D-Link DIR-505 Router TFTP File Upload Stack-Based Buffer Overflow (CVE-2026-120189) Technical Analysis and Mitigation Guide

1. Vulnerability Overview & Root Cause

A critical stack-based buffer overflow vulnerability (CVE-2026-120189) has been identified in the TFTP daemon (/usr/bin/tftpd) of the D-Link DIR-505 router. The root cause lies within the save_upload_file and mp_upload_file functions, which handle file upload requests. These functions fail to validate the length of the user-supplied filename parameter before copying it into a fixed-size stack buffer. This lack of boundary checking allows an attacker to overwrite adjacent stack memory, including the saved return address (CWE-121, CWE-119).

2. Exploitation Mechanics & Attack Scenario

To exploit this vulnerability, an attacker must have network access to the target device's TFTP service (typically UDP port 69). The attacker sends a malicious TFTP Write Request (WRQ) containing an excessively long filename. When the /usr/bin/tftpd process processes this request, the oversized filename overflows the stack buffer and overwrites the return address. By redirecting the execution flow to a shellcode payload or a Return-Oriented Programming (ROP) chain, the attacker can bypass authentication and execute arbitrary commands with root privileges. A public exploit is currently available, increasing the likelihood of active exploitation.

3. Infrastructure & Business Impact

Since the D-Link DIR-505 is commonly deployed as a gateway or access point in SOHO environments, compromise of this device exposes the entire internal network. Attackers can leverage root access to intercept network traffic, perform lateral movement, or recruit the device into a botnet for DDoS campaigns. This poses severe risks to data confidentiality, network integrity, and business continuity.