CVE-2026-4211 Overview
A stack-based buffer overflow vulnerability has been identified in multiple D-Link Network Attached Storage (NAS) devices affecting the Local_Backup_Info function within the /cgi-bin/local_backup_mgr.cgi endpoint. This vulnerability allows remote attackers to exploit improper handling of the f_idx argument, potentially leading to arbitrary code execution or denial of service on affected devices.
The vulnerability affects a wide range of D-Link DNS and DNR series NAS devices with firmware versions up to 20260205. Given that the exploit has been made publicly available, organizations using these devices face immediate risk of compromise if exposed to untrusted networks.
Critical Impact
Remote attackers with low privileges can trigger a stack-based buffer overflow via network access, potentially achieving complete device compromise including confidentiality, integrity, and availability impacts.
Affected Products
- D-Link DNS-120, DNS-315L, DNS-320, DNS-320L, DNS-320LW
- D-Link DNS-321, DNS-323, DNS-325, DNS-326, DNS-327L
- D-Link DNS-340L, DNS-343, DNS-345, DNS-726-4
- D-Link DNS-1100-4, DNS-1200-05, DNS-1550-04
- D-Link DNR-202L, DNR-322L, DNR-326
- Firmware versions up to 20260205
Discovery Timeline
- 2026-03-16 - CVE-2026-4211 published to NVD
- 2026-03-19 - Last updated in NVD database
Technical Details for CVE-2026-4211
Vulnerability Analysis
This vulnerability resides in the Local_Backup_Info function of the local backup management CGI script (/cgi-bin/local_backup_mgr.cgi). The function fails to properly validate the length of user-supplied input passed through the f_idx argument before copying it to a fixed-size stack buffer. When an attacker provides an oversized value for this parameter, it overwrites adjacent memory on the stack, including the return address.
The attack can be initiated remotely over the network and requires only low-level privileges to execute. The vulnerability enables attackers to potentially gain complete control over the affected NAS device, compromising stored data confidentiality, system integrity, and device availability.
Root Cause
The root cause is a classic stack-based buffer overflow stemming from insufficient input validation in the Local_Backup_Info function. The vulnerable code copies user-controlled data from the f_idx HTTP parameter into a stack-allocated buffer without verifying that the input length does not exceed the buffer's capacity. This is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) and CWE-787 (Out-of-bounds Write).
Attack Vector
The attack is network-accessible through HTTP requests to the /cgi-bin/local_backup_mgr.cgi endpoint. An authenticated attacker with low privileges can craft a malicious HTTP request containing an oversized f_idx parameter value. When the vulnerable function processes this request, the excessive data overflows the stack buffer, allowing the attacker to overwrite the return address and potentially redirect execution to attacker-controlled code.
The exploitation technique typically involves:
- Identifying the target NAS device and accessible CGI endpoint
- Crafting an HTTP request with a malicious f_idx parameter containing overflow payload
- Triggering the Local_Backup_Info function to process the malformed input
- Achieving code execution through return address overwrite
For detailed technical analysis of the vulnerability, refer to the GitHub Vulnerability Documentation.
Detection Methods for CVE-2026-4211
Indicators of Compromise
- Unexpected HTTP requests to /cgi-bin/local_backup_mgr.cgi with abnormally long f_idx parameter values
- NAS device crashes, unexpected reboots, or unresponsive behavior following network activity
- Anomalous outbound network connections from NAS devices to unknown external hosts
- Unauthorized file access or modifications on NAS storage volumes
Detection Strategies
- Deploy network intrusion detection rules to identify HTTP requests targeting /cgi-bin/local_backup_mgr.cgi with oversized parameters
- Monitor NAS device logs for CGI script errors, segmentation faults, or abnormal process terminations
- Implement web application firewall rules to block requests with excessively long parameter values to known vulnerable endpoints
- Conduct regular vulnerability scanning of network-accessible D-Link NAS devices
Monitoring Recommendations
- Enable comprehensive logging on D-Link NAS devices and forward logs to a centralized SIEM platform
- Configure alerts for repeated failed authentication attempts followed by successful access to CGI endpoints
- Monitor network traffic patterns for unusual data exfiltration from NAS device IP addresses
- Establish baseline behavioral profiles for NAS devices and alert on deviations
How to Mitigate CVE-2026-4211
Immediate Actions Required
- Isolate affected D-Link NAS devices from untrusted networks and the public internet immediately
- Review access controls and ensure only necessary users have network access to NAS management interfaces
- Disable remote management features if not required for business operations
- Implement network segmentation to limit exposure of NAS devices to internal trusted networks only
Patch Information
At the time of publication, organizations should check the D-Link Official Website for firmware updates addressing this vulnerability. Given that many of the affected models are legacy devices, some may have reached end-of-life status and may not receive security patches.
For additional vulnerability details and tracking, refer to:
Workarounds
- Place affected NAS devices behind a properly configured firewall that blocks external access to CGI endpoints
- Implement a reverse proxy with request filtering to limit parameter lengths on requests to /cgi-bin/local_backup_mgr.cgi
- Configure access control lists (ACLs) to restrict management interface access to specific trusted IP addresses
- Consider replacing end-of-life devices with currently supported models that receive security updates
# Example firewall rule to restrict NAS management access (iptables)
# Allow access only from trusted management subnet
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
