CVE-2026-5830: Tenda AC15 Buffer Overflow Vulnerability

CVE-2026-5830 Overview

CVE-2026-5830 is a stack-based buffer overflow vulnerability in the Tenda AC15 router firmware version 15.03.05.18. The flaw exists in the websGetVar function processing the /goform/SysToolChangePwd endpoint. Attackers manipulate the oldPwd, newPwd, or cfmPwd parameters to overflow the stack buffer. The issue is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). Exploitation occurs remotely over the network, and a public exploit reference has been disclosed. Successful exploitation can compromise confidentiality, integrity, and availability of the affected device.

Critical Impact

Remote attackers with low-privilege access can overflow the stack via password change parameters, potentially achieving arbitrary code execution on the router.

Affected Products

• Tenda AC15 router hardware
• Tenda AC15 firmware version 15.03.05.18
• Deployments exposing the web management interface at /goform/SysToolChangePwd

Discovery Timeline

• 2026-04-09 - CVE-2026-5830 published to NVD
• 2026-04-30 - Last updated in NVD database

Technical Details for CVE-2026-5830

Vulnerability Analysis

The vulnerability resides in the password change handler of the Tenda AC15 web management interface. The /goform/SysToolChangePwd endpoint calls the websGetVar helper to retrieve user-supplied values for oldPwd, newPwd, and cfmPwd. These values are copied into fixed-size stack buffers without proper length validation. An attacker supplies oversized strings to overwrite adjacent stack memory, including saved return addresses and local variables.

The attack is reachable over the network and requires only low-privilege authentication to the management interface. A public technical writeup referenced through VulDB indicates the exploit is available, increasing the risk of opportunistic abuse against exposed devices.

Root Cause

The root cause is the absence of bounds checking when copying HTTP form parameters into stack-allocated buffers within the SysToolChangePwd handler. The websGetVar function returns attacker-controlled data, which is then handed to unsafe string-copy routines. Because the firmware lacks modern stack protections such as canaries or ASLR enforcement, overwriting the return address yields control of execution flow.

Attack Vector

An authenticated attacker sends a crafted HTTP POST request to /goform/SysToolChangePwd containing overlong values for any of the oldPwd, newPwd, or cfmPwd parameters. The malformed request corrupts the stack frame of the handler, leading to denial of service or, with carefully shaped payloads, code execution on the MIPS-based device. The vulnerability is exploitable across the network, including from any host that can reach the router's management interface.

No verified proof-of-concept code is reproduced here. Refer to the VulDB advisory for CVE-2026-5830 for technical details.

Detection Methods for CVE-2026-5830

Indicators of Compromise

• HTTP POST requests to /goform/SysToolChangePwd containing unusually long oldPwd, newPwd, or cfmPwd parameter values exceeding typical password lengths
• Unexpected reboots, watchdog resets, or httpd process crashes on Tenda AC15 devices following web management traffic
• Outbound connections from the router to unknown hosts after a suspicious management session
• Configuration changes or new administrative sessions originating from untrusted source IP addresses

Detection Strategies

• Inspect web server and reverse-proxy logs for requests to /goform/SysToolChangePwd with payload sizes inconsistent with normal password change activity
• Deploy network IDS signatures that flag POST bodies to the affected endpoint where parameter length exceeds a sane threshold such as 64 bytes
• Correlate web request anomalies with router availability metrics to identify exploitation attempts that cause service disruption

Monitoring Recommendations

• Forward router syslog and HTTP access logs to a central analytics platform for retention and search
• Alert on access to the management interface from networks outside expected administrative ranges
• Track firmware versions across the device fleet to identify hosts still running 15.03.05.18

How to Mitigate CVE-2026-5830

Immediate Actions Required

• Restrict access to the router web management interface to a dedicated management VLAN or trusted IP allowlist
• Disable remote WAN-side management on all Tenda AC15 devices until a fixed firmware is applied
• Rotate administrative credentials and review router configuration for unauthorized changes
• Inventory all Tenda AC15 devices running firmware 15.03.05.18 and prioritize them for replacement or update

Patch Information

At the time of publication, no vendor advisory or fixed firmware version is listed in the NVD record for CVE-2026-5830. Monitor the Tenda official website for firmware updates addressing the SysToolChangePwd handler. Where a patched firmware is unavailable, treat the device as end-of-support and consider replacement with a supported model.

Workarounds

• Place the router behind a network segment that blocks inbound HTTP/HTTPS access to the management interface from untrusted zones
• Enforce strong, unique credentials on the management account to raise the bar for the low-privilege precondition
• Disable Universal Plug and Play (UPnP) and remote administration features that may expose /goform/ endpoints
• Deploy a web application firewall or reverse proxy rule that drops requests to /goform/SysToolChangePwd containing parameter values longer than 64 bytes

# Example iptables rule limiting management access to a trusted subnet
iptables -A INPUT -p tcp --dport 80 -s 192.0.2.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 192.0.2.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP