CVE-2026-3040: Draytek Vigor300b Firmware RCE Vulnerability

CVE-2026-3040 Overview

A critical OS command injection vulnerability has been identified in DrayTek Vigor 300B routers running firmware versions up to 1.5.1.6. This vulnerability affects the cgiGetFile function within the /cgi-bin/mainfunction.cgi/uploadlangs endpoint of the Web Management Interface. An authenticated attacker with administrative privileges can remotely manipulate the File argument to execute arbitrary operating system commands on the affected device.

Critical Impact

This vulnerability allows authenticated attackers to achieve remote command execution on DrayTek Vigor 300B devices through the web management interface. The vendor has confirmed this product is End-of-Life (EoL) and will not be patched.

Affected Products

• DrayTek Vigor 300B firmware versions up to 1.5.1.6
• DrayTek Vigor 300B hardware devices
• All DrayTek Vigor300b Firmware installations on affected hardware

Discovery Timeline

• 2026-02-23 - CVE-2026-3040 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-3040

Vulnerability Analysis

This vulnerability is classified as an OS command injection flaw (CWE-78) and command injection (CWE-77). The affected function cgiGetFile in the Web Management Interface fails to properly sanitize user-supplied input through the File argument. When processing requests to the /cgi-bin/mainfunction.cgi/uploadlangs endpoint, the application passes user-controlled data directly to system command execution functions without adequate validation or escaping.

The vulnerability requires network access and high privileges (administrative authentication) to exploit, which limits the attack surface but does not eliminate the risk, particularly in environments where administrative credentials may be compromised or shared. The exploit is publicly available, increasing the likelihood of exploitation in the wild.

Root Cause

The root cause of this vulnerability lies in improper input validation and sanitization within the cgiGetFile function. The Web Management Interface directly incorporates user-supplied values from the File parameter into system commands without proper escaping or validation. This allows specially crafted input containing shell metacharacters to break out of the intended command context and execute arbitrary OS commands with the privileges of the web server process.

Attack Vector

The attack vector is network-based, requiring an authenticated attacker to send malicious HTTP requests to the /cgi-bin/mainfunction.cgi/uploadlangs endpoint. By manipulating the File argument with command injection payloads, an attacker can execute arbitrary commands on the underlying operating system.

An attacker would typically:

1. Obtain valid administrative credentials for the DrayTek Vigor 300B web interface
2. Craft a malicious HTTP request targeting the vulnerable endpoint
3. Include OS command injection payload within the File parameter
4. The injected commands execute with the privileges of the web server process

The vulnerability mechanism involves passing unsanitized user input from the File parameter to system command execution. Shell metacharacters such as semicolons, pipes, or backticks can be used to chain additional commands. See the GitHub Issue Tracker for additional technical details.

Detection Methods for CVE-2026-3040

Indicators of Compromise

• Unusual HTTP requests to /cgi-bin/mainfunction.cgi/uploadlangs containing shell metacharacters in the File parameter
• Unexpected processes spawned by the web server process on DrayTek devices
• Anomalous network connections originating from the router to external hosts
• Presence of unauthorized files or modifications to system configuration

Detection Strategies

• Monitor web server logs for requests to /cgi-bin/mainfunction.cgi/uploadlangs with suspicious File parameter values containing characters like ;, |, &, or backticks
• Implement network intrusion detection rules to identify command injection patterns in HTTP traffic to DrayTek management interfaces
• Deploy endpoint detection and response (EDR) solutions capable of monitoring router management interfaces for anomalous behavior

Monitoring Recommendations

• Enable comprehensive logging on DrayTek Vigor 300B devices and forward logs to a centralized SIEM for analysis
• Establish baseline behavior for web management interface access and alert on deviations
• Monitor for unauthorized administrative login attempts and credential brute-force attacks

How to Mitigate CVE-2026-3040

Immediate Actions Required

• Replace DrayTek Vigor 300B devices with currently supported hardware models as the vendor has confirmed this is an End-of-Life product with no planned patches
• Restrict access to the Web Management Interface to trusted internal networks only; do not expose the management interface to the internet
• Implement strong, unique administrative credentials and enable multi-factor authentication if supported
• Deploy network segmentation to isolate vulnerable devices from critical network assets

Patch Information

The vendor (DrayTek) has explicitly confirmed that the Vigor 300B is End-of-Life (EoL) and that they do not plan to release a security patch for this authenticated vulnerability. Organizations must plan for hardware replacement as the only definitive remediation strategy.

For additional vulnerability details, refer to VulDB #347394 and the VulDB Submission #757126.

Workarounds

• Disable remote management access entirely and only allow local console administration
• Implement firewall rules to block external access to the Web Management Interface on port 80/443
• Use a VPN to access the management interface rather than exposing it directly
• Consider deploying a web application firewall (WAF) to filter malicious requests if the device must remain in service temporarily

# Example firewall rule to restrict management interface access (implementation varies by firewall)
# Block external access to DrayTek management interface
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.1.0/24 -j DROP