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Project Design Document: frp (Fast Reverse Proxy) for Threat Modeling

Version: 1.1 Date: 2023-10-27 Author: AI Expert

1. Project Overview

1.1. Project Name

frp (Fast Reverse Proxy)

1.2. Project Goal

frp (Fast Reverse Proxy) is designed to facilitate access to internal network services from the public internet, bypassing NAT and firewalls. Its primary objective is to enable secure and efficient reverse proxying, making services behind restrictive network boundaries accessible externally. Key use cases include:

  • Publicly Exposing Web Applications: Making internal HTTP/HTTPS web servers accessible over the internet for demos, testing, or public access.
  • Remote Access to Applications: Enabling access to various TCP/UDP-based applications running on private networks, such as databases, game servers, or custom applications.
  • Secure Shell (SSH) Tunneling: Providing a secure and convenient way to access internal network resources via SSH from external locations.
  • Development and Testing: Allowing developers to expose local development environments for collaboration, testing with external services, or showcasing progress.

1.3. Target Audience

frp is intended for a diverse range of users, including:

  • Software Developers: For exposing local development servers, testing webhooks, and collaborating on projects.
  • System Administrators & DevOps Engineers: For managing remote servers, providing secure access to internal infrastructure, and simplifying network configurations.
  • Home and Small Office Users: For accessing home servers, NAS devices, or security cameras from outside their local network.
  • Organizations of all sizes: For securely exposing internal applications to external partners, clients, or remote employees without complex VPN infrastructure.

1.4. Key Features

frp's core functionalities are built around providing flexible and secure reverse proxying:

  • Multi-Protocol Support: Handles TCP, UDP, HTTP, and HTTPS traffic, catering to a wide range of application types.
  • Encrypted Communication Options: Offers TLS encryption for control and proxy connections, and STCP/SUDP for encrypted proxy channels, enhancing data confidentiality.
  • Traffic Compression: Supports data compression to optimize performance, especially over networks with limited bandwidth.
  • Authentication Mechanisms: Includes auth_token based authentication to secure communication between frps and frpc.
  • Subdomain and Virtual Host Support: Enables flexible routing for HTTP/HTTPS proxies using subdomains and virtual hosts.
  • Port Forwarding and Mapping: Allows mapping public ports on the frps server to internal ports on the frpc client's network.
  • Optional Web Dashboard: Provides a community-contributed web UI for monitoring and managing frp instances (note: security implications should be carefully considered).
  • Configuration Flexibility: Configuration is managed through INI files, offering a straightforward way to define server and client settings, and proxy rules.
  • Cross-Platform Compatibility: Written in Go, frp is easily compiled and runs on various operating systems (Linux, Windows, macOS, etc.).

2. Architecture Diagram

graph LR
    subgraph "Public Internet"
    "Internet Client" as A
    end

    subgraph "frp Server (frps) - Public Network"
    "frps Listener (TCP/UDP)" as B
    "frps Control Connection Handler" as C
    "frps Proxy Handler" as D
    "frps Configuration" as E
    end

    subgraph "Private Network (Behind NAT/Firewall)"
    "frpc Client (frpc)" as F
    "frpc Control Connection" as G
    "frpc Proxy Connection" as H
    "Internal Application (e.g., Web Server, SSH)" as I
    end

    A -- "Public Port (e.g., 80, 443, custom)" --> B
    B -- "Control Connection Request" --> C
    C -- "frps Configuration" --> E
    C -- "Proxy Request Routing" --> D
    D -- "Proxy Connection" --> F
    F -- "Control Connection (frps)" --> G
    F -- "Proxy Connection (frps)" --> H
    H -- "Internal Application Port" --> I
Loading

Diagram Description:

This diagram illustrates the fundamental architecture of frp, highlighting the interaction between its core components across public and private network boundaries. The frp server (frps) acts as a bridge, deployed on a publicly accessible network, while the frp client (frpc) resides within the private network, alongside the internal applications to be exposed.

