Tenda A21 stack overflow (CVE-2026-2870)

Vendor-confirmed - CVE-2026-2870 is a high criticality stack-based buffer overflow in the Tenda A21 Wi-Fi router firmware 1.0.0.0 that grants remote unauthenticated code execution and full device compromise via the set_qosMib_list function. A public exploit is available, and no patch currently exists, so immediately isolate the device or replace it.

Overview

A critical security vulnerability has been identified in the Tenda A21 Wi-Fi router, firmware version 1.0.0.0. This flaw allows a remote attacker to execute arbitrary code on the device by sending a specially crafted network request, potentially leading to a complete compromise.

Vulnerability Details

The vulnerability exists in a specific function (set_qosMib_list) within the router’s web management interface. This function handles Quality of Service (QoS) bandwidth settings. Due to insufficient input validation, an attacker can send an overly long argument (named “list”) that overflows a fixed-size memory buffer on the device’s stack.

This type of flaw, known as a stack-based buffer overflow, can corrupt the router’s memory and allow an attacker to hijack its normal operation. Crucially, this attack can be performed remotely over the internet or a local network without requiring any authentication from the attacker. A functional exploit for this vulnerability is publicly available, increasing the immediate risk.

Potential Impact

If successfully exploited, this vulnerability can have severe consequences:

• Full Device Compromise: An attacker could gain complete control over the router, allowing them to intercept, modify, or redirect all network traffic passing through it (e.g., login credentials, financial data, personal communications).
• Network Instability: The attack can cause the router to crash, leading to a denial of service for all connected devices.
• Persistence: An attacker could install malicious firmware or backdoors, making the compromise difficult to remove and providing long-term access to the network.
• Launchpad for Further Attacks: The compromised router could be used to attack other devices on the internal network that were previously protected.

Remediation and Mitigation

Immediate action is required to protect affected networks.

Primary Solution - Update Firmware:

1. Check the official Tenda support website for an updated firmware release that addresses CVE-2026-2870.
2. If an update is available, install it immediately following the vendor’s instructions. As of this advisory, no patched firmware is known; users must monitor for its release.

Immediate Mitigations (If No Patch Exists):

• Isolate the Device: If possible, disconnect the Tenda A21 router from the public internet. Place it behind another, patched firewall/router if it must remain in use.
• Segment the Network: Use VLANs or a separate physical network to isolate critical devices (servers, workstations with sensitive data) from the vulnerable router and any devices connected to it.
• Replace the Device: Given the high severity and public exploit availability, consider replacing the router with a model from a vendor that provides active security support and timely patches.

General Best Practice:

• Never expose your router’s administrative web interface (typically on port 80 or 443) to the public internet. Ensure it is only accessible from your local network.