CVE-2026-5213: D-Link DNR-202L Buffer Overflow Flaw

CVE-2026-5213 Overview

A stack-based buffer overflow vulnerability has been identified in multiple D-Link Network Attached Storage (NAS) devices. The vulnerability exists in the cgi_adduser_to_session function within the /cgi-bin/account_mgr.cgi file. By manipulating the read_list argument, an attacker can trigger a stack-based buffer overflow condition that may lead to remote code execution or denial of service.

The exploit has been publicly disclosed and may be utilized by threat actors. Given that this vulnerability affects the account management functionality of these NAS devices, successful exploitation could allow attackers to gain unauthorized access to stored data or use compromised devices as pivot points for further network intrusion.

Critical Impact

Remote attackers with low-level privileges can exploit this stack-based buffer overflow in D-Link NAS devices to potentially achieve arbitrary code execution, compromising data confidentiality, integrity, and availability across affected network storage systems.

Affected Products

• D-Link DNS-120, DNS-315L, DNS-320, DNS-320L, DNS-320LW NAS devices
• D-Link DNS-321, DNS-323, DNS-325, DNS-326, DNS-327L NAS devices
• D-Link DNS-340L, DNS-343, DNS-345, DNS-726-4 NAS devices
• D-Link DNS-1100-4, DNS-1200-05, DNS-1550-04 NAS devices
• D-Link DNR-202L, DNR-322L, DNR-326 Network Video Recorders
• Firmware versions up to 20260205

Discovery Timeline

• 2026-03-31 - CVE-2026-5213 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-5213

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) and CWE-787 (Out-of-bounds Write). The affected function cgi_adduser_to_session in the /cgi-bin/account_mgr.cgi endpoint fails to properly validate the length of user-supplied input passed through the read_list argument before copying it to a fixed-size stack buffer.

The network-accessible nature of this vulnerability is particularly concerning for organizations deploying these NAS devices on networks accessible from the internet or shared network segments. An authenticated attacker with low privileges can craft a malicious request to the account management CGI endpoint, providing an oversized read_list parameter that exceeds the expected buffer boundaries.

Root Cause

The root cause of this vulnerability is improper bounds checking in the cgi_adduser_to_session function. The function allocates a fixed-size buffer on the stack to store the read_list parameter value but does not validate that the incoming data fits within this allocated space. When an attacker supplies data exceeding the buffer size, the overflow overwrites adjacent stack memory, potentially including saved return addresses and other critical control data.

This represents a classic stack-based buffer overflow pattern commonly found in embedded device firmware where memory-safe programming practices may not be consistently applied.

Attack Vector

The attack vector is network-based, targeting the web management interface of affected D-Link NAS devices. An attacker must have low-level authenticated access to the device to exploit this vulnerability. The attack workflow involves:

1. Authenticating to the D-Link NAS web interface with valid credentials (even low-privilege user accounts)
2. Crafting an HTTP request to /cgi-bin/account_mgr.cgi with a maliciously oversized read_list parameter
3. The overflow corrupts stack memory, potentially allowing control flow hijacking
4. Depending on memory protections present (or absent) on the embedded device, this could lead to arbitrary code execution

Technical details and proof-of-concept information have been documented in the GitHub Vulnerability Documentation.

Detection Methods for CVE-2026-5213

Indicators of Compromise

• Unusual HTTP POST requests to /cgi-bin/account_mgr.cgi containing abnormally large read_list parameter values
• Web server crash logs or unexpected service restarts on D-Link NAS devices
• Anomalous outbound connections from NAS devices to unknown external IP addresses
• Unexpected user account creation or modification events in NAS administration logs

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block HTTP requests with oversized parameters targeting /cgi-bin/account_mgr.cgi
• Deploy network intrusion detection signatures to identify exploitation attempts against D-Link NAS management interfaces
• Monitor for patterns consistent with buffer overflow attacks such as NOP sleds or shellcode signatures in HTTP traffic
• Utilize SentinelOne Singularity platform to detect post-exploitation behaviors including unauthorized process spawning and anomalous network connections

Monitoring Recommendations

• Enable verbose logging on D-Link NAS devices and forward logs to a centralized SIEM for correlation and analysis
• Set up alerts for authentication failures followed by requests to administrative CGI endpoints
• Monitor network traffic flows from NAS devices for unusual data exfiltration patterns or command-and-control communications
• Regularly audit NAS device configurations and user accounts for unauthorized changes

How to Mitigate CVE-2026-5213

Immediate Actions Required

• Restrict network access to D-Link NAS management interfaces using firewall rules, limiting access to trusted IP addresses only
• Disable remote web management if not required, or place NAS devices behind a VPN for remote access
• Audit user accounts on affected devices and remove any unnecessary or suspicious accounts
• Implement network segmentation to isolate NAS devices from general network traffic
• Monitor affected devices for signs of compromise while awaiting vendor guidance

Patch Information

At the time of publication, specific patch information from D-Link has not been confirmed. Organizations should monitor the D-Link Official Website for security advisories and firmware updates addressing this vulnerability. Given that many of the affected models may be end-of-life products, replacement with currently supported devices may be necessary.

Additional vulnerability details are available through VulDB #354350 and the original VulDB Vulnerability Submission.

Workarounds

• Block external access to port 80/443 on affected NAS devices using perimeter firewalls or access control lists
• Implement IP-based access restrictions within the NAS device configuration to limit management interface access
• Deploy a reverse proxy with request filtering capabilities to inspect and sanitize requests before they reach the NAS device
• Consider disabling the web management interface entirely and manage devices through local console access where possible

# Example: Block external access to NAS management interface using iptables
# Run on network gateway/firewall protecting the NAS subnet

# Block HTTP/HTTPS access to NAS devices from external networks
iptables -A FORWARD -d 192.168.1.0/24 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.0/24 -p tcp --dport 443 -j DROP

# Allow management access only from trusted admin workstation
iptables -I FORWARD -s 192.168.1.100 -d 192.168.1.0/24 -p tcp --dport 80 -j ACCEPT
iptables -I FORWARD -s 192.168.1.100 -d 192.168.1.0/24 -p tcp --dport 443 -j ACCEPT