CVE-2026-5629: Belkin F9k1015 Buffer Overflow Vulnerability

CVE-2026-5629 Overview

CVE-2026-5629 is a stack-based buffer overflow [CWE-119] in the Belkin F9K1015 wireless router running firmware version 1.00.10. The flaw resides in the formSetFirewall function exposed through the /goform/formSetFirewall endpoint. An attacker can manipulate the webpage argument to overflow a fixed-size stack buffer. The attack is remotely executable and a public proof-of-concept exists. Belkin was notified prior to disclosure but did not respond, leaving the device unpatched.

Critical Impact

Remote attackers with low-privilege access can corrupt the router's stack and potentially execute arbitrary code, compromising confidentiality, integrity, and availability of the device.

Affected Products

• Belkin F9K1015 router (hardware)
• Belkin F9K1015 firmware version 1.00.10
• Any deployment exposing the /goform/formSetFirewall web interface to untrusted networks

Discovery Timeline

• 2026-04-06 - CVE-2026-5629 published to the National Vulnerability Database (NVD)
• 2026-04-30 - Entry last modified in NVD

Technical Details for CVE-2026-5629

Vulnerability Analysis

The Belkin F9K1015 web management interface exposes the /goform/formSetFirewall handler, which is processed by the formSetFirewall C function within the router's HTTP daemon. The handler reads the webpage HTTP parameter and copies it into a fixed-size stack buffer without validating the input length. Submitting an overlong webpage value overruns the buffer and overwrites adjacent stack data, including the saved return address.

The vulnerability requires only low-level privileges on the web interface and no user interaction. Successful exploitation results in process crash at minimum and arbitrary code execution in the worst case, since MIPS-based SOHO routers typically lack stack canaries, address space layout randomization (ASLR), and non-executable stack protections. Public technical details are available in the GitHub PoC repository and the VulDB Vulnerability #355417 entry.

Root Cause

The root cause is the absence of length validation on the webpage parameter before it is copied into a stack-allocated buffer. The use of unbounded string copy primitives such as strcpy or sprintf on attacker-controlled input is the classic pattern flagged by [CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer].

Attack Vector

An attacker reaches the vulnerable code path by sending a crafted HTTP request to /goform/formSetFirewall on the router's management interface. The request must include an oversized value for the webpage parameter. Exploitation is feasible across the LAN by default and across the WAN where the management interface is exposed to the internet. The EPSS score is 0.108% with a percentile of 28.591, reflecting limited observed exploitation activity at the time of publication.

No verified exploitation code is reproduced here. Refer to the GitHub PoC repository for technical reproduction details.

Detection Methods for CVE-2026-5629

Indicators of Compromise

• HTTP POST requests to /goform/formSetFirewall containing abnormally long webpage parameter values
• Unexpected reboots, web interface crashes, or httpd process restarts on the router
• Outbound connections from the router to unknown hosts following web interface activity
• New or modified firewall rules that were not initiated by an administrator

Detection Strategies

• Inspect HTTP traffic destined to the router management interface for webpage parameter lengths exceeding typical client values (a few hundred bytes)
• Alert on repeated requests to /goform/formSetFirewall originating from a single source within a short interval
• Correlate router syslog entries showing httpd crashes or watchdog-triggered reboots with preceding inbound HTTP traffic

Monitoring Recommendations

• Forward router syslog and HTTP access logs to a centralized log analytics platform for retention and correlation
• Baseline normal administrative traffic to the router and alert on deviations in request size, frequency, and source
• Monitor egress traffic from the router subnet for signs of post-exploitation command-and-control activity

How to Mitigate CVE-2026-5629

Immediate Actions Required

• Restrict access to the router web management interface to trusted management VLANs and disable remote (WAN-side) management
• Place the F9K1015 behind a network segment that blocks untrusted HTTP traffic to its management port
• Plan replacement of the F9K1015 with a vendor-supported device, as Belkin has not responded to the disclosure
• Rotate administrative credentials and review router configuration for unauthorized changes

Patch Information

No vendor patch is available. According to the disclosure, Belkin was contacted prior to public release but did not respond. Organizations should treat the device as unsupported and prioritize replacement or compensating network controls.

Workarounds

• Disable the web administration interface on untrusted interfaces where the router supports this configuration
• Apply upstream firewall ACLs to allow inbound TCP to the router's HTTP/HTTPS management ports only from designated administrator hosts
• Segment the router onto a management network isolated from general user and IoT traffic to reduce attack surface
• Replace the device with a currently supported model that receives security updates

# Example upstream firewall ACL restricting management access to the router
# Replace ROUTER_IP and ADMIN_HOST with site-specific values
iptables -A FORWARD -p tcp -d ROUTER_IP --dport 80  -s ADMIN_HOST -j ACCEPT
iptables -A FORWARD -p tcp -d ROUTER_IP --dport 443 -s ADMIN_HOST -j ACCEPT
iptables -A FORWARD -p tcp -d ROUTER_IP --dport 80  -j DROP
iptables -A FORWARD -p tcp -d ROUTER_IP --dport 443 -j DROP