CVE-2024-2809: Tenda AC15 Buffer Overflow Vulnerability

CVE-2024-2809 Overview

A critical stack-based buffer overflow vulnerability has been identified in the Tenda AC15 router firmware. The vulnerability exists in the formSetFirewallCfg function within the /goform/SetFirewallCfg endpoint. Improper handling of the firewallEn argument allows remote attackers to overflow the stack buffer, potentially leading to remote code execution or denial of service. The exploit has been publicly disclosed, and the vendor did not respond to early disclosure attempts.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow without authentication to potentially execute arbitrary code or crash the device, compromising network security at the router level.

Affected Products

• Tenda AC15 Firmware version 15.03.05.18
• Tenda AC15 Firmware version 15.03.20_multi
• Tenda AC15 Hardware version 1.0

Discovery Timeline

• 2024-03-22 - CVE-2024-2809 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-2809

Vulnerability Analysis

This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow), a memory corruption flaw that occurs when user-controlled input exceeds the bounds of a stack-allocated buffer. In the case of CVE-2024-2809, the formSetFirewallCfg function in the Tenda AC15 firmware fails to properly validate the length of the firewallEn argument before copying it to a fixed-size buffer on the stack.

The network-accessible attack vector means that any attacker with network access to the router's web interface can potentially exploit this vulnerability. No authentication is required, and no user interaction is needed, making this a highly exploitable flaw. Successful exploitation can result in complete compromise of confidentiality, integrity, and availability of the affected device.

Root Cause

The root cause of this vulnerability lies in inadequate input validation within the formSetFirewallCfg function. When processing HTTP POST requests to /goform/SetFirewallCfg, the firmware copies the firewallEn parameter value directly into a stack buffer without verifying that the input length does not exceed the buffer's allocated size. This classic buffer overflow pattern allows attackers to overwrite adjacent stack memory, including return addresses, enabling control flow hijacking.

Attack Vector

The attack can be conducted remotely over the network by sending a specially crafted HTTP POST request to the vulnerable endpoint /goform/SetFirewallCfg. An attacker would submit an oversized value for the firewallEn parameter, overflowing the stack buffer. By carefully crafting the overflow payload, an attacker can overwrite the function's return address and redirect execution to malicious shellcode, potentially gaining full control over the router.

The vulnerability is particularly dangerous on IoT devices like routers because:

• They often run with elevated privileges
• They serve as network gateway devices with access to all traffic
• Firmware updates may be infrequent or unavailable
• Exploitation can enable persistent backdoor access to the network

For detailed technical analysis, see the GitHub vulnerability documentation and the VulDB entry #257664.

Detection Methods for CVE-2024-2809

Indicators of Compromise

• Unexpected device reboots or crashes of the Tenda AC15 router
• Unusual network traffic patterns originating from or directed to the router's management interface
• HTTP POST requests to /goform/SetFirewallCfg with abnormally large firewallEn parameter values
• Signs of unauthorized configuration changes or firmware modifications

Detection Strategies

• Implement network monitoring to detect HTTP POST requests targeting /goform/SetFirewallCfg with oversized payloads
• Deploy intrusion detection/prevention systems (IDS/IPS) with rules to identify buffer overflow exploitation attempts against Tenda devices
• Monitor for anomalous behavior from router devices including unexpected outbound connections or process spawning
• Analyze network traffic logs for exploitation patterns matching known Tenda AC15 attack signatures

Monitoring Recommendations

• Enable logging on network perimeter devices to capture traffic to and from Tenda router management interfaces
• Implement network segmentation to isolate IoT and router management interfaces from general network traffic
• Configure alerting for any unexpected access attempts to router configuration endpoints
• Regularly audit router configurations and firmware versions to ensure compliance with security baselines

How to Mitigate CVE-2024-2809

Immediate Actions Required

• Restrict access to the router's web management interface to trusted IP addresses only
• Place the router management interface on a separate, isolated network segment
• Implement firewall rules to block external access to /goform/SetFirewallCfg and other sensitive endpoints
• Consider replacing affected Tenda AC15 devices with alternative hardware from vendors with better security response practices
• Monitor network traffic for exploitation attempts while awaiting a vendor patch

Patch Information

No official patch has been released by Tenda. The vendor was contacted about this disclosure but did not respond. Users should consider replacing affected devices or implementing strict network access controls to mitigate risk. Monitor the VulDB entry for updates on vendor response or patch availability.

Workarounds

• Disable remote management access to the router's web interface if not required
• Implement access control lists (ACLs) to limit management interface access to specific trusted hosts
• Deploy a web application firewall (WAF) or reverse proxy in front of the management interface to filter malicious requests
• Use VPN-only access for router administration to prevent direct exposure of the management interface

# Example: Restrict management interface access via iptables on upstream device
# Block external access to the Tenda AC15 management interface (adjust IP as needed)
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 management access only from trusted admin workstation
iptables -I FORWARD -s 192.168.1.100 -d 192.168.1.1 -p tcp --dport 80 -j ACCEPT