CVE-2026-5611: Belkin F9K1015 Buffer Overflow Vulnerability

CVE-2026-5611 Overview

CVE-2026-5611 is a stack-based buffer overflow vulnerability in the Belkin F9K1015 wireless router running firmware version 1.00.10. The flaw resides in the formCrossBandSwitch function handling requests to /goform/formCrossBandSwitch. An attacker can manipulate the webpage argument to corrupt stack memory on the device. The issue is remotely exploitable and a public exploit description has been released. Belkin was contacted prior to disclosure but did not respond, leaving devices unpatched.

Critical Impact

Remote attackers with low-level access can trigger a stack-based buffer overflow in the router's web management interface, leading to memory corruption and potential remote code execution on affected Belkin F9K1015 devices.

Affected Products

• Belkin F9K1015 router (hardware)
• Belkin F9K1015 firmware version 1.00.10
• Devices exposing the /goform/formCrossBandSwitch endpoint

Discovery Timeline

• 2026-04-06 - CVE-2026-5611 published to NVD
• 2026-04-30 - Last updated in NVD database

Technical Details for CVE-2026-5611

Vulnerability Analysis

The vulnerability is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer). The Belkin F9K1015 web administration interface exposes a CGI-style handler at /goform/formCrossBandSwitch that processes HTTP requests for switching wireless band configuration. The formCrossBandSwitch function reads the webpage parameter from the request and copies it into a fixed-size stack buffer without validating the input length. Supplying a value longer than the destination buffer overwrites adjacent stack data, including saved registers and the return address. Successful exploitation can corrupt control flow on the embedded MIPS or ARM device, resulting in denial of service or arbitrary code execution within the web server process, which typically runs with root privileges on consumer routers.

Root Cause

The root cause is the absence of bounds checking on the webpage HTTP parameter prior to copying it into a stack-allocated buffer. The handler relies on unsafe string operations such as strcpy or sprintf instead of length-limited equivalents. This pattern is common in legacy SOHO router firmware where input validation is delegated to client-side scripts.

Attack Vector

The attack requires network access to the router's management interface and a low-privilege authenticated session. An attacker sends a crafted HTTP POST request to /goform/formCrossBandSwitch containing an oversized webpage argument. Because the vulnerability is reachable over the network and a public proof-of-concept exists, opportunistic exploitation against exposed devices is plausible. The current EPSS probability sits at 0.108%, reflecting limited observed exploitation activity.

No verified exploit code is republished here. Refer to the GitHub Vulnerability Report and VulDB #355402 Details for technical analysis.

Detection Methods for CVE-2026-5611

Indicators of Compromise

• HTTP POST requests targeting /goform/formCrossBandSwitch with abnormally long webpage parameter values.
• Unexpected reboots, web management interface crashes, or watchdog resets on the F9K1015 device.
• Outbound connections from the router to unknown hosts following management interface activity.

Detection Strategies

• Inspect network traffic to the router's HTTP administration port for requests containing oversized parameters or non-printable byte sequences in the webpage field.
• Correlate router log entries with web requests to identify malformed formCrossBandSwitch submissions.
• Apply intrusion detection signatures that flag HTTP request bodies exceeding expected lengths for /goform/ endpoints.

Monitoring Recommendations

• Monitor router management plane traffic from non-administrative subnets and flag unauthorized access attempts.
• Track repeated authentication events to the router web UI followed by malformed POST requests.
• Alert on router availability changes that coincide with HTTP requests to /goform/formCrossBandSwitch.

How to Mitigate CVE-2026-5611

Immediate Actions Required

• Restrict access to the router's web management interface to trusted internal management VLANs only.
• Disable remote (WAN-side) administration on the F9K1015 immediately.
• Rotate administrative credentials and enforce strong, unique passwords on the device.
• Evaluate replacement of the F9K1015 given the lack of vendor response and the public disclosure of the exploit.

Patch Information

No vendor patch is available. According to the disclosure, Belkin was contacted before publication but did not respond. Customers should monitor the Belkin support site for any future firmware update addressing CVE-2026-5611 and consider migrating to a supported router model.

Workarounds

• Place the F9K1015 behind a network segment that blocks untrusted clients from reaching the management interface.
• Use host firewall rules or upstream ACLs to permit HTTP access to the router only from authorized administrator workstations.
• Disconnect or replace the device if it must be exposed to untrusted networks, since no patch exists.

# Example: restrict access to the router management interface using iptables on an upstream gateway
iptables -A FORWARD -p tcp -d 192.168.1.1 --dport 80 -s 192.168.1.10 -j ACCEPT
iptables -A FORWARD -p tcp -d 192.168.1.1 --dport 80 -j DROP