CVE-2021-20124: Draytek VigorConnect Path Traversal Flaw

CVE-2021-20124 Overview

A local file inclusion (LFI) vulnerability exists in Draytek VigorConnect 1.6.0-B3 in the file download functionality of the WebServlet endpoint. An unauthenticated attacker could leverage this vulnerability to download arbitrary files from the underlying operating system with root privileges. This vulnerability allows malicious actors to access sensitive system files, configuration data, and credentials without requiring any authentication.

Critical Impact

This vulnerability is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Unauthenticated attackers can download arbitrary files with root privileges, potentially exposing sensitive system configurations, credentials, and proprietary data.

Affected Products

• Draytek VigorConnect 1.6.0-B3 (Beta 3)
• Draytek VigorConnect versions prior to patched releases

Discovery Timeline

• 2021-10-13 - CVE-2021-20124 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2021-20124

Vulnerability Analysis

This Local File Inclusion (LFI) vulnerability resides in the WebServlet endpoint's file download functionality within Draytek VigorConnect. The vulnerability arises from improper validation of user-supplied input in file path parameters, allowing attackers to traverse directory structures and access files outside the intended scope.

The vulnerability is classified under CWE-22 (Improper Limitation of a Pathname to a Restricted Directory, also known as 'Path Traversal'). An attacker can exploit this flaw remotely without requiring any authentication, making it particularly dangerous for internet-exposed VigorConnect installations.

When successfully exploited, the attacker gains the ability to read arbitrary files from the underlying operating system with root-level privileges. This could include sensitive configuration files, private keys, database credentials, and other critical system data. The inclusion in CISA's Known Exploited Vulnerabilities catalog confirms that this vulnerability has been actively weaponized by threat actors.

Root Cause

The root cause of CVE-2021-20124 is inadequate input validation in the file download functionality of the WebServlet endpoint. The application fails to properly sanitize or validate file path parameters, allowing attackers to use directory traversal sequences (such as ../) to escape the intended directory and access arbitrary files on the system.

The WebServlet component processes file download requests but does not implement proper path canonicalization or boundary checks to ensure requested files remain within authorized directories. This oversight enables path traversal attacks that can reach any file readable by the application's execution context, which runs with root privileges.

Attack Vector

The attack is conducted over the network and requires no authentication or user interaction. An attacker can craft malicious HTTP requests to the WebServlet endpoint containing path traversal sequences to target specific files on the system.

The attack flow typically involves:

1. Identifying a Draytek VigorConnect instance exposed to the network
2. Crafting HTTP requests to the WebServlet file download endpoint with directory traversal payloads
3. Injecting path traversal sequences (e.g., ../../etc/passwd) into the file path parameter
4. Receiving the contents of the targeted file in the server response

Since the application runs with root privileges, attackers can access highly sensitive system files including /etc/shadow, private keys, application configuration files containing database credentials, and other critical data. For detailed technical analysis, see the Tenable Security Research Report.

Detection Methods for CVE-2021-20124

Indicators of Compromise

• HTTP requests to WebServlet endpoints containing directory traversal sequences such as ../, ..%2f, or ..%252f
• Unusual file access patterns in application logs targeting system files like /etc/passwd, /etc/shadow, or configuration files
• Network traffic from external sources targeting VigorConnect management interfaces with path traversal payloads
• Evidence of sensitive file exfiltration in network traffic or proxy logs

Detection Strategies

• Deploy web application firewall (WAF) rules to detect and block path traversal sequences in HTTP requests
• Monitor HTTP access logs for requests containing encoded or unencoded directory traversal patterns (../, %2e%2e%2f, etc.)
• Implement intrusion detection system (IDS) signatures for known VigorConnect exploitation attempts
• Configure file integrity monitoring on critical system files to detect unauthorized access attempts

Monitoring Recommendations

• Enable verbose logging on Draytek VigorConnect installations and forward logs to a SIEM for centralized analysis
• Set up alerts for any access attempts to the WebServlet file download functionality from external IP addresses
• Monitor for anomalous outbound data transfers that may indicate successful file exfiltration
• Regularly audit network traffic to and from VigorConnect management interfaces

How to Mitigate CVE-2021-20124

Immediate Actions Required

• Immediately update Draytek VigorConnect to the latest patched version available from the vendor
• Restrict network access to VigorConnect management interfaces to trusted IP addresses only
• Place VigorConnect instances behind a properly configured firewall that blocks direct internet exposure
• Review system logs for indicators of prior exploitation attempts

Patch Information

Organizations should apply the security update from Draytek that addresses this vulnerability. Consult the CISA Known Exploited Vulnerability Catalog for remediation guidance and compliance deadlines. The Tenable Security Research Report provides additional technical context for the vulnerability.

Given this vulnerability's presence in the CISA KEV catalog, federal agencies and organizations following CISA guidance are required to remediate within specified timeframes.

Workarounds

• Implement network segmentation to isolate VigorConnect instances from untrusted networks
• Deploy a reverse proxy or web application firewall in front of VigorConnect to filter malicious requests containing path traversal sequences
• Disable or restrict access to the WebServlet file download functionality if not operationally required
• Consider taking vulnerable instances offline until patches can be applied if they are internet-facing

# Example: Restrict access to VigorConnect management interface using iptables
# Allow only trusted administrative network
iptables -A INPUT -p tcp --dport 443 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP

# Block common path traversal patterns at the network level (defense in depth)
# Note: This is a supplementary control, not a replacement for patching