CVE-2026-6015: Tenda AC9 Buffer Overflow Vulnerability

CVE-2026-6015 Overview

A stack-based buffer overflow vulnerability has been identified in Tenda AC9 router firmware version 15.03.02.13. The vulnerability exists in the formQuickIndex function within the /goform/QuickIndex endpoint of the POST Request Handler component. Attackers can exploit this flaw by manipulating the PPPOEPassword argument, potentially leading to remote code execution or denial of service conditions on affected devices.

Critical Impact

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

Affected Products

• Tenda AC9 Firmware version 15.03.02.13
• Tenda AC9 routers with vulnerable /goform/QuickIndex endpoint
• Network environments utilizing affected Tenda AC9 devices

Discovery Timeline

• 2026-04-10 - CVE-2026-6015 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-6015

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The formQuickIndex function in the Tenda AC9 router firmware fails to properly validate the length of input provided through the PPPOEPassword parameter before copying it into a fixed-size stack buffer.

When a POST request is sent to the /goform/QuickIndex endpoint with an oversized PPPOEPassword value, the function copies user-supplied data beyond the allocated buffer boundaries. This overflow can overwrite adjacent stack memory, including saved return addresses and other critical control data.

The vulnerability is remotely exploitable over the network and requires only low-level privileges to trigger. No user interaction is necessary for exploitation, making this a significant threat to network infrastructure.

Root Cause

The root cause of this vulnerability is insufficient bounds checking in the formQuickIndex function. The function processes the PPPOEPassword argument from incoming POST requests without validating that the input length does not exceed the allocated buffer size. This lack of input validation allows attackers to supply arbitrarily long strings that overflow the stack buffer.

The underlying issue stems from unsafe memory handling practices commonly found in embedded device firmware, where functions like strcpy() or similar unbounded copy operations are used without proper length verification.

Attack Vector

The attack vector for CVE-2026-6015 is network-based. An attacker with network access to the Tenda AC9 router's web management interface can craft a malicious HTTP POST request targeting the /goform/QuickIndex endpoint.

The exploitation involves sending a POST request with an excessively long PPPOEPassword parameter value. The oversized input triggers the stack-based buffer overflow in the formQuickIndex function, potentially allowing the attacker to:

1. Overwrite the return address on the stack to redirect execution flow
2. Inject and execute arbitrary code on the device
3. Cause a denial of service by crashing the router

For additional technical details on this vulnerability, refer to the Tenda AC9 QuickIndex Guide and the VulDB entry.

Detection Methods for CVE-2026-6015

Indicators of Compromise

• Unusual or malformed POST requests to /goform/QuickIndex endpoint containing oversized PPPOEPassword values
• Unexpected router crashes or reboots indicating potential exploitation attempts
• Anomalous outbound connections from the router suggesting successful compromise
• Modified router configurations or unauthorized administrative changes

Detection Strategies

• Monitor HTTP traffic to the router's web interface for abnormally large POST request bodies targeting /goform/QuickIndex
• Implement intrusion detection rules to alert on requests with PPPOEPassword parameters exceeding normal length thresholds
• Deploy network-based anomaly detection to identify unusual traffic patterns to router management ports
• Review router access logs for repeated failed or suspicious authentication attempts

Monitoring Recommendations

• Enable logging on the Tenda AC9 router's web management interface if supported
• Configure network monitoring tools to track all traffic destined for router management ports
• Set up alerts for unexpected router reboots or availability issues that may indicate exploitation
• Regularly audit network traffic for signs of lateral movement from compromised network devices

How to Mitigate CVE-2026-6015

Immediate Actions Required

• Restrict network access to the router's web management interface to trusted IP addresses only
• Disable remote management capabilities if not required for operations
• Place the router's management interface behind a firewall or VPN
• Monitor for any firmware updates from Tenda addressing this vulnerability

Patch Information

At the time of publication, no official patch has been released by Tenda for this vulnerability. Organizations should monitor the Tenda Official Website for security updates and firmware releases addressing CVE-2026-6015.

Refer to the VulDB entry #356571 and CTI Analysis for ongoing updates regarding this vulnerability.

Workarounds

• Implement network segmentation to isolate vulnerable Tenda AC9 devices from untrusted networks
• Configure access control lists (ACLs) to restrict web management access to specific administrator IP addresses
• Consider replacing affected devices with alternatives that have active security support if no patch becomes available
• Deploy a web application firewall (WAF) or reverse proxy to filter malicious requests before they reach the router

# Example: Restrict management access using firewall rules
# Block external access to router management port (example using iptables)
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP

# Verify current management access settings
# Check your network firewall configuration to ensure
# only trusted subnets can reach the router's web interface