CVE-2026-75976: TRENDnet TEW-823DRU Buffer Overflow Flaw

CVE-2026-75976 Overview

CVE-2026-75976 is a stack-based buffer overflow in the TRENDnet TEW-823DRU wireless router running firmware version 1.1.02b01. The flaw resides in the strcpy call within /cgi-bin/wan.cgi, part of the NVRAM handling component. Attackers can trigger the overflow by manipulating the wan_l2tp_password argument. The vulnerability is exploitable over the network and a public proof-of-concept has been released. Successful exploitation can corrupt the stack, hijack control flow, and lead to remote code execution on the affected device. The weakness maps to [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer).

Critical Impact
Remote attackers with low privileges can trigger a stack-based buffer overflow in wan.cgi, potentially executing arbitrary code on affected TRENDnet TEW-823DRU routers.

Affected Products

TRENDnet TEW-823DRU firmware 1.1.02b01
/cgi-bin/wan.cgi handler within the router web management interface
NVRAM configuration component processing wan_l2tp_password

Discovery Timeline

2026-08-19 - CVE-2026-75976 published to NVD
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-75976

Vulnerability Analysis

The TEW-823DRU exposes a CGI endpoint at /cgi-bin/wan.cgi that parses WAN configuration parameters submitted through the web interface. When the handler processes the wan_l2tp_password field, it copies the attacker-controlled string into a fixed-size stack buffer using strcpy. Because strcpy performs no length validation, an oversized value overwrites adjacent stack data, including the saved return address. An authenticated attacker on the network can submit a crafted request to overflow the buffer and redirect execution to injected shellcode or a ROP chain. The public proof-of-concept demonstrates the overflow using a Python script that sends a long password value to the vulnerable endpoint. See the GitHub PoC Repository for details.

Root Cause

The root cause is unsafe use of strcpy in the wan.cgi handler that writes user-controlled NVRAM parameters to a bounded stack buffer without length checks. The firmware also lacks server-side validation of the wan_l2tp_password length before invoking the copy. This is a classic [CWE-119] memory safety failure common in embedded router firmware written in C.

Attack Vector

Attackers reach the vulnerable endpoint over the network with low-privileged credentials to the device management interface. Where the interface is exposed to the internet or shared across untrusted network segments, the attack surface expands significantly. A single crafted HTTP request to /cgi-bin/wan.cgi with an oversized wan_l2tp_password value is sufficient to trigger the overflow. Reference material for the exploitation flow is available at the VulDB CVE Details page.

The vulnerability manifests when the wan.cgi handler passes a user-supplied string directly to strcpy targeting a fixed-size stack buffer. See the GitHub Code Repository for the published proof-of-concept.

Detection Methods for CVE-2026-75976

Indicators of Compromise

HTTP POST requests to /cgi-bin/wan.cgi containing abnormally long wan_l2tp_password values.
Unexpected reboots, watchdog resets, or crashes of the router web management daemon following inbound WAN configuration requests.
Outbound connections from the router to unfamiliar hosts, indicating post-exploitation implant activity.

Detection Strategies

Inspect web server and NVRAM logs on the device for malformed WAN configuration submissions and truncated parameter values.
Deploy network intrusion detection signatures that flag oversized form fields targeting /cgi-bin/wan.cgi.
Correlate authentication events with configuration change requests to identify low-privileged accounts attempting WAN modifications.

Monitoring Recommendations

Monitor management-plane traffic to router administrative interfaces from untrusted networks.
Alert on router firmware crashes and unscheduled reboots reported through syslog or SNMP traps.
Track outbound DNS and TCP connections originating from router interfaces for signs of compromise.

How to Mitigate CVE-2026-75976

Immediate Actions Required

Restrict access to the router's web management interface to trusted management VLANs and remove any exposure to the public internet.
Rotate administrative credentials and disable unused management accounts on the TEW-823DRU.
Audit routers running firmware 1.1.02b01 and prioritize replacement or firmware updates as they become available.

Patch Information

No vendor patch is referenced in the enriched CVE data at the time of publication. Administrators should consult the VulDB Vulnerability Overview and TRENDnet support channels for firmware update availability. Where a fixed firmware version is not yet released, apply compensating controls and consider replacing end-of-life hardware.

Workarounds

Disable remote WAN administration and restrict /cgi-bin/wan.cgi access to a dedicated management subnet.
Place vulnerable routers behind an upstream firewall that filters unauthenticated or oversized HTTP requests to the device.
Segment guest and IoT networks from the router management interface to reduce lateral movement risk.

bash

# Example ACL restricting router management to a trusted subnet
iptables -A INPUT -p tcp --dport 80 -s 10.10.10.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 10.10.10.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP