CVE-2026-71958: D-Link DWR-M961 Buffer Overflow Vulnerability

CVE-2026-71958 Overview

CVE-2026-71958 is a buffer overflow vulnerability affecting D-Link DWR-M961 routers running hardware version C1 with software version 1.1.2_C1_202602110044. The flaw resides in the quicksetup.cgi interface, where the test4, ssid2, and username fields fail to enforce input length boundaries. A remote attacker can submit overly long strings to these fields to trigger memory corruption. With a crafted payload, the attacker can execute arbitrary commands on the device or force it to crash. The weakness is classified under CWE-120, buffer copy without checking size of input.

Critical Impact
Unauthenticated remote attackers can execute arbitrary commands on affected DWR-M961 routers or induce a denial-of-service condition through the quicksetup.cgi endpoint.

Affected Products

D-Link DWR-M961 (Hardware Version C1)
D-Link DWR-M961 Software Version 1.1.2_C1_202602110044
4G AC1200 LTE Router product line

Discovery Timeline

2026-08-08 - CVE-2026-71958 published to NVD
2026-08-10 - Last updated in NVD database

Technical Details for CVE-2026-71958

Vulnerability Analysis

The vulnerability lives in the quicksetup.cgi handler responsible for processing initial router configuration requests. The handler reads user-supplied values from the test4, ssid2, and username HTTP parameters and copies them into fixed-size stack buffers without validating input length. When an attacker submits strings that exceed the destination buffer size, adjacent stack memory is overwritten, including saved return addresses and function pointers.

Exploitation results in one of two outcomes. A carefully crafted payload aligns overwritten memory to redirect control flow into attacker-supplied shellcode or an existing gadget chain, enabling arbitrary command execution with the privileges of the CGI process. A malformed payload corrupts memory unpredictably and causes the CGI worker or the entire device to crash. Because DWR-M961 CGI processes typically run with elevated privileges on the embedded Linux system, successful code execution grants full control of the router.

Root Cause

The root cause is missing length validation on parameters read from HTTP requests inside quicksetup.cgi. The affected code path invokes an unbounded string copy operation such as strcpy or sprintf targeting a fixed-size stack buffer. No pre-checks on input length exist before the copy, and the CGI does not require authentication for the vulnerable endpoint. This combination of missing bounds checking and unauthenticated network access produces the CWE-120 buffer overflow.

Attack Vector

An attacker sends a single HTTP request to the router's management interface targeting quicksetup.cgi. The request populates test4, ssid2, or username with an oversized string containing an architecture-appropriate ROP chain or shellcode. If the router exposes its web management interface to the WAN, exploitation is possible from the internet. On LAN-only deployments, exploitation requires network adjacency, which can be achieved through a compromised host or a malicious wireless client. See the VulnCheck Advisory D-Link Buffer Overflow for exploitation details.

// No verified proof-of-concept code has been published.
// See the VulnCheck advisory linked above for technical analysis.

Detection Methods for CVE-2026-71958

Indicators of Compromise

HTTP POST or GET requests to /quicksetup.cgi containing abnormally long values (greater than 256 bytes) in the test4, ssid2, or username parameters.
Unexpected reboots or web management interface crashes on DWR-M961 devices following inbound HTTP traffic.
Outbound connections from the router to unknown IP addresses shortly after inbound requests to quicksetup.cgi.
Presence of shellcode-like byte patterns (NOP sleds, architecture-specific opcodes) in captured HTTP request bodies.

Detection Strategies

Inspect network traffic destined for router management interfaces and flag requests to quicksetup.cgi that contain parameter values exceeding typical field length limits.
Deploy IDS/IPS signatures that identify oversized test4, ssid2, and username field values in HTTP requests to D-Link CGI endpoints.
Correlate router availability outages with preceding HTTP traffic patterns to identify probable exploitation attempts.

Monitoring Recommendations

Enable and centralize syslog forwarding from DWR-M961 devices to detect crash and restart events.
Monitor firewall logs for inbound WAN-side connections to router management ports (typically TCP 80 and 443).
Track configuration changes on the router that were not initiated by administrators.

How to Mitigate CVE-2026-71958

Immediate Actions Required

Disable remote (WAN-side) access to the DWR-M961 web management interface until a patched firmware version is installed.
Restrict LAN-side access to quicksetup.cgi using network segmentation and firewall access control lists that permit only trusted administrative hosts.
Change administrative credentials and audit router configuration for unauthorized modifications.
Review the D-Link Security Advisory SAP10512 for the vendor's official guidance and firmware availability.

Patch Information

Refer to the D-Link Security Advisory SAP10512 for firmware update availability and instructions. Administrators should validate firmware integrity and apply updates through the vendor's supported channels. Consult the D-Link Product Information Page to confirm the current supported release for hardware version C1.

Workarounds

Block external access to router management ports at the upstream firewall or ISP-side network device.
Place the router behind a network segment that requires VPN authentication for management access.
If the device is end-of-support or a patch is not yet available, consider replacing the affected router with a supported model.

bash

# Example: restrict router management access to a trusted admin subnet
# Apply on an upstream firewall in front of the DWR-M961
iptables -A FORWARD -p tcp -d <router-ip> --dport 80 -s 10.10.0.0/24 -j ACCEPT
iptables -A FORWARD -p tcp -d <router-ip> --dport 80 -j DROP
iptables -A FORWARD -p tcp -d <router-ip> --dport 443 -s 10.10.0.0/24 -j ACCEPT
iptables -A FORWARD -p tcp -d <router-ip> --dport 443 -j DROP