CVE-2026-6137: Tenda F451 Buffer Overflow Vulnerability

CVE-2026-6137 Overview

CVE-2026-6137 is a stack-based buffer overflow vulnerability affecting the Tenda F451 router running firmware version 1.0.0.7_cn_svn7958. The flaw resides in the fromAdvSetWan function within the /goform/AdvSetWan endpoint. Attackers can trigger the overflow by manipulating the wanmode or PPPOEPassword parameters. The vulnerability is remotely exploitable across the network, and public exploit details are already available. Successful exploitation can corrupt the device stack, enabling arbitrary code execution or full denial of service on the affected router [CWE-119].

Critical Impact

Remote attackers with low-privileged network access can overflow the stack in the Tenda F451 web management interface, compromising confidentiality, integrity, and availability of the device.

Affected Products

• Tenda F451 router (hardware)
• Tenda F451 firmware version 1.0.0.7_cn_svn7958
• Deployments exposing the /goform/AdvSetWan administrative endpoint

Discovery Timeline

• 2026-04-13 - CVE-2026-6137 published to the National Vulnerability Database (NVD)
• 2026-04-30 - Last updated in NVD database

Technical Details for CVE-2026-6137

Vulnerability Analysis

The Tenda F451 web management daemon exposes the /goform/AdvSetWan handler, which invokes the fromAdvSetWan function to process WAN configuration changes. The handler reads attacker-supplied values for the wanmode and PPPOEPassword parameters and copies them into fixed-size stack buffers without enforcing a length boundary. When either argument exceeds the buffer size, adjacent stack memory including saved return addresses is overwritten. Because the vulnerable endpoint accepts requests over the network, an attacker with valid low-level credentials can deliver the payload remotely with a single crafted HTTP request. Public technical details have been published, increasing the risk of opportunistic exploitation against exposed devices.

Root Cause

The root cause is improper restriction of operations within the bounds of a memory buffer [CWE-119]. The fromAdvSetWan routine performs an unchecked string copy of user-controlled HTTP parameters into a stack-allocated buffer. The firmware does not validate parameter length, sanitize input, or apply modern compiler-level stack protections that would mitigate the overflow.

Attack Vector

Exploitation requires network reachability to the router's HTTP management interface and a valid low-privileged session. The attacker sends a POST request to /goform/AdvSetWan containing an oversized wanmode or PPPOEPassword value. The overflow corrupts saved registers and return addresses on the stack, allowing the attacker to redirect execution flow. On embedded MIPS or ARM SoHo routers such as the F451, the absence of address space layout randomization (ASLR) and non-executable stack enforcement makes reliable code execution feasible. The vulnerability mechanism is documented in the GitHub CVE Issue #22 and the VulDB #357001 advisory.

Detection Methods for CVE-2026-6137

Indicators of Compromise

• HTTP POST requests to /goform/AdvSetWan containing abnormally long wanmode or PPPOEPassword parameter values
• Unexpected reboots, crashes, or watchdog resets of the Tenda F451 device following administrative HTTP traffic
• Outbound connections from the router to unknown hosts, indicating possible post-exploitation implant activity
• Modifications to WAN configuration that do not correspond to authorized administrative actions

Detection Strategies

• Inspect HTTP traffic to the router management interface for parameter lengths exceeding reasonable thresholds for wanmode and PPPOEPassword
• Deploy network signatures matching POST requests to /goform/AdvSetWan with payload sizes inconsistent with legitimate configuration values
• Correlate authentication events on the router with subsequent administrative requests to identify suspicious low-privilege sessions

Monitoring Recommendations

• Forward router syslog and HTTP access logs to a centralized analytics platform for anomaly detection
• Alert on repeated requests to /goform/AdvSetWan from a single source within a short interval
• Track device availability metrics to identify crash-loop behavior caused by failed exploitation attempts

How to Mitigate CVE-2026-6137

Immediate Actions Required

• Restrict access to the Tenda F451 web management interface to trusted internal networks only and disable remote WAN-side administration
• Rotate administrative credentials and enforce strong, unique passwords to limit the pool of attackers who can reach the authenticated endpoint
• Place affected devices behind a firewall or VLAN segment that blocks untrusted HTTP traffic to the router
• Monitor the Tenda Security Resource for firmware updates addressing CVE-2026-6137

Patch Information

At the time of publication, no vendor-supplied patch for Tenda F451 firmware 1.0.0.7_cn_svn7958 has been referenced in the advisory. Administrators should consult the VulDB #357001 CTI entry and the Tenda Security Resource for updated firmware releases. Until an official fix is published, compensating network controls remain the primary defense.

Workarounds

• Disable the WAN-side HTTP and HTTPS management interface so the /goform/AdvSetWan endpoint is not reachable from untrusted networks
• Apply access control lists (ACLs) on upstream network equipment to permit management traffic only from designated administrative hosts
• Replace end-of-life or unpatched Tenda F451 devices with currently supported hardware when no firmware fix is available

# Example iptables rule restricting router management access to a trusted subnet
iptables -A INPUT -p tcp --dport 80 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP