CVE-2021-20124: Draytek VigorConnect Path Traversal Vulnerability

CVE-2021-20124 is a path traversal vulnerability in DrayTek VigorConnect that affects the file download functionality of the WebServlet endpoint. An unauthenticated attacker can use it to download arbitrary files from the underlying operating system with root privileges. This matters because VigorConnect is typically deployed to manage network devices, so successful exploitation can expose sensitive configuration data, credentials, or other system files and give an attacker a foothold for further compromise of the management infrastructure.

Defenders should treat this as a high-priority issue for any environment running the product, confirm exact impact and fixed versions against the vendor advisory, and follow CISA guidance to apply mitigations or discontinue use if mitigations are unavailable.

How it works

The flaw is classified as CWE-22 (Improper Limitation of a Pathname to a Restricted Directory). In the WebServlet endpoint's file download feature, insufficient validation of user-supplied path information allows an attacker to traverse outside the intended directory. Because the request does not require authentication, a remote attacker can craft requests that resolve to arbitrary locations on the host filesystem and retrieve the contents of those files with the privileges of the service, which run as root.

No public exploit code or detailed request patterns are provided in the available facts; any observed exploitation would involve unauthenticated HTTP requests targeting the download functionality. Confirm the precise attack surface and request handling against the vendor advisory rather than assuming specific payloads.

Am I affected? How to find it in your systems

DrayTek VigorConnect is management software commonly used to administer DrayTek routers and related network appliances, often running on Windows or Linux hosts inside enterprise or managed-service environments. Inventory all instances by searching asset databases, configuration management tools, and network scans for hosts advertising VigorConnect services or listening on the ports used by its web interface.

• Identify installed versions and compare them to the fixed releases listed in the vendor advisory; any unpatched installation that exposes the WebServlet endpoint is potentially vulnerable.
• Review web server and application logs for unauthenticated requests to the file download functionality that contain directory traversal sequences (such as "../" patterns) or that request files outside the expected download directory.
• Look for anomalous outbound file transfers or sudden access to sensitive system paths (for example, configuration or credential stores) originating from the VigorConnect process.
• If the product is internet-facing or reachable from untrusted networks, treat it as higher risk and prioritize discovery.

Telemetry from network intrusion detection, web application firewalls, or endpoint detection can surface these indicators; correlate them with the CISA description of root-privileged arbitrary file download.

How to remediate

The primary remediation is to apply the vendor-supplied update that addresses CVE-2021-20124. Follow the installation and verification steps in the official DrayTek advisory, then confirm the WebServlet endpoint no longer permits path traversal. After patching, restart the service and re-test the download functionality under controlled conditions to ensure the fix is effective.

In addition to the patch, harden the deployment for this class of weakness:

• Restrict the service account and process privileges so that even if a similar flaw reappears the impact is limited.
• Disable or tightly control any unused file-download or file-management features.
• Ensure the application runs with the least privilege necessary and that sensitive directories are not readable by the service user.
• Keep the underlying operating system and supporting libraries current.

If the vendor indicates that mitigations are unavailable, discontinue use of the product as directed by CISA.

If you can't patch immediately

Until the vendor update can be applied, reduce exposure with compensating controls. Place VigorConnect hosts behind network segmentation so that only authorized management stations can reach the web interface; block all unauthenticated internet access. Deploy a web application firewall or reverse-proxy rule that rejects requests containing path traversal sequences aimed at the WebServlet download endpoint. If the file-download feature is not required for operations, disable it entirely. Increase monitoring for the log and telemetry signs described earlier and alert on any unauthenticated access attempts. These measures lower risk but do not eliminate it; schedule the official patch or product retirement as soon as possible.

If your data may have been exposed

Actively exploited path-traversal flaws of this type can lead to the theft of configuration files, credentials, and other sensitive data that later enable broader breaches. Although ransomware use is not documented for this CVE, any confirmed exploitation should trigger incident-response procedures: isolate the host, preserve logs, rotate potentially exposed credentials, and assess lateral movement. As a quick check for prior exposure of personal or organizational email addresses, run a free exposure scan against known breach data sets to determine whether related accounts appear in public dumps.