CVE-2026-4903: Tenda AC5 Buffer Overflow Vulnerability

CVE-2026-4903 Overview

A stack-based buffer overflow vulnerability has been discovered in Tenda AC5 router firmware version 15.03.06.47. This vulnerability affects the formQuickIndex function within the /goform/QuickIndex endpoint of the POST Request Handler component. Remote attackers can exploit this flaw by manipulating the PPPOEPassword argument to trigger a buffer overflow condition, potentially leading to arbitrary code execution or denial of service on the affected device.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow vulnerability to compromise Tenda AC5 routers, potentially gaining full control of the device or causing service disruption. The network-based attack vector and published exploit increase the urgency for remediation.

Affected Products

• Tenda AC5 Router Firmware version 15.03.06.47
• Devices running the vulnerable /goform/QuickIndex POST Request Handler

Discovery Timeline

• 2026-03-26 - CVE-2026-4903 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-4903

Vulnerability Analysis

This vulnerability is classified as CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw exists in the formQuickIndex function which processes POST requests to the /goform/QuickIndex endpoint. When handling the PPPOEPassword parameter, the function fails to properly validate the length of user-supplied input before copying it to a fixed-size stack buffer.

The network-accessible nature of this vulnerability means an authenticated attacker with low privileges can exploit it remotely without user interaction. Successful exploitation can result in high impact to confidentiality, integrity, and availability of the affected device. The exploit has been published publicly, increasing the likelihood of exploitation attempts in the wild.

Root Cause

The root cause stems from insufficient bounds checking in the formQuickIndex function when processing the PPPOEPassword parameter. The function allocates a fixed-size buffer on the stack to store the password value but does not verify that incoming data fits within this allocated space. When an attacker supplies an oversized value, the excess data overwrites adjacent stack memory, corrupting return addresses and potentially enabling control flow hijacking.

Attack Vector

The attack can be initiated remotely over the network by sending a specially crafted POST request to the /goform/QuickIndex endpoint. The attacker must have low-level privileges to access the endpoint. By including an excessively long PPPOEPassword parameter value, the attacker triggers the stack-based buffer overflow condition.

The vulnerability manifests in the boundary validation of the PPPOEPassword argument within the formQuickIndex function. An attacker sends a malicious POST request to /goform/QuickIndex containing an oversized PPPOEPassword value that exceeds the expected buffer size, causing stack memory corruption. For detailed technical analysis, refer to the Notion Tutorial on Tenda AC5 and VulDB entry #353654.

Detection Methods for CVE-2026-4903

Indicators of Compromise

• Unusual POST requests to /goform/QuickIndex containing excessively long PPPOEPassword values
• Router crashes or unexpected reboots following web interface access
• Anomalous network traffic patterns targeting the router's web management interface
• Evidence of unauthorized configuration changes on Tenda AC5 devices

Detection Strategies

• Monitor web server logs for POST requests to /goform/QuickIndex with abnormally large payload sizes
• Implement intrusion detection rules to flag requests containing oversized PPPOEPassword parameters
• Deploy network-based anomaly detection to identify buffer overflow exploitation attempts against IoT devices
• Review router access logs for repeated failed requests followed by service interruptions

Monitoring Recommendations

• Enable detailed logging on Tenda AC5 devices if supported by firmware
• Configure network monitoring tools to alert on traffic anomalies targeting router management interfaces
• Implement SIEM rules to correlate potential exploitation attempts across multiple IoT devices
• Regularly audit device behavior and compare against baseline performance metrics

How to Mitigate CVE-2026-4903

Immediate Actions Required

• Restrict access to the router's web management interface to trusted networks only
• Implement firewall rules to block external access to /goform/QuickIndex and related endpoints
• Consider isolating affected Tenda AC5 devices on a separate network segment
• Monitor for firmware updates from Tenda that address this vulnerability

Patch Information

No official patch information is available at this time. Organizations should monitor the Tenda Official Website for security updates addressing this vulnerability. The VulDB submission #777380 contains additional tracking information for this vulnerability.

Workarounds

• Disable remote web management access on affected Tenda AC5 devices
• Use access control lists (ACLs) to restrict management interface access to specific trusted IP addresses
• Implement a VPN requirement for any remote administration needs
• Consider replacing vulnerable devices with models that receive regular security updates

# Network isolation configuration example (on upstream firewall)
# Block external access to Tenda router management ports
iptables -A FORWARD -d <TENDA_ROUTER_IP> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <TENDA_ROUTER_IP> -p tcp --dport 443 -j DROP

# Allow management access only from trusted admin subnet
iptables -A FORWARD -s <TRUSTED_ADMIN_SUBNET> -d <TENDA_ROUTER_IP> -p tcp --dport 80 -j ACCEPT