CVE-2026-5815: D-Link DIR-645 Buffer Overflow Vulnerability

CVE-2026-5815 Overview

A stack-based buffer overflow vulnerability has been identified in D-Link DIR-645 routers affecting firmware versions 1.01, 1.02, and 1.03. The vulnerability exists in the hedwigcgi_main function within the /cgi-bin/hedwig.cgi file. An authenticated attacker can exploit this flaw remotely to cause a buffer overflow, potentially leading to arbitrary code execution or denial of service on affected devices. The exploit for this vulnerability is now publicly available, increasing the risk of active exploitation. Notably, this vulnerability only affects end-of-life products that are no longer supported by D-Link.

Critical Impact

Remote attackers with low privileges can exploit this stack-based buffer overflow to achieve high impact on confidentiality, integrity, and availability of the affected D-Link DIR-645 routers.

Affected Products

• D-Link DIR-645 Firmware Version 1.01
• D-Link DIR-645 Firmware Version 1.02
• D-Link DIR-645 Firmware Version 1.03

Discovery Timeline

• April 9, 2026 - CVE-2026-5815 published to NVD
• April 9, 2026 - Last updated in NVD database

Technical Details for CVE-2026-5815

Vulnerability Analysis

This vulnerability is classified as a stack-based buffer overflow (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw resides in the hedwigcgi_main function within the hedwig.cgi CGI script, which is responsible for handling web-based configuration requests on D-Link DIR-645 routers.

When specially crafted input is submitted to the vulnerable CGI endpoint, the application fails to properly validate the length of user-supplied data before copying it into a fixed-size stack buffer. This allows an attacker to overwrite adjacent memory on the stack, including the return address, potentially redirecting program execution to attacker-controlled code.

Since the D-Link DIR-645 is an end-of-life product with no vendor support, no security patches are expected to be released, making this vulnerability particularly dangerous for organizations still using these devices.

Root Cause

The root cause of this vulnerability is insufficient bounds checking in the hedwigcgi_main function when processing user-supplied input. The function allocates a fixed-size buffer on the stack and copies input data without validating that the input length does not exceed the buffer capacity. This classic stack-based buffer overflow pattern allows attackers to corrupt stack memory and potentially hijack control flow.

Attack Vector

The attack can be executed remotely over the network. An attacker with low-level privileges (authenticated access) can send a malicious HTTP request to the /cgi-bin/hedwig.cgi endpoint containing oversized input data. The attack does not require user interaction, making it suitable for automated exploitation.

The vulnerability mechanism involves manipulating input parameters sent to the hedwigcgi_main function. When the function processes the oversized input without proper bounds checking, it overwrites critical stack data including saved return addresses. For detailed technical analysis and proof-of-concept information, refer to the GitHub PoC Repository.

Detection Methods for CVE-2026-5815

Indicators of Compromise

• Unusual or malformed HTTP requests targeting /cgi-bin/hedwig.cgi with abnormally large parameter values
• Router crashes, reboots, or unexpected behavior following web management access
• Suspicious outbound network connections originating from the router
• Unexpected changes to router configuration or firmware

Detection Strategies

• Deploy network intrusion detection systems (IDS) with signatures for buffer overflow attacks targeting D-Link DIR-645 devices
• Monitor HTTP traffic to the /cgi-bin/hedwig.cgi endpoint for requests with abnormally large payloads
• Implement web application firewall (WAF) rules to block requests exceeding normal parameter lengths
• Use SentinelOne Singularity to monitor network segments containing vulnerable IoT devices for anomalous behavior

Monitoring Recommendations

• Enable comprehensive logging on network perimeter devices to capture traffic to and from D-Link routers
• Set up alerts for repeated access attempts to the vulnerable CGI endpoint
• Perform regular network scans to identify D-Link DIR-645 devices still in use on your network
• Monitor for signs of device compromise such as unusual DNS queries or C2 communication patterns

How to Mitigate CVE-2026-5815

Immediate Actions Required

• Identify and inventory all D-Link DIR-645 devices on your network immediately
• Isolate affected devices from untrusted networks, particularly the internet
• Replace end-of-life D-Link DIR-645 routers with currently supported alternatives
• If replacement is not immediately possible, restrict administrative access to trusted internal networks only

Patch Information

D-Link DIR-645 is an end-of-life product and no longer receives security updates from the vendor. D-Link has not released and will not release a patch for this vulnerability. The only permanent remediation is to replace affected devices with supported router models. For more information, visit the D-Link Official Website.

Workarounds

• Disable remote management and restrict access to the web interface from the LAN only
• Implement network segmentation to isolate affected routers from critical systems
• Use a firewall to block external access to the router's management interface on ports 80 and 443
• Consider placing affected devices behind a reverse proxy with input validation capabilities

# Example: Block external access to management interface using iptables on upstream device
iptables -A FORWARD -d <router_ip> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <router_ip> -p tcp --dport 443 -j DROP

# Allow management access only from trusted admin subnet
iptables -I FORWARD -s 192.168.1.0/24 -d <router_ip> -p tcp --dport 80 -j ACCEPT