CVE-2026-4961: Tenda AC6 Buffer Overflow Vulnerability

CVE-2026-4961 Overview

CVE-2026-4961 is a stack-based buffer overflow in the Tenda AC6 wireless router running firmware version 15.03.05.16. The flaw resides in the formQuickIndex function handling POST requests to /goform/QuickIndex. An attacker who manipulates the PPPOEPassword parameter can overflow a fixed-size stack buffer, corrupting adjacent memory and the saved return address. The vulnerability is exploitable over the network and the exploit details have been publicly disclosed. Successful exploitation can lead to arbitrary code execution on the router or a denial-of-service condition affecting the device.

Critical Impact

Remote attackers with low-privileged access can corrupt the stack of the embedded HTTP server through a crafted PPPOEPassword value, potentially achieving code execution on the router.

Affected Products

• Tenda AC6 hardware revision 1.0
• Tenda AC6 firmware version 15.03.05.16
• POST request handler component /goform/QuickIndex

Discovery Timeline

• 2026-03-27 - CVE-2026-4961 published to the National Vulnerability Database
• 2026-03-31 - Last updated in NVD database

Technical Details for CVE-2026-4961

Vulnerability Analysis

The vulnerability is classified as a stack-based buffer overflow under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer) and [CWE-787] (Out-of-bounds Write). The defect resides in the formQuickIndex handler exposed at the /goform/QuickIndex endpoint of the embedded web administration interface. When a POST request supplies the PPPOEPassword field, the handler copies its contents into a fixed-size stack buffer without enforcing length validation. The Exploit Prediction Scoring System currently places exploitation probability at 0.092%, but a public exploit description has been released, raising the practical risk for exposed devices.

Root Cause

The root cause is the absence of bounds checking when the formQuickIndex function processes the PPPOEPassword argument from the POST body. The handler uses an unsafe string copy routine — typical in Tenda's httpd binary — that writes attacker-controlled bytes directly onto the stack. Oversized input overruns the local buffer, overwriting saved registers and the return address used by the MIPS or ARM SoC.

Attack Vector

The attack is remote and network-based, requiring only the ability to reach the router's HTTP management interface. An authenticated session with low privileges is sufficient to invoke the vulnerable endpoint. Attackers send a crafted POST request to /goform/QuickIndex containing an oversized PPPOEPassword value to trigger the overflow. Successful exploitation can hijack control flow of the embedded server process, which typically runs with root privileges on Tenda devices.

No verified proof-of-concept code is republished here. Technical details are documented in the public write-up linked from the Notion Security Index and the VulDB Incident Report.

Detection Methods for CVE-2026-4961

Indicators of Compromise

• POST requests to /goform/QuickIndex containing abnormally long PPPOEPassword values
• Unexpected reboots or crashes of the Tenda httpd process on AC6 devices
• New outbound connections from the router to unknown infrastructure after configuration changes
• HTTP requests targeting the management interface from external or untrusted IP ranges

Detection Strategies

• Inspect web server and router syslog entries for malformed POST bodies referencing the QuickIndex endpoint
• Apply network IDS signatures that flag oversized form parameters submitted to Tenda /goform/ URIs
• Baseline the management interface traffic and alert on POST payloads exceeding expected parameter sizes

Monitoring Recommendations

• Forward router logs to a central SIEM for retention and correlation with endpoint telemetry
• Monitor for repeated authentication followed by POSTs to /goform/QuickIndex from the same source
• Track firmware version inventory across deployed Tenda devices and alert on unpatched 15.03.05.16 instances

How to Mitigate CVE-2026-4961

Immediate Actions Required

• Restrict access to the router's web administration interface so it is reachable only from trusted internal management hosts
• Disable remote WAN-side management on all Tenda AC6 devices running firmware 15.03.05.16
• Rotate administrative credentials to reduce the value of any compromised low-privilege account
• Place vulnerable devices behind a network segment with strict egress filtering until a fix is applied

Patch Information

At the time of publication, no vendor advisory or fixed firmware build has been linked to this CVE in the NVD record. Administrators should monitor the Tenda Official Website for updated firmware releases addressing the formQuickIndex flaw and apply them as soon as they become available.

Workarounds

• Block inbound TCP traffic to the router management port from untrusted networks at the perimeter firewall
• Enforce strong, unique administrator passwords to prevent credential reuse that would enable the low-privileged precondition
• Replace end-of-support Tenda AC6 hardware with a vendor-supported router model if no patch is released

# Example perimeter rule blocking external access to the AC6 management UI
iptables -A FORWARD -p tcp -d <router_ip> --dport 80 ! -s <trusted_mgmt_subnet> -j DROP
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 ! -s <trusted_mgmt_subnet> -j DROP