CVE-2021-3019: Lanproxy Path Traversal Vulnerability

CVE-2021-3019 Overview

CVE-2021-3019 is a Directory Traversal vulnerability affecting ffay lanproxy version 0.1. This vulnerability allows remote attackers to read arbitrary files on the server by exploiting path traversal sequences to access /../conf/config.properties, potentially exposing credentials used for intranet connections. Lanproxy is a tool that exposes local servers behind a NAT or firewall to the public internet, making this credential exposure particularly dangerous as it could grant attackers access to internal network resources.

Critical Impact

Unauthorized access to sensitive configuration files containing intranet connection credentials, potentially enabling lateral movement into protected internal networks.

Affected Products

• lanproxy_project lanproxy version 0.1
• Lanproxy deployments exposing internal services to the internet
• Systems relying on lanproxy for NAT traversal and port forwarding

Discovery Timeline

• 2021-01-05 - CVE CVE-2021-3019 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3019

Vulnerability Analysis

This Directory Traversal vulnerability (CWE-22) allows unauthenticated remote attackers to escape the intended web root directory and access sensitive files on the server. The core issue lies in the improper sanitization of user-supplied input in file path handling, enabling attackers to use path traversal sequences like ../ to navigate the file system hierarchy.

The primary target of this attack is the config.properties file located in the conf directory, which contains authentication credentials for intranet connections. Since lanproxy is specifically designed to expose internal services through NAT boundaries, these credentials provide direct access to protected internal network resources—significantly amplifying the impact of this vulnerability.

The vulnerability can be exploited with minimal complexity, requiring no authentication or user interaction. Successful exploitation results in complete compromise of confidential configuration data.

Root Cause

The root cause of CVE-2021-3019 is insufficient input validation in the file serving mechanism of lanproxy. The application fails to properly sanitize path traversal sequences (such as ../) from user-supplied file paths before concatenating them with the base directory. This allows attackers to construct malicious requests that traverse outside the intended web root to access arbitrary files readable by the lanproxy process.

Attack Vector

The attack can be conducted remotely over the network by any unauthenticated attacker with network access to the lanproxy service. The attacker crafts an HTTP request containing path traversal sequences targeting the /../conf/config.properties path. When the server processes this request, it fails to canonicalize or validate the path, resulting in the disclosure of the configuration file containing sensitive credentials.

The attacker does not require any privileges or authentication to execute this attack, and no user interaction is needed. The attack specifically targets the confidentiality of sensitive configuration data, with no direct impact on system integrity or availability.

Detection Methods for CVE-2021-3019

Indicators of Compromise

• HTTP request logs containing path traversal patterns such as ../, ..%2f, or %2e%2e/ targeting lanproxy endpoints
• Unusual access attempts to /conf/config.properties or similar configuration file paths
• Repeated failed or successful requests with encoded directory traversal sequences in URL parameters
• Evidence of credential misuse for intranet connections following unexplained configuration file access

Detection Strategies

• Deploy web application firewalls (WAF) with rules to detect and block path traversal sequences in HTTP requests
• Configure intrusion detection systems (IDS) to alert on requests containing ../ patterns or URL-encoded variants
• Implement log monitoring to identify requests attempting to access files outside the intended web root
• Enable file integrity monitoring on sensitive configuration files including config.properties

Monitoring Recommendations

• Monitor lanproxy access logs for requests containing directory traversal indicators
• Alert on any successful access to configuration files from external IP addresses
• Track authentication events for intranet services using credentials stored in config.properties
• Implement anomaly detection for unusual file access patterns on the lanproxy server

How to Mitigate CVE-2021-3019

Immediate Actions Required

• Upgrade lanproxy to the latest available version with security patches applied
• Restrict network access to the lanproxy management interface to trusted IP addresses only
• Rotate all credentials stored in config.properties immediately if exposure is suspected
• Review access logs for indicators of prior exploitation attempts

Patch Information

Security updates addressing this vulnerability are available through the official lanproxy repository. Administrators should review the GitHub Commits Overview for the latest security patches. Additional technical details about this vulnerability can be found in the GitHub Issue Discussion.

Workarounds

• Place lanproxy behind a reverse proxy that strips or rejects path traversal sequences
• Implement network-level access controls to limit exposure of the lanproxy service
• Configure web application firewall rules to block requests containing ../ or encoded variants
• Move sensitive configuration files outside directories accessible through the web service
• Consider disabling the vulnerable service until patches can be applied in production environments