Data Flow Explanation:

  1. Initiating Connection: An "Internet Client" (A) attempts to access a service by connecting to the "frps Listener" (B) on a designated public port.
  2. Control Connection Handling: The "frps Listener" (B) directs the incoming connection request to the "frps Control Connection Handler" (C).
  3. Configuration Lookup: The "frps Control Connection Handler" (C) consults the "frps Configuration" (E) to determine the appropriate proxy rules and routing based on the requested port or domain.
  4. Proxy Request Routing: Based on the configuration, the "frps Control Connection Handler" (C) routes the request to the relevant "frps Proxy Handler" (D).
  5. Establishing Proxy Channel: The "frps Proxy Handler" (D) establishes a "Proxy Connection" to the "frpc Client" (F) through the persistent "Control Connection" (G) that was previously established by the frpc.
  6. Forwarding to Internal Application: The "frpc Client" (F) receives the proxy connection and forwards the traffic via the "Proxy Connection" (H) to the designated "Internal Application" (I) on the private network, using the configured internal port.
  7. Response Path (Reverse Flow): Responses from the "Internal Application" (I) traverse the same path in reverse, ultimately reaching the "Internet Client" (A).

3. Component Description

3.1. frps (frp server)

3.1.1. Functionality Breakdown

The frp server (frps) is the central component, responsible for managing connections, routing traffic, and enforcing security policies. Its core functionalities include:

  • Listening and Connection Acceptance: frps listens on configured network interfaces and ports (defined by bind_addr and bind_port) for incoming control connections from frpc clients and potentially direct proxy connections (e.g., for STCP).
  • Control Connection Management: Handles the establishment, authentication, and maintenance of persistent control connections from frpc clients. These connections are used for proxy registration, heartbeat signals, and proxy data transmission.
  • Proxy Configuration Management: Loads and manages proxy configurations defined in frps.ini. This includes defining proxy types, public ports, backend routing rules, and security settings.
  • Proxy Request Routing and Handling: Receives incoming traffic from public clients, identifies the target proxy based on configured rules (port, subdomain, etc.), and routes the traffic to the appropriate frpc client through the established control connection.
  • Traffic Forwarding: Acts as a relay, forwarding data between public clients and frpc clients for all active proxy connections.
  • Security Enforcement: Enforces security policies defined in the configuration, such as authentication (auth_token), TLS encryption (tls_enable), and port restrictions (allow_ports).
  • Optional Dashboard (Community Contributed): May include an optional web dashboard for monitoring server status, connection metrics, and proxy configurations (if enabled).

3.1.2. Security-Relevant Configuration Parameters (frps.ini)

These configuration parameters directly impact the security posture of the frps instance:

  • bind_addr: Specifies the IP address for frps to listen on. Restricting this to a specific public IP (instead of 0.0.0.0) can limit exposure.
  • bind_port: The port for control connections. Changing the default port (7000) can offer a minor degree of security through obscurity.
  • auth_token: Critical for authentication. A strong, randomly generated secret shared between frps and frpc clients. Must be kept confidential.
  • tcp_mux: Enables TCP multiplexing on a single connection. While improving performance, it might complicate traffic analysis and potentially impact security monitoring.
  • subdomain_host: Base domain for subdomain-based proxies. Requires careful DNS management and can be a target for subdomain takeover attacks if misconfigured.
  • vhost_http_port, vhost_https_port: Ports for virtual host HTTP/HTTPS proxies. Ensure these ports are appropriately firewalled and secured.
  • dashboard_port, dashboard_user, dashboard_pwd: Security risk if enabled without strong protection. The dashboard should be disabled in production unless absolutely necessary and secured with strong credentials and network access controls. Consider using alternative monitoring solutions.
  • log_level, log_file: Logging is crucial for security auditing and incident response. Configure appropriate logging levels and secure log storage.
  • tls_enable: Highly recommended. Enables TLS encryption for control and proxy connections, protecting data in transit. Use with strong TLS configurations.
  • allow_ports: Essential for limiting attack surface. Restricts the range of ports that frpc clients can request to expose. Define a strict whitelist of allowed ports.
  • max_pool_count: Limits the number of proxy connections, mitigating potential resource exhaustion attacks.
  • max_ports_per_client: Limits the number of ports a single client can expose, preventing abuse by compromised clients.
  • kcp_bind_port: Port for KCP protocol (if enabled). KCP is a UDP-based protocol that can improve performance in lossy networks but may have different security characteristics than TCP.

