CVE-2021-26747

What it actually does

The flaw is an unauthenticated network-accessible command injection vulnerability in affected Netis router firmware. The device’s ping diagnostic functionality improperly passes user-controlled input to an operating-system shell without sufficient sanitization, allowing shell metacharacters to break out of the intended ping command and execute arbitrary commands with the privileges of the web management process, commonly root or another highly privileged embedded-system account. Successful exploitation can provide full device compromise, including configuration theft, traffic interception or redirection, persistent malware installation, and denial of service.

Why it matters to the business

Compromised edge routers can give attackers control over network traffic, DNS settings, firewall behavior, and administrative access paths, creating risk of outages, credential theft, surveillance, botnet enrollment, or use of the device as a foothold into internal networks. The CVSS score is critical and exploit code is available; the EPSS score is also high and stable, indicating meaningful likelihood of exploitation in the wild even though the vulnerability is not currently listed in CISA KEV. Because these devices are often internet-exposed and may be difficult to monitor centrally, unpatched systems should be treated as high-priority remediation targets.

How an attack unfolds

An attacker identifies an exposed Netis WF2780 or WF2411 management interface, typically by scanning for router web services and fingerprinting model or firmware indicators. The attacker then sends a crafted request to the ping diagnostic endpoint with shell metacharacters embedded in the host or address parameter. Instead of treating the input solely as a ping target, the device shell interprets the injected characters as additional commands. The attacker may first run harmless commands to confirm execution, then retrieve system information, download a payload, alter DNS or firewall settings, add persistence, pivot through the LAN, or enroll the router in a botnet. Because the vulnerability is remotely exploitable with low complexity and no authentication or user interaction, exploitation can be automated at internet scale.

How to fix it

Upgrade affected Netis WF2780 and WF2411 devices to a vendor firmware version that corrects command injection in diagnostic functions, if available from the vendor or device distributor. The provided advisory data does not include specific fixed versions, so organizations should verify current firmware directly against official Netis release notes and support channels. If no fixed firmware is available, replace the affected device with a supported model. As immediate mitigations, disable internet exposure of the web management interface, restrict management access to trusted internal administration networks or VPNs, disable remote administration, change default credentials, block inbound access to router administration ports from untrusted networks, and monitor or restrict outbound traffic from the router to prevent payload retrieval and command-and-control communications. Rebooting alone is not sufficient if configuration changes or persistent malware were installed.

How to confirm the fix worked

Confirm device inventory includes all Netis WF2780 and WF2411 routers and record firmware versions. Verify that WF2780 devices are not running firmware 2.3.40404 and WF2411 devices are not running firmware 1.1.29629 unless a vendor backport explicitly states the issue is fixed. Attempt to access the management interface only from approved administrative networks and confirm remote administration is disabled from the internet. Validate firewall rules block untrusted access to HTTP/HTTPS/Telnet/SSH management services. Where safe and authorized, use a non-destructive test payload against the ping diagnostic function to confirm shell metacharacters are rejected, escaped, or treated as literal input. Review configuration after remediation to ensure DNS, port-forwarding, admin credentials, and startup settings have not been altered.

Typical attack path

1. Scan the internet or local network for Netis WF2780 or WF2411 router management interfaces.
2. Fingerprint the device model and firmware version through HTTP banners, login pages, static assets, or management responses.
3. Submit a crafted ping diagnostic request containing shell metacharacters such as command separators or command substitution syntax.
4. Confirm command execution by triggering an outbound network request, creating a file, or observing command output in the HTTP response.
5. Execute follow-on commands to retrieve device configuration, change DNS/firewall settings, download malware, or start a reverse shell.
6. Establish persistence through startup scripts, modified configuration, scheduled tasks where available, or recurring downloaded payloads.
7. Use the compromised router for traffic interception, botnet activity, denial of service, credential capture, or pivoting toward internal systems.

How to detect it

Look for evidence of suspicious requests to the router web interface, especially ping or diagnostic endpoint requests containing shell metacharacters, command separators, URL-encoded command syntax, or payload-download commands. Useful log sources include router administrative logs, web access logs if exported, upstream reverse proxy or firewall logs, IDS/IPS events, DNS resolver logs, DHCP lease history, netflow records, and egress firewall logs for the router’s management or WAN IP. Investigate outbound connections from router IPs to unfamiliar external hosts, especially HTTP, TFTP, FTP, Telnet, IRC, or high-numbered TCP ports following administrative web requests. On the device, where forensic access is possible, inspect configuration for unauthorized DNS servers, new port forwards, enabled remote administration, modified startup scripts, unexpected files in temporary directories, and unknown processes. Because many embedded routers have limited logging and volatile filesystems, absence of logs does not prove absence of compromise; correlate network telemetry around the device with configuration review and consider factory reset or replacement if suspicious activity is observed.

• ;wget
• ; curl
• ;curl
• |wget
• | curl
• `wget
• `curl
• $(wget
• $(curl
• /bin/sh -c
• /bin/busybox
• busybox wget
• busybox telnetd
• tftp -g
• /tmp/
• /var/tmp/
• /dev/shm/
• ping.cgi
• diagnostic
• host=;
• host=%3B
• ip=;
• ip=%3B
• addr=;
• addr=%3B
• ping_addr=;
• ping_addr=%3B
• 8.8.8.8;
• 127.0.0.1;
• /etc/rc.local
• /etc/init.d/
• telnetd -l /bin/sh

What your security team should do

• Prioritize discovery of all Netis WF2780 and WF2411 devices, especially those exposed to the internet or used at branch, small-office, or remote locations.
• Upgrade to fixed vendor firmware where available; if no fixed firmware exists, plan replacement with a supported device.
• Immediately disable WAN-side management and restrict router administration to trusted networks or VPN-only access.
• Hunt for prior compromise by reviewing router logs, upstream firewall logs, DNS telemetry, DHCP records, and netflow for suspicious outbound connections from router IP addresses.
• Reset administrative credentials and review router configuration for unauthorized DNS servers, port forwards, firewall changes, new users, or remote-management settings.
• If compromise is suspected, perform a factory reset, reinstall trusted firmware, reconfigure from known-good settings rather than backup files, and rotate credentials that may have traversed or been stored on the device.
• Add perimeter monitoring for command-injection patterns targeting router diagnostic endpoints and for unexpected outbound wget, curl, tftp, busybox, or telnet behavior from network infrastructure devices.
• Treat exploit availability and high EPSS as urgency drivers even without KEV listing; internet-facing devices should be remediated or isolated as soon as operationally feasible.