CVE-2021-20123: Draytek VigorConnect Path Traversal Vulnerability

CVE-2021-20123 is a path traversal vulnerability in DrayTek VigorConnect that allows an unauthenticated attacker to download arbitrary files from the underlying operating system with root privileges via the DownloadFileServlet endpoint. This matters because VigorConnect is often used to manage network devices and infrastructure; successful abuse can expose sensitive configuration files, credentials, or other system data without any authentication barrier, increasing the risk of further compromise of the management platform and connected assets.

Defenders should treat this as a high-priority issue for any deployment of the product until the vendor's recommended actions are confirmed and applied. Public detail is limited to the facts above; always verify exact scope against the vendor advisory.

How it works

The flaw belongs to the path traversal class (CWE-22). In this weakness, an application fails to properly sanitize user-supplied input that is used to construct file paths. An attacker can insert directory traversal sequences to escape the intended directory and reach files elsewhere on the filesystem.

According to the available summary, the vulnerable component is the DownloadFileServlet endpoint in DrayTek VigorConnect. An unauthenticated remote attacker can leverage the path traversal to request and retrieve arbitrary files from the operating system, executing the read with root privileges. No further exploit mechanics are provided in the public record; specifics of request format or payload construction must be confirmed against the vendor advisory rather than assumed.

Am I affected? How to find it in your systems

DrayTek VigorConnect is management software typically deployed on servers or appliances used to administer DrayTek networking equipment. It may appear in enterprise or service-provider environments that centralize configuration and monitoring of routers, firewalls, or VPN devices.

• Inventory systems by searching asset databases, configuration management tools, and network scans for hosts running VigorConnect services or listening on ports associated with its management interface.
• Check software inventories, package lists, and application directories for the presence of VigorConnect; confirm the exact build or release against the vendor advisory to determine whether the DownloadFileServlet component is present and unpatched.
• Review network exposure: identify any instances reachable from untrusted networks, as the vulnerability requires no authentication.
• For signs of exploitation, examine web server or application logs for anomalous requests to the DownloadFileServlet endpoint that contain path traversal patterns or requests for sensitive system files. Correlate with unexpected file access events or privilege-related activity on the host. Telemetry is product-specific; confirm log locations and indicators with the vendor documentation.

How to remediate

Patch first. Apply the vendor update or mitigations named in the official advisory for DrayTek VigorConnect. CISA directs organizations to apply mitigations per vendor instructions or to discontinue use of the product if mitigations are unavailable.

• After applying the update, verify that the DownloadFileServlet endpoint no longer accepts traversal sequences and that file download functionality is restricted to authorized paths and users.
• Harden the deployment: run the service with least privilege where possible (noting the root-level impact described), restrict management interfaces to trusted networks only, and enforce strong authentication and access controls on all administrative endpoints.
• Review and rotate any credentials or secrets that may have been stored on the host, and ensure configuration backups are protected.

Confirm all version and configuration guidance directly from the vendor advisory before declaring systems remediated.

If you can't patch immediately

Implement compensating controls to reduce exposure until the vendor update can be applied.

• Segment the VigorConnect host so that it is reachable only from a tightly controlled management network; block inbound access from the internet and untrusted segments at the firewall.
• If a web application firewall or reverse proxy sits in front of the service, apply virtual patching rules that detect and block path traversal sequences in requests to DownloadFileServlet and similar endpoints. Test rules carefully to avoid breaking legitimate functionality.
• Disable or restrict the file download feature if the product configuration allows it and business needs permit.
• Increase monitoring: alert on any access to the DownloadFileServlet endpoint, especially requests containing traversal indicators or targeting sensitive paths, and forward host and application logs to a central SIEM for correlation.
• If mitigations cannot be implemented and the risk is unacceptable, follow the CISA guidance to discontinue use of the product until a fix is available.

If your data may have been exposed

Actively exploited vulnerabilities of this class can lead to data breaches when attackers retrieve sensitive files. Known ransomware use of this CVE is not documented. If you suspect exposure, preserve logs, isolate affected systems, and follow your incident response process. You can also run a free exposure scan of your email address to check whether it appears in known breach data sets as one additional indicator of compromise.