3.1.3. Network Protocols

  • TCP: Primarily used for control connections, HTTP/HTTPS proxies, TCP proxies, and STCP proxies.
  • UDP: Used for UDP proxies and SUDP proxies.
  • TLS (Transport Layer Security): Optional but strongly recommended for encrypting TCP-based control and proxy connections when tls_enable = true.
  • KCP (Kernel Congestion Protocol): Optional, UDP-based protocol for potentially improved performance in specific network conditions.

3.1.4. Data Storage and Persistence

frps primarily operates in memory. Persistent data storage is minimal:

  • Configuration File (frps.ini): Stores server configuration, including security settings and proxy definitions. Secure storage and access control for this file are important.
  • Logs: Logs are written to files as configured, providing audit trails and debugging information. Secure log management is essential.
  • In-Memory State: frps maintains in-memory state for active connections, proxy configurations, and client information. This state is lost upon server restart.

3.2. frpc (frp client)

3.2.1. Functionality Breakdown

The frpc client runs on the private network and acts as the agent that connects to the frps server and establishes proxies for internal applications. Key functionalities include:

  • Control Connection Establishment: Initiates and maintains a persistent control connection to the configured frps server (server_addr, server_port).
  • Authentication with frps: Authenticates with the frps server using the shared auth_token.
  • Proxy Configuration Registration: Registers proxy configurations with the frps server, defining the internal services to be exposed, proxy types, and associated settings (e.g., local ports, remote ports, domains).
  • Proxy Connection Handling (Accepting Proxy Requests): Listens for and accepts proxy connection requests from the frps server over the control channel.
  • Traffic Forwarding (to Internal Applications): Forwards traffic received through proxy connections to the specified "Internal Application" on the private network.
  • Heartbeat Mechanism: Sends periodic heartbeat signals to the frps server to maintain the control connection and ensure its liveness.
  • Configuration Loading: Loads client configuration from frpc.ini, defining server connection details, authentication credentials, and proxy definitions.

3.2.2. Security-Relevant Configuration Parameters (frpc.ini)

These parameters in frpc.ini are crucial for client-side security:

  • server_addr: The public IP address or hostname of the frps server. Ensure this is correctly configured and points to the legitimate frps server.
  • server_port: The port of the frps server. Must match the bind_port on the frps server.
  • auth_token: Must match the auth_token on the frps server. Critical for authentication. Securely manage and store this token.
  • tcp_mux: Should match the tcp_mux setting on the frps server for compatibility.
  • tls_enable: Should match the tls_enable setting on the frps server to enable TLS encryption for the control and proxy connections.
  • Proxy Definitions (e.g., [ssh], [web], etc.): Each proxy definition section controls the exposure of an internal service.
    • type: Proxy type (tcp, udp, http, https, stcp, sudp). Choose the appropriate type for the application.
    • local_ip, local_port: IP address and port of the internal application. Ensure these are correctly specified and only expose intended services.
    • remote_port: Public port on the frps server (for TCP/UDP proxies). Carefully choose public ports and avoid conflicts.
    • custom_domains, subdomain: Domain/subdomain for HTTP/HTTPS proxies. Manage DNS records securely and prevent subdomain takeover risks.
    • use_encryption, use_compression: Enables encryption and compression for proxy connections (STCP/SUDP). Recommended for enhanced security and performance.
    • sk (for STCP/SUDP): Shared secret key for STCP/SUDP encryption. Important for STCP/SUDP security. Keep this secret confidential.
    • plugin: Allows using plugins for extended functionality. Exercise caution when using plugins from untrusted sources, as they can introduce security vulnerabilities.

