Tenda A21 Buffer Overflow RCE (CVE-2026-2873)

Vendor-confirmed - CVE-2026-2873 is a high unauthenticated remote code execution vulnerability in the Tenda A21 Wi-Fi router firmware version 1.0.0.0 that grants attackers complete device control, DNS hijacking, and traffic interception via a stack overflow in the setSchedWifi function. No firmware patch is available, making immediate replacement or network isolation critical.

Overview

A critical security flaw has been identified in the Tenda A21 Wi-Fi router, firmware version 1.0.0.0. This vulnerability allows a remote attacker to send specially crafted data to the device’s web management interface, triggering a stack-based buffer overflow.

Vulnerability Explanation

In simple terms, the router has a specific function for setting up scheduled Wi-Fi access (setSchedWifi). The code that processes the start and end times for this schedule (schedStartTime/schedEndTime) does not properly check the size of the data it receives. By sending an overly long string of characters instead of a normal time, an attacker can overflow a fixed-size memory buffer (the “stack”). This corruption can crash the device or, more critically, allow the attacker to run their own malicious code on the router.

Impact

The primary risk is that an unauthenticated attacker on the same network could exploit this flaw to take full control of the router. Consequences include:

• Complete Device Compromise: An attacker could change DNS settings, redirecting your internet traffic to malicious websites to steal passwords or banking details.
• Network Eavesdropping: They could intercept and monitor all data passing through the router.
• Persistence: Malicious code could survive a router reboot, making it difficult to remove.
• Denial of Service: The attack could simply crash the router, causing a network outage. This is a high-severity issue with a public exploit, making attacks more likely.

Remediation and Mitigation

Immediate action is required to protect affected networks.

1. Primary Solution: Firmware Update

• Check the official Tenda website or your router’s admin interface for a firmware update that addresses CVE-2026-2873.
• If an update is available, install it immediately. If Tenda has not released a patch, consider the mitigations below and evaluate replacing the device.

2. Immediate Mitigations (If No Patch Exists)

• Disable Remote Administration: Ensure the router’s web management interface is only accessible from your local network, not from the internet. This setting is usually found under “Administration” or “System” in the router’s settings.
• Segment Your Network: Place the Tenda A21 on an isolated network segment if possible, separate from critical devices like computers and servers.
• Monitor for Replacement: As the exploit is public, if the vendor does not provide a timely security update, plan to replace the router with a model from a vendor with a strong track record of security updates.

General Advice: Always change the router’s default admin password and disable features you do not use to reduce the overall attack surface.