CVE-2026-9429: Tenda F1202 Buffer Overflow Vulnerability

CVE-2026-9429 Overview

CVE-2026-9429 is a stack-based buffer overflow vulnerability in the Tenda F1202 router running firmware version 1.2.0.20(408). The flaw resides in the formWrlExtraSet function handling requests to the /goform/WrlExtraSet endpoint. Attackers can trigger the overflow by manipulating the delno parameter, which is processed without proper bounds checking. The vulnerability is reachable over the network and a public exploit has been disclosed, increasing the likelihood of opportunistic abuse against exposed devices. The weakness is classified under CWE-119, improper restriction of operations within the bounds of a memory buffer.

Critical Impact

Remote attackers with low privileges can corrupt stack memory on Tenda F1202 routers, leading to denial of service or potential arbitrary code execution on the device.

Affected Products

• Tenda F1202 router, firmware 1.2.0.20(408)
• Web management interface endpoint /goform/WrlExtraSet
• The formWrlExtraSet handler function within the device firmware

Discovery Timeline

• 2026-05-25 - CVE-2026-9429 published to NVD
• 2026-05-26 - Last updated in NVD database

Technical Details for CVE-2026-9429

Vulnerability Analysis

The vulnerability exists in the formWrlExtraSet function exposed through the router's web administration interface at /goform/WrlExtraSet. The handler accepts the delno parameter from HTTP requests and copies its contents into a fixed-size stack buffer without validating the input length. When an attacker supplies a delno value exceeding the buffer capacity, adjacent stack memory is overwritten, including saved return addresses and frame pointers. Successful exploitation typically results in a process crash or hijacked control flow on the embedded MIPS or ARM platform used by Tenda devices. Because the exploit is publicly available, automated scanners and botnets targeting consumer-grade routers can readily incorporate it.

Root Cause

The root cause is the absence of length validation on the delno argument before it is copied into a stack-allocated buffer. The function relies on unsafe string handling routines that do not enforce destination buffer boundaries. This pattern is consistent with [CWE-119] memory corruption flaws frequently observed across Tenda's goform request handlers.

Attack Vector

Exploitation requires network reachability to the router's HTTP management interface and low-level credentials to submit the request. An attacker crafts a POST request to /goform/WrlExtraSet containing an oversized delno value. Once parsed by formWrlExtraSet, the malformed input overwrites stack memory and can redirect execution flow. The attack does not require user interaction, and devices with the management interface exposed to untrusted networks are at greatest risk.

No verified proof-of-concept code is reproduced here. Technical details and reproduction steps are documented in the GitHub Vulnerability Report and the corresponding VulDB #365410 entry.

Detection Methods for CVE-2026-9429

Indicators of Compromise

• HTTP POST requests to /goform/WrlExtraSet containing abnormally long delno parameter values
• Unexpected reboots, watchdog resets, or service crashes on Tenda F1202 routers
• Outbound connections from the router to unfamiliar hosts following inbound requests to the management interface

Detection Strategies

• Inspect web server and router syslog data for repeated requests to /goform/WrlExtraSet originating from external or untrusted segments
• Deploy network intrusion detection signatures matching oversized delno parameters in HTTP request bodies
• Correlate router crash events with preceding HTTP traffic to identify exploitation attempts

Monitoring Recommendations

• Forward router and edge firewall logs to a centralized analytics platform for long-term retention and pattern analysis
• Alert on any administrative interface exposure from the WAN side of the network
• Track firmware versions across deployed Tenda devices to identify unpatched assets

How to Mitigate CVE-2026-9429

Immediate Actions Required

• Restrict access to the router management interface to trusted internal hosts only and disable WAN-side administration
• Place affected Tenda F1202 devices behind a segmented network boundary with strict ingress filtering
• Change default and weak administrative credentials to limit attackers who require low-privilege access

Patch Information

No vendor patch has been referenced in the public disclosure at this time. Administrators should monitor the Tenda Official Website for firmware updates addressing CVE-2026-9429. Until a fixed firmware release is available, compensating controls are required.

Workarounds

• Disable remote management on the router and permit administration only from a wired LAN interface
• Apply ACLs on upstream firewalls to block inbound HTTP and HTTPS traffic to the router management port
• Retire end-of-life Tenda F1202 units and replace them with actively supported hardware where feasible

# Example iptables rule to block external access to the router management interface
iptables -A INPUT -p tcp --dport 80 -i wan0 -j DROP
iptables -A INPUT -p tcp --dport 443 -i wan0 -j DROP