3.2.3. Network Protocols

  • TCP: Primarily used for control connections and most proxy types.
  • UDP: Used for UDP proxying.
  • TLS (Transport Layer Security): Optional but recommended for encrypting TCP-based control and proxy connections when tls_enable = true.

3.2.4. Data Storage and Persistence

Similar to frps, frpc has minimal persistent data storage:

  • Configuration File (frpc.ini): Stores client configuration, including server connection details, authentication tokens, and proxy definitions. Secure storage and access control are crucial.
  • Logs: Logs are written to files as configured, aiding in debugging and monitoring. Secure log management is important.
  • In-Memory State: frpc maintains in-memory state for active connections and proxy configurations, which is lost on client restart.

3.3. Proxy Types - Detailed Security Considerations

frp's various proxy types offer different levels of security and features. Understanding their nuances is crucial for threat modeling:

  • TCP Proxy:

    • Functionality: Forwards raw TCP traffic. Simple and versatile.
    • Security: Least secure in terms of built-in encryption. Relies on TLS for control channel encryption and application-level encryption (if any).
    • Use Cases: Generic TCP applications, databases (consider STCP for databases).

  • UDP Proxy:

    • Functionality: Forwards raw UDP traffic.
    • Security: Similar to TCP proxy, relies on TLS for control channel and application-level security. UDP is inherently connectionless and may be more susceptible to certain attacks (e.g., UDP flooding).
    • Use Cases: Gaming servers, VoIP, other UDP-based applications (consider SUDP for sensitive UDP traffic).

  • HTTP Proxy:

    • Functionality: Optimized for HTTP traffic. Supports virtual hosts, subdomain routing, header rewriting.
    • Security: Can handle HTTP traffic. Security depends on TLS for control channel and application-level HTTPS. Vulnerable to web application attacks if backend is not secure.
    • Use Cases: Exposing HTTP web applications.

  • HTTPS Proxy:

    • Functionality: Handles HTTPS traffic. Can terminate TLS at frps or forward encrypted traffic (TLS passthrough).
    • Security: Offers better security for web traffic by handling HTTPS. TLS termination at frps requires certificate management on the server. TLS passthrough maintains end-to-end encryption but might limit frps's ability to inspect traffic (for WAF-like features, if any were to be added in future).
    • Use Cases: Exposing HTTPS web applications securely.

  • STCP (Secret TCP):

    • Functionality: Encrypted TCP proxy. Encrypts the proxy connection itself between frps and frpc using a shared secret key (sk), in addition to potential TLS on the control channel.
    • Security: Provides an extra layer of encryption for proxy data, even if TLS on the control channel is compromised or not used. Relies on the strength and secrecy of the sk.
    • Use Cases: Exposing sensitive TCP applications where enhanced proxy data encryption is required (e.g., databases, internal APIs).

  • SUDP (Secret UDP):

    • Functionality: Encrypted UDP proxy, similar to STCP but for UDP traffic.
    • Security: Provides encryption for UDP proxy data using a shared secret key (sk). Addresses the inherent lack of encryption in UDP.
    • Use Cases: Exposing sensitive UDP applications where UDP encryption is needed (e.g., secure VoIP, encrypted gaming traffic).

4. Security Considerations - Expanded

4.1. Authentication and Authorization - Deep Dive

  • frps to frpc Authentication (auth_token):

    • Mechanism: A pre-shared secret key (auth_token) configured on both frps and frpc. frpc presents this token during control connection establishment.
    • Strengths: Simple to implement and configure. Provides basic authentication to prevent unauthorized frpc clients from connecting to the frps server.
    • Weaknesses: Shared secret vulnerability. If the auth_token is compromised, unauthorized clients can connect. No user-level authorization within frp itself. Susceptible to brute-force attacks if not combined with rate limiting or other protective measures (though not directly brute-forceable against the control channel itself, but against potential misconfigurations or information leaks).
    • Best Practices: Use a strong, randomly generated auth_token. Rotate the token periodically. Securely store and transmit the token during configuration.

