CVE-2020-15415: DrayTek Vigor Firmware RCE Vulnerability

CVE-2020-15415 Overview

CVE-2020-15415 is a critical command injection vulnerability affecting DrayTek Vigor3900, Vigor2960, and Vigor300B enterprise routers. The vulnerability exists in the cgi-bin/mainfunction.cgi/cvmcfgupload endpoint, which allows remote attackers to execute arbitrary commands via shell metacharacters embedded in a filename parameter when the text/x-python-script content type is specified. This is a distinct vulnerability from CVE-2020-14472.

Critical Impact

This vulnerability enables unauthenticated remote command execution on affected DrayTek enterprise routers. CISA has confirmed active exploitation in the wild, and organizations using vulnerable firmware versions face significant risk of complete device compromise, network infiltration, and lateral movement by attackers.

Affected Products

• DrayTek Vigor3900 (firmware versions before 1.5.1)
• DrayTek Vigor2960 (firmware versions before 1.5.1)
• DrayTek Vigor300B (firmware versions before 1.5.1)

Discovery Timeline

• 2020-06-30 - CVE-2020-15415 published to NVD
• 2025-11-07 - Last updated in NVD database

Technical Details for CVE-2020-15415

Vulnerability Analysis

This vulnerability is classified as CWE-78 (Improper Neutralization of Special Elements used in an OS Command), commonly known as OS Command Injection. The flaw resides in the CGI script handling of file upload functionality on DrayTek Vigor enterprise routers.

The vulnerable endpoint cgi-bin/mainfunction.cgi/cvmcfgupload fails to properly sanitize user-supplied input when processing uploaded filenames. When an attacker sends a request with the text/x-python-script content type and includes shell metacharacters (such as backticks, semicolons, or pipe characters) within the filename parameter, the underlying system processes these metacharacters, resulting in arbitrary command execution with the privileges of the web server process.

The network-based attack vector requires no authentication and no user interaction, making this vulnerability particularly dangerous for internet-exposed devices. Successful exploitation grants attackers complete control over the affected router, potentially compromising the entire network perimeter.

Root Cause

The root cause of this vulnerability is insufficient input validation in the cvmcfgupload CGI function. The application passes user-supplied filename data directly to system shell commands without proper sanitization or escaping of shell metacharacters. This allows attackers to break out of the intended command context and inject arbitrary OS commands.

The specific trigger requires the text/x-python-script content type to be set, which bypasses certain validation checks that may be present for other content types, creating an exploitable code path.

Attack Vector

The attack is conducted remotely over the network by sending a specially crafted HTTP request to the vulnerable CGI endpoint. The attacker embeds shell metacharacters within the filename parameter of a multipart file upload request while specifying the text/x-python-script content type.

For example, an attacker could inject commands using standard shell metacharacters such as command substitution ($(command) or backticks), command chaining (;, &&, ||), or piping (|). The injected commands execute with the privileges of the web application, typically root on embedded devices, allowing full system compromise.

Technical details and proof-of-concept information can be found in the GitHub Vigor Command Injection repository.

Detection Methods for CVE-2020-15415

Indicators of Compromise

• Unusual HTTP POST requests to /cgi-bin/mainfunction.cgi/cvmcfgupload with text/x-python-script content type
• Filenames containing shell metacharacters (;, |, $(), backticks) in upload requests to DrayTek management interfaces
• Unexpected outbound connections from DrayTek router devices to unknown external IP addresses
• New or unauthorized user accounts created on router devices
• Modified router configurations or firewall rules without administrative action

Detection Strategies

• Implement network intrusion detection rules to monitor for requests to /cgi-bin/mainfunction.cgi/cvmcfgupload containing shell metacharacters
• Deploy web application firewall (WAF) rules to block requests with suspicious content types targeting DrayTek CGI endpoints
• Monitor router syslog output for unexpected command execution or process spawning
• Analyze network traffic for anomalous patterns originating from DrayTek devices

Monitoring Recommendations

• Enable verbose logging on DrayTek router management interfaces and forward logs to a centralized SIEM
• Configure alerts for any access attempts to CGI endpoints from external IP addresses
• Implement baseline monitoring for DrayTek device behavior and alert on deviations
• Regularly audit router configurations and compare against known-good baselines

How to Mitigate CVE-2020-15415

Immediate Actions Required

• Update affected DrayTek Vigor3900, Vigor2960, and Vigor300B devices to firmware version 1.5.1 or later immediately
• If immediate patching is not possible, restrict management interface access to trusted internal networks only
• Disable remote management access from the internet until patches are applied
• Review router logs for evidence of exploitation attempts or compromise
• Implement network segmentation to isolate network management devices from general user traffic

Patch Information

DrayTek has released firmware version 1.5.1 which addresses this vulnerability. Organizations should download the patched firmware directly from the DrayTek Security Advisory page and apply updates following the manufacturer's recommended procedures.

Given that this vulnerability is listed in the CISA Known Exploited Vulnerabilities Catalog, federal agencies are required to remediate this vulnerability within mandated timeframes. All organizations should treat patching as an urgent priority.

Workarounds

• Restrict access to the router management interface to trusted IP addresses only using ACLs
• Disable remote management access completely if not required for operations
• Place management interfaces behind a VPN to prevent direct internet exposure
• Implement network-level filtering to block requests containing shell metacharacters to CGI endpoints
• Deploy an intrusion prevention system (IPS) with signatures for CVE-2020-15415 exploitation attempts

# Example: Restrict management access to internal network only
# Access DrayTek router administration panel
# Navigate to System Maintenance > Management
# Set "Allow management from Internet" to "Disable"
# Configure "Management Port Access Control List" to include only trusted internal IP ranges
# Example: 192.168.1.0/24 for management access