CVE-2026-41927: WDR201A WiFi Extender Buffer Overflow

CVE-2026-41927 Overview

CVE-2026-41927 is a stack-based buffer overflow [CWE-121] in the WDR201A WiFi Extender running hardware version 2.1 and firmware LFMZX28040922V1.02. The flaw resides in the firewall.cgi and makeRequest.cgi binaries, which fail to validate input length before copying request data into a fixed-size stack buffer. Unauthenticated attackers can send a crafted POST request with a Content-Length header exceeding 512 bytes to overwrite the saved return address. Successful exploitation enables arbitrary code execution through return-oriented programming (ROP) or return-to-libc techniques.

Critical Impact
Unauthenticated network attackers can achieve arbitrary code execution on affected WDR201A WiFi Extenders by overwriting the saved return address through a single crafted POST request.

Affected Products

WDR201A WiFi Extender, hardware version 2.1
Firmware version LFMZX28040922V1.02
firewall.cgi and makeRequest.cgi request handlers

Discovery Timeline

2026-05-04 - CVE-2026-41927 published to NVD
2026-05-05 - Last updated in NVD database

Technical Details for CVE-2026-41927

Vulnerability Analysis

The vulnerability is a classic stack-based buffer overflow in the request-handling code of two CGI binaries on the WDR201A. Both firewall.cgi and makeRequest.cgi parse incoming HTTP POST bodies using fgets() without enforcing a maximum length consistent with the destination buffer. The destination buffer on the stack is 512 bytes, but the read operation accepts data sized by the attacker-controlled Content-Length header. When the request body exceeds 512 bytes, the surplus data overwrites adjacent stack frames including the saved return address.

Because the device is an embedded MIPS or ARM Linux system, classic mitigations such as Address Space Layout Randomization (ASLR), stack canaries, and non-executable stacks are typically absent or partially enforced on consumer-grade IoT firmware. This makes return-oriented programming and return-to-libc reliable paths to code execution.

Root Cause

The root cause is insufficient length validation in the fgets() call inside the affected CGI handlers. The size argument passed to fgets() does not match the bounded capacity of the stack buffer, allowing a single oversized request to corrupt control flow data. No authentication is required to reach the vulnerable code path.

Attack Vector

An attacker on the same network as the device, or one able to reach its HTTP management interface, sends a POST request to firewall.cgi or makeRequest.cgi with a Content-Length header greater than 512 bytes. The body contains a ROP chain or shellcode that overwrites the saved return address. Upon function return, execution transfers to attacker-controlled gadgets, granting code execution in the context of the web service.

No authenticated public exploit code is referenced in NVD. Technical details are documented in the VulnCheck Advisory: WDR201A Exploit and the GitHub AI-Assisted Vulnerability Disclosure.

Detection Methods for CVE-2026-41927

Indicators of Compromise

Inbound HTTP POST requests to /firewall.cgi or /makeRequest.cgi with Content-Length values greater than 512 bytes.
Request bodies containing repeated padding patterns followed by binary payloads consistent with ROP chains.
Unexpected outbound connections originating from the WiFi extender after receiving large POST requests.

Detection Strategies

Inspect HTTP traffic destined for the device management interface for oversized POST bodies targeting CGI endpoints.
Apply intrusion detection rules that flag non-printable byte sequences in POST bodies addressed to embedded device administration paths.
Correlate device crashes or reboots with preceding HTTP requests to identify potential exploitation attempts.

Monitoring Recommendations

Capture HTTP request and response logs at the network perimeter or upstream router for IoT management interfaces.
Alert on traffic to consumer extenders originating from untrusted network segments such as guest Wi-Fi.
Track DNS and outbound connection telemetry from IoT subnets for anomalous post-exploitation callbacks.

How to Mitigate CVE-2026-41927

Immediate Actions Required

Restrict access to the WDR201A management interface to a dedicated administrative VLAN or trusted hosts only.
Block inbound HTTP requests to firewall.cgi and makeRequest.cgi from untrusted networks at the upstream firewall.
Power down or replace exposed devices if a vendor patch is unavailable, especially in environments handling sensitive traffic.

Patch Information

No vendor advisory or firmware patch is referenced in the NVD entry at the time of publication. Affected operators should consult the VulnCheck Advisory: WDR201A Exploit for updates and review any newer firmware releases issued by the device manufacturer.

Workarounds

Segment IoT and consumer networking equipment onto isolated VLANs with no inbound access from user or guest networks.
Disable remote administration features and ensure the management web service is not exposed to the WAN interface.
Replace end-of-life or unpatched WiFi extenders with devices that receive active firmware support and include modern exploit mitigations.

bash

# Example iptables rule to block external access to vulnerable CGI endpoints
iptables -A FORWARD -p tcp -d <extender_ip> --dport 80 \
  -m string --algo bm --string "/firewall.cgi" -j DROP
iptables -A FORWARD -p tcp -d <extender_ip> --dport 80 \
  -m string --algo bm --string "/makeRequest.cgi" -j DROP