  • Authorization Model:

    • Limited Authorization: frp's authorization is primarily implicit and configuration-based. Authorization is determined by the proxy definitions in frps.ini and frpc.ini.
    • allow_ports for Port Restriction: allow_ports on frps provides a form of authorization by limiting the ports that frpc clients can expose.
    • No User-Specific Authorization: frp does not inherently support user-level authorization or access control lists (ACLs) for proxies. Authorization is at the client level (via auth_token) and proxy definition level.
    • External Authorization: For more complex authorization requirements, consider integrating frp with external authorization mechanisms at the application level (e.g., application-level authentication and authorization within the proxied web application).

  • Dashboard Authentication (Optional):

    • Basic Authentication: The optional web dashboard typically uses basic username/password authentication.
    • Security Risks: Basic authentication is vulnerable to brute-force attacks and credential stuffing. If enabled, use strong, unique credentials and consider network-level access restrictions (e.g., only allow access from specific IP ranges). Strongly consider disabling the dashboard in production environments unless absolutely necessary and properly secured.

4.2. Encryption - Detailed Analysis

  • TLS Encryption (tls_enable):

    • Scope: Encrypts the control connection and proxy connections between frps and frpc.
    • Benefits: Protects data in transit from eavesdropping and man-in-the-middle attacks. Essential for security, especially over public networks.
    • Configuration: Enabled by setting tls_enable = true in both frps.ini and frpc.ini.
    • Considerations: Ensure proper TLS configuration on the frps server, including certificate management if using HTTPS proxies with TLS termination at frps.

  • STCP/SUDP Encryption (use_encryption, sk):

    • Scope: Encrypts the proxy data itself for STCP and SUDP proxies, independently of TLS on the control channel.
    • Benefits: Adds an extra layer of encryption for sensitive proxy data. Useful in scenarios where control channel TLS might be compromised or not desired for proxy data encryption.
    • Mechanism: Uses a shared secret key (sk) configured in frpc.ini proxy definitions.
    • Considerations: sk must be securely generated, distributed, and managed. Strength of encryption depends on the algorithm used by STCP/SUDP (implementation details need to be reviewed in frp codebase).

  • End-to-End Encryption:

    • frp as a Facilitator: frp can facilitate end-to-end encryption if the proxied application itself uses encryption (e.g., HTTPS for web services, SSH for remote access).
    • Transparency: frp generally operates transparently in terms of application-level encryption. It forwards encrypted traffic without decrypting it (unless TLS termination is configured for HTTPS proxies at frps).
    • Best Practice: Encourage and enforce end-to-end encryption for sensitive applications proxied through frp.

4.3. Access Control - In-Depth

  • Network Firewalls (frps Server and Private Network):

    • Essential Layer of Defense: Firewalls are critical for controlling network access to frps and internal applications.
    • frps Server Firewall:
      • Inbound Rules: Restrict inbound access to only necessary ports: bind_port (for control connections), vhost_http_port, vhost_https_port, and any other explicitly exposed proxy ports. Block all other inbound ports.
      • Outbound Rules: Ideally, restrict outbound access as well, allowing only necessary outbound connections (e.g., to logging servers, monitoring systems, if applicable).
    • Private Network Firewall:
      • Inbound Rules: Strictly restrict inbound access to internal applications. Only allow traffic from the frpc client (and potentially other trusted internal sources). Block all external inbound access to internal applications.
      • Outbound Rules: Control outbound traffic from the private network as needed, but ensure frpc can connect to the frps server.

  • allow_ports Configuration (frps):

    • Port Whitelisting: allow_ports acts as a port whitelist on the frps server. It restricts the ports that frpc clients can request to expose.
    • Principle of Least Privilege: Configure allow_ports to only permit the minimum necessary ports to be exposed. Avoid allowing wide port ranges.
    • Regular Review: Periodically review and update allow_ports to ensure it remains aligned with security requirements.

