risk of opportunistic scanning and weaponization against exposed devices.
Root Cause
The root cause is the absence of length validation on the priDns POST parameter before it is written into a fixed-size buffer inside setWanConfig. The routine relies on unsafe string handling without enforcing maximum input size. Standard secure coding practices for embedded HTTP handlers — bounded copies, explicit length checks, and rejection of oversized fields — are not applied to this code path.
Attack Vector
The attack vector is network-based. An authenticated attacker with low privileges sends a crafted POST request to /cgi-bin/cstecgi.cgi with an oversized priDns value targeting the setWanConfig action. The malformed request triggers the overflow during request processing. On routers exposing the management interface to untrusted networks, the attack surface extends beyond the local LAN. Refer to the VulDB entry for vulnerability #360924 and the setWanConfig technical writeup for detailed reproduction information.
No verified exploit code is reproduced here. See the linked references for technical details.
Detection Methods for CVE-2026-7749
Indicators of Compromise
- POST requests to /cgi-bin/cstecgi.cgi containing an unusually long priDns parameter value
- Repeated setWanConfig requests from a single source within a short time window
- Unexpected reboots, crashes, or service interruptions on the router's management daemon following inbound HTTP traffic
- Outbound connections from the router to unknown hosts after WAN reconfiguration events
Detection Strategies
- Inspect HTTP request bodies destined for cstecgi.cgi and flag priDns values exceeding expected DNS string length
- Apply network IDS signatures targeting oversized parameters in TOTOLINK CGI endpoints
- Correlate router log entries for setWanConfig invocations with crash or watchdog reset events
Monitoring Recommendations
- Enable centralized logging of router management plane activity and forward syslog to a monitoring platform
- Alert on any external-source HTTP traffic reaching the router's administrative interface
- Monitor DNS configuration changes on edge devices and verify them against an approved baseline
How to Mitigate CVE-2026-7749
Immediate Actions Required
- Restrict access to the Totolink N300RH web management interface to trusted management VLANs only
- Disable WAN-side administration if it is currently enabled on the device
- Rotate administrator credentials, since exploitation requires low-privilege authentication
- Audit firmware versions across the fleet and identify all devices running 3.2.4-B20220812
Patch Information
No vendor patch is referenced in the published advisory data at the time of writing. Monitor the TOTOLINK official website for firmware updates addressing the setWanConfig buffer overflow. Until a fix is released, treat the device as vulnerable and apply compensating controls.
Workarounds
- Place the router behind a network firewall that blocks untrusted access to TCP ports hosting the web interface
- Segment management traffic from user networks using VLANs and ACLs
- Replace end-of-support hardware with a current device if no patched firmware becomes available
# Configuration example: restrict management interface to a single admin host
iptables -A INPUT -p tcp --dport 80 -s 192.0.2.10 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 192.0.2.10 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
