CVE-2026-41926: WDR201A WiFi Extender RCE Vulnerability

CVE-2026-41926 Overview

CVE-2026-41926 is an operating system command injection vulnerability [CWE-78] affecting the WDR201A WiFi Extender (hardware version V2.1, firmware LFMZX28040922V1.02). The flaw resides in the firewall.cgi binary, where five request handlers fail to validate input before passing parameters to shell execution. Attackers can supply crafted values to parameters such as websURLFilter, websHostFilter, portForward, singlePortForward, and ipportFilter to inject arbitrary shell commands. Injected payloads persist in non-volatile RAM (NVRAM) and re-execute on every subsequent firewall.cgi request, providing persistent command execution on the device.

Critical Impact
Unauthenticated network attackers can achieve persistent remote code execution as the device process owner, fully compromising the WiFi extender and any traffic it handles.

Affected Products

WDR201A WiFi Extender, Hardware Version V2.1
WDR201A Firmware LFMZX28040922V1.02
Devices exposing the firewall.cgi management interface

Discovery Timeline

2026-05-04 - CVE-2026-41926 published to the National Vulnerability Database
2026-05-05 - Last updated in NVD database

Technical Details for CVE-2026-41926

Vulnerability Analysis

The firewall.cgi binary handles configuration requests for firewall, URL filtering, host filtering, and port forwarding features. Five distinct request handlers parse user-supplied parameters and incorporate them into shell command strings without sanitization or argument separation. Because the handlers invoke a shell to perform configuration work, any metacharacters in the request body are interpreted by the shell rather than treated as data.

Attackers exploit this by inserting subshell syntax such as backticks or $(...) constructs, or by appending command separators like ; and && to parameter values. The injected commands execute with the privileges of the CGI process, which on consumer-grade routers typically runs as root.

A distinguishing characteristic of this flaw is persistence. The vulnerable parameters are stored in NVRAM as part of normal device configuration. Each subsequent invocation of firewall.cgi re-reads the stored values and reissues the malicious shell command, giving attackers a durable foothold that survives reboots.

Root Cause

The root cause is missing input validation in five firewall.cgi handlers. Parameter values flow directly into shell command construction without escaping, allow-listing, or use of safe APIs such as execve with separated arguments. The design also conflates configuration storage with executable command context by persisting raw user input to NVRAM.

Attack Vector

The vulnerability is reachable over the network through the device's HTTP management interface. No authentication or user interaction is required to submit requests to firewall.cgi. An attacker with network access to the management plane sends an HTTP request containing a malicious value in one of the five vulnerable parameters. The device stores the payload and executes it during configuration processing, then re-executes it on every later request to firewall.cgi.

The vulnerability mechanism is described in detail in the VulnCheck Advisory WDR201A and the GitHub Vulnerability Disclosure. No verified proof-of-concept code is reproduced here.

Detection Methods for CVE-2026-41926

Indicators of Compromise

HTTP POST requests to firewall.cgi containing shell metacharacters (`, $(), ;, &&, |) in websURLFilter, websHostFilter, portForward, singlePortForward, or ipportFilter parameters
Outbound connections from the WDR201A to unexpected internet hosts, indicating callback or download activity from injected commands
Unexpected child processes spawned by the firewall.cgi process, such as wget, curl, tftp, nc, or shell interpreters
NVRAM configuration entries containing command syntax rather than expected filter or port values

Detection Strategies

Inspect HTTP request bodies destined for the device management interface and alert on metacharacters in the five named parameters
Baseline normal outbound traffic from IoT devices and flag deviations originating from the extender's IP address
Periodically export and review device configuration to detect command payloads stored in firewall and port-forward fields

Monitoring Recommendations

Place the WDR201A on an isolated management VLAN with packet capture enabled for forensic review
Forward router and gateway logs to a central log platform and correlate requests to firewall.cgi with subsequent egress activity
Monitor for repeated requests to firewall.cgi that consistently trigger outbound connections, indicating re-execution of stored payloads

How to Mitigate CVE-2026-41926

Immediate Actions Required

Remove the WDR201A from any network segment that handles sensitive traffic until a fixed firmware is available
Block external access to the device's HTTP management interface at the upstream router or firewall
Reset the device to factory defaults to clear any malicious payload persisted in NVRAM, then change administrative credentials
Audit firewall, URL filter, host filter, and port forwarding configuration entries for shell metacharacters

Patch Information

At the time of publication, no vendor patch is referenced in the NVD entry or in the VulnCheck Advisory WDR201A. Operators should consult the device vendor for firmware updates and replace the device if updates are not produced.

Workarounds

Restrict the management interface to a dedicated administrative host using ACLs on the upstream switch or router
Disable remote management features and ensure the management interface is not bound to the WAN
Replace the WDR201A with a device that receives security maintenance if vendor updates are not forthcoming

bash

# Example upstream ACL restricting WDR201A management access
# Replace 192.0.2.10 with the extender IP and 192.0.2.5 with the admin host
iptables -I FORWARD -d 192.0.2.10 -p tcp --dport 80 -j DROP
iptables -I FORWARD -s 192.0.2.5 -d 192.0.2.10 -p tcp --dport 80 -j ACCEPT