CVE-2026-3978: D-Link DIR-513 Buffer Overflow Vulnerability

CVE-2026-3978 Overview

CVE-2026-3978 is a stack-based buffer overflow vulnerability affecting the D-Link DIR-513 router running firmware version 1.10. The flaw resides in the /goform/formEasySetupWizard3 endpoint, where the wan_connected argument is processed without proper bounds checking. Attackers can trigger the overflow remotely across the network, corrupting stack memory on the device. Public exploit information is available, increasing the likelihood of opportunistic exploitation against exposed devices. The DIR-513 is a legacy consumer router model, and affected units may not receive vendor patches.

Critical Impact
Remote attackers can corrupt stack memory on D-Link DIR-513 routers through the wan_connected parameter, potentially leading to arbitrary code execution or device compromise.

Affected Products

D-Link DIR-513 hardware
D-Link DIR-513 firmware version 1.10
Legacy D-Link DIR-513 deployments still exposed to untrusted networks

Discovery Timeline

2026-03-12 - CVE-2026-3978 published to the National Vulnerability Database (NVD)
2026-03-16 - Last updated in NVD database

Technical Details for CVE-2026-3978

Vulnerability Analysis

The vulnerability is classified under [CWE-119], improper restriction of operations within the bounds of a memory buffer. It affects an unknown handler within the /goform/formEasySetupWizard3 web interface endpoint exposed by the DIR-513 management service. The handler processes the wan_connected request argument and writes attacker-controlled data into a fixed-size stack buffer. Because the input length is not validated against the destination buffer, oversized values overwrite adjacent stack memory, including saved return addresses and frame pointers. Successful exploitation can divert control flow on the MIPS or ARM-based embedded processor used by the router.

The attack requires network access to the router's web management interface and low privileges per the CVSS vector. Public disclosure of exploit details lowers the technical barrier to weaponization. While not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, the EPSS probability of 0.039% reflects limited observed exploitation at publication time.

Root Cause

The root cause is missing length validation on the wan_connected parameter before it is copied into a stack-allocated buffer inside the request handler. The use of unbounded string-copy primitives such as strcpy or sprintf on attacker-controlled input is a recurring pattern in D-Link formEasySetupWizard* handlers.

Attack Vector

An attacker sends a crafted HTTP POST request to /goform/formEasySetupWizard3 containing an overlong wan_connected value. The request can originate from any network-adjacent host that can reach the router's HTTP service, including LAN clients and, where exposed, the WAN interface. The malformed input overflows the stack and may be used to redirect execution to attacker-supplied shellcode or ROP gadgets. See the GitHub vulnerability notes and VulDB entry #350413 for the published proof-of-concept details.

No verified exploit code is reproduced here. Refer to the linked references for the disclosed request structure.

Detection Methods for CVE-2026-3978

Indicators of Compromise

HTTP POST requests to /goform/formEasySetupWizard3 containing abnormally long wan_connected parameter values
Unexpected reboots, watchdog resets, or httpd crashes on DIR-513 devices coinciding with inbound web requests
Configuration changes to WAN settings that were not initiated by an administrator
Outbound connections from the router to unfamiliar hosts following suspicious web traffic

Detection Strategies

Inspect web server and router syslog entries for repeated requests to formEasySetupWizard3 from a single source
Apply network IDS signatures that flag oversized URL-encoded values targeting the wan_connected argument
Correlate router crash telemetry with HTTP request logs to identify exploitation attempts

Monitoring Recommendations

Forward router logs and network flow data to a centralized SIEM for long-term retention and correlation
Alert on management-interface traffic originating from outside trusted administrative subnets
Monitor for new HTTP scanning activity targeting /goform/ endpoints across the environment

How to Mitigate CVE-2026-3978

Immediate Actions Required

Restrict access to the DIR-513 web management interface to trusted management VLANs only
Disable remote (WAN-side) administration on all DIR-513 units
Inventory affected DIR-513 devices and prioritize replacement, as the model is end-of-life consumer hardware
Place affected routers behind an upstream firewall that filters requests to /goform/formEasySetupWizard3

Patch Information

At the time of publication, no vendor patch was referenced in the CVE record. The DIR-513 is a legacy D-Link product, and customers should consult the D-Link Security Information page for current advisories and replacement guidance. Where no firmware update is available, replacing the device with a supported model is the recommended remediation.

Workarounds

Block external access to TCP port 80 and 443 on the router's WAN interface
Apply ACLs on upstream network equipment to drop HTTP requests containing formEasySetupWizard3 paths from untrusted sources
Segment the router's LAN so only authorized administrative hosts can reach the management interface
Disable the easy setup wizard functionality where the device configuration permits

bash

# Example upstream firewall rule (iptables) to block external access to the router admin interface
iptables -A FORWARD -p tcp -d <ROUTER_IP> --dport 80 -m string \
  --string "formEasySetupWizard3" --algo bm -j DROP
iptables -A FORWARD -p tcp -d <ROUTER_IP> --dport 80 \
  -s <TRUSTED_MGMT_SUBNET> -j ACCEPT
iptables -A FORWARD -p tcp -d <ROUTER_IP> --dport 80 -j DROP