CVE-2026-4961: Tenda AC6 Buffer Overflow Vulnerability

CVE-2026-4961 Overview

A critical stack-based buffer overflow vulnerability has been identified in Tenda AC6 router firmware version 15.03.05.16. This vulnerability affects the formQuickIndex function within the /goform/QuickIndex POST Request Handler component. An attacker can exploit this flaw by manipulating the PPPOEPassword argument, causing a stack-based buffer overflow that could lead to arbitrary code execution or denial of service.

The vulnerability is remotely exploitable, requiring only low-privilege authentication to trigger. Exploit information has been publicly disclosed, increasing the risk of active exploitation against vulnerable devices.

Critical Impact

Remote attackers with low privileges can exploit this stack-based buffer overflow to potentially execute arbitrary code or crash the device, compromising network security and availability.

Affected Products

• Tenda AC6 Firmware version 15.03.05.16
• Tenda AC6 Hardware version 1.0

Discovery Timeline

• 2026-03-27 - CVE-2026-4961 published to NVD
• 2026-03-31 - Last updated in NVD database

Technical Details for CVE-2026-4961

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) and CWE-787 (Out-of-bounds Write). The formQuickIndex function fails to properly validate the length of user-supplied input through the PPPOEPassword parameter before copying it to a fixed-size stack buffer.

When an attacker submits an oversized PPPOEPassword value via a POST request to /goform/QuickIndex, the function writes beyond the allocated stack buffer boundaries. This memory corruption can overwrite critical stack data including return addresses and saved registers, potentially allowing an attacker to hijack program execution flow.

The network-accessible nature of this endpoint combined with low authentication requirements makes this vulnerability particularly dangerous for internet-facing Tenda AC6 routers.

Root Cause

The root cause is improper input validation in the formQuickIndex function. The code does not implement adequate bounds checking when processing the PPPOEPassword argument from POST requests. Without length verification, arbitrarily long input strings are copied to a fixed-size stack buffer, causing a classic stack-based buffer overflow condition.

This represents a fundamental secure coding failure where user-controlled data is trusted without validation before being used in memory operations.

Attack Vector

The attack can be executed remotely over the network by sending a crafted HTTP POST request to the /goform/QuickIndex endpoint. The attacker must have low-level authentication to access this endpoint.

The exploitation process involves:

1. Authenticating to the router's web interface with minimal credentials
2. Crafting a POST request to /goform/QuickIndex with an oversized PPPOEPassword parameter
3. The malicious payload overwrites the stack buffer and adjacent memory
4. Depending on the payload, this can result in code execution or device crash

Technical details and proof-of-concept information are available through the Notion Quick Index Resource published by security researchers. Additional vulnerability intelligence can be found in the VulDB Entry #353838.

Detection Methods for CVE-2026-4961

Indicators of Compromise

• Unusual HTTP POST requests to /goform/QuickIndex with excessively long PPPOEPassword parameters
• Router crashes, reboots, or unresponsive behavior following web management access
• Unexpected modifications to router configuration or firmware
• Anomalous outbound network traffic originating from the router device

Detection Strategies

• Monitor network traffic for POST requests to /goform/QuickIndex containing abnormally large payloads (greater than 256 bytes in the PPPOEPassword field)
• Implement web application firewall (WAF) rules to detect and block buffer overflow patterns targeting Tenda device endpoints
• Deploy intrusion detection signatures to identify exploitation attempts against Tenda AC6 routers
• Enable logging on network perimeter devices to capture suspicious requests to router management interfaces

Monitoring Recommendations

• Configure SIEM alerts for repeated authentication attempts followed by large POST requests to Tenda router management endpoints
• Establish baseline behavior for router management traffic and alert on deviations
• Monitor for firmware integrity changes using file integrity monitoring where supported
• Review router logs regularly for signs of exploitation attempts or unauthorized configuration changes

How to Mitigate CVE-2026-4961

Immediate Actions Required

• Restrict access to the router's web management interface to trusted networks only
• Disable remote management features if not required for operations
• Place vulnerable Tenda AC6 devices behind a properly configured firewall
• Implement network segmentation to isolate IoT and network devices from critical infrastructure
• Monitor vendor channels for firmware updates addressing this vulnerability

Patch Information

As of the last update on 2026-03-31, no official patch has been released by Tenda. Organizations should monitor the Tenda Official Website for security advisories and firmware updates. Additional vulnerability tracking information is available through VulDB CTI Report #353838.

SentinelOne Singularity platform provides network-level visibility that can help detect exploitation attempts against vulnerable IoT devices like the Tenda AC6 router.

Workarounds

• Disable the web management interface entirely if not required for daily operations
• Implement IP-based access control lists (ACLs) to limit management interface access to specific administrator IP addresses
• Deploy a reverse proxy with input validation in front of the router management interface
• Consider replacing vulnerable devices with alternatives that have better security track records
• Use VPN connections for all remote router administration rather than exposing the management interface directly

# Example: Restrict management access via iptables on upstream device
# Block external access to router management ports
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 443 -j DROP
# Allow only from trusted admin workstation
iptables -I FORWARD -s 192.168.1.100 -d 192.168.1.1 -p tcp --dport 80 -j ACCEPT