CVE-2020-15415: DrayTek Multiple Vigor Routers OS Command Injection Vulnerability

CVE-2020-15415 is an OS command injection flaw affecting certain DrayTek Vigor routers. It allows an attacker to achieve remote code execution by supplying shell metacharacters in a filename when a specific content type is used against a CGI endpoint. Because these devices often sit at the network edge, successful exploitation can give an attacker a foothold for further lateral movement or persistent access. Confirm exact impact and affected configurations against the vendor advisory.

CISA notes that the vulnerability is present in the cgi-bin/mainfunction.cgi/cvmcfgupload path on listed models and can lead to remote code execution. Known ransomware use is not documented for this CVE. Organizations should treat any unpatched edge router as high priority for review.

How it works

This issue is classified as CWE-78 (OS Command Injection). The vulnerable component fails to properly sanitize user-controlled input before it is passed to a system shell. According to the CISA summary, an attacker can inject shell metacharacters into a filename parameter when the request uses the text/x-python-script content type against the cvmcfgupload handler. The injected characters are interpreted by the underlying operating system, allowing arbitrary command execution with the privileges of the web service process.

No further exploit mechanics are provided in the public summary. Attackers typically need network reachability to the management or CGI interface; whether authentication is required or whether the interface is exposed by default must be verified against the vendor advisory and your own configuration. Once code execution is obtained, the attacker can run commands, install persistence, or pivot into the internal network.

Am I affected? How to find it in your systems

DrayTek Vigor routers are commonly deployed as small-to-medium business or branch-office edge devices providing VPN, firewall, and WAN connectivity. The CISA summary specifically names Vigor3900, Vigor2960, and Vigor300B. Inventory all DrayTek Vigor appliances in your environment by:

• Querying asset management or network discovery tools for DrayTek vendor OUI or model strings.
• Reviewing management consoles, configuration backups, and DHCP/lease records for Vigor hostnames or firmware banners.
• Scanning internal and external IP ranges for open HTTP/HTTPS management ports and identifying the CGI path cgi-bin/mainfunction.cgi.

Once located, compare the installed firmware version and enabled features against the vendor advisory. Pay particular attention to any configuration that exposes the cvmcfgupload functionality or accepts the text/x-python-script content type. Log sources that may indicate probing or exploitation include web server access logs showing requests to /cgi-bin/mainfunction.cgi/cvmcfgupload, unusual filenames containing shell metacharacters, or unexpected process execution on the device. Telemetry from network IDS/IPS looking for command-injection patterns on those paths can also help, but absence of alerts does not prove safety.

How to remediate

The primary remediation is to apply the mitigations or firmware updates specified by the vendor. CISA’s required action is to apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable. Obtain the official advisory, download the corrected firmware only from DrayTek’s authenticated channels, and follow the documented upgrade procedure. After patching, verify the new version is running and re-test that the vulnerable endpoint no longer accepts the previously dangerous input.

In addition to the vendor fix, harden the device class by:

• Disabling any unused CGI or management features that are not required for operations.
• Restricting management-plane access to a dedicated, authenticated administrative network or VPN.
• Enforcing strong authentication and, where supported, multi-factor authentication for administrative interfaces.
• Ensuring the device is running a currently supported firmware branch and that automatic update or notification mechanisms are enabled if available.

Document the change and retain pre- and post-upgrade configuration snapshots for audit.

If you can't patch immediately

If an immediate firmware update is not possible, implement compensating controls to reduce exposure until the official fix can be applied:

• Segment the router so that its management and CGI interfaces are reachable only from a tightly controlled jump host or management VLAN; block all other sources at the firewall.
• If a web application firewall or reverse proxy sits in front of the device, create a temporary virtual-patch rule that blocks or sanitizes requests containing shell metacharacters in filenames and that rejects the text/x-python-script content type for the cvmcfgupload path.
• Disable the affected upload or configuration feature entirely if business requirements allow.
• Increase monitoring: forward device logs to a SIEM, alert on any access to the vulnerable CGI path, and watch for anomalous outbound connections originating from the router itself.
• If the product cannot be adequately mitigated, plan to replace or decommission it in accordance with CISA guidance.

These measures lower risk but do not eliminate the underlying vulnerability; schedule the vendor-recommended remediation as soon as operationally feasible.

If your data may have been exposed

Actively exploited edge-device vulnerabilities frequently serve as initial access vectors that lead to broader network compromise and data exposure. Even though ransomware use is not documented for this specific CVE, any successful remote code execution should be treated as a potential breach. Review device logs, network flows, and endpoint telemetry for signs of post-exploitation activity. If you believe credentials or data may have been taken, rotate secrets that traversed the device and consider a free exposure scan of organizational email addresses against known breach data sets to identify previously compromised accounts that attackers might reuse.