  • Proxy Configuration (frpc.ini):

    • Careful Proxy Definitions: Proxy definitions in frpc.ini directly control which internal services are exposed and how.
    • Minimize Exposure: Only expose necessary services and ports. Avoid exposing entire internal networks or unnecessary applications.
    • Regular Audits: Regularly audit proxy configurations to identify and remove any unintended or unnecessary exposures.

4.4. Input Validation and Data Handling

  • Configuration File Parsing:

    • Robust Parsing: frp needs to robustly parse frps.ini and frpc.ini to prevent vulnerabilities like injection attacks or denial-of-service through malformed configuration files.
    • Validation: Implement input validation for configuration parameters to ensure they are within expected ranges and formats.

  • Protocol Handling (TCP, UDP, HTTP, etc.):

    • Secure Protocol Implementations: Ensure secure and robust implementations of TCP, UDP, HTTP, and other protocols to prevent protocol-level attacks (e.g., buffer overflows, protocol manipulation).
    • Defense in Depth: While frp primarily forwards traffic, consider potential vulnerabilities in its protocol handling logic.

  • Limited Input Validation on Proxy Data:

    • Application Responsibility: Input validation on the content of proxied traffic is primarily the responsibility of the proxied "Internal Application," not frp itself.
    • frp's Role: frp's role is to securely transport traffic. It generally does not perform deep packet inspection or content-based filtering (unless potentially through future plugin mechanisms).
    • WAF Considerations: For web applications, consider using a Web Application Firewall (WAF) in front of frps or the backend web application for content-based security.

4.5. Logging and Monitoring - Enhanced

  • Comprehensive Logging:

    • frps Logging: Configure frps to log important events, including:
      • Control connection attempts (successful and failed).
      • Proxy connection requests and establishment.
      • Errors and warnings.
      • Configuration changes.
      • Security-related events (e.g., authentication failures, TLS errors).
    • frpc Logging: Configure frpc to log:
      • Control connection establishment and maintenance.
      • Proxy registration and status.
      • Errors and warnings.
      • Security-related events.

  • Log Analysis and Monitoring:

    • Centralized Logging: Consider centralizing logs from frps and frpc instances for easier analysis and correlation.
    • Security Information and Event Management (SIEM): Integrate frp logs into a SIEM system for real-time monitoring, anomaly detection, and security alerting.
    • Monitoring Dashboards: Create monitoring dashboards to visualize frp server and client status, connection metrics, and potential security indicators.

4.6. Update and Patch Management

  • Manual Updates: frp updates are typically manual, requiring downloading new binaries and restarting frps and frpc processes.
  • Timely Updates - Critical: Staying up-to-date with the latest frp releases is essential to patch security vulnerabilities. Monitor frp release notes and security advisories.
  • Update Procedures: Establish clear procedures for testing and deploying frp updates in a timely manner.
  • Vulnerability Scanning: Consider periodically scanning frp binaries and infrastructure for known vulnerabilities.

5. Deployment Scenarios and Security Implications - Expanded Threat Scenarios

This section expands on deployment scenarios, focusing on potential threats and mitigations for each:

5.1. Exposing Web Services (HTTP/HTTPS) - Threat Scenarios

  • Scenario: Exposing a web application behind frp.

  • Threats:

    • Web Application Exploits: Vulnerabilities in the web application (SQL injection, XSS, etc.) become directly exploitable from the internet.
    • DDoS Attacks: The frps server's public IP becomes a target for Distributed Denial of Service (DDoS) attacks, potentially disrupting access to all proxied services.
    • Subdomain Takeover: If subdomain routing is used and DNS is misconfigured, attackers might be able to take over subdomains and redirect traffic to malicious sites.
    • frps Misconfiguration Exploits: Weak auth_token, disabled TLS, overly permissive allow_ports, or exposed dashboard can be exploited to compromise the frps server or gain unauthorized access to proxied services.
    • Information Disclosure: Misconfigured HTTP headers or error pages on the backend web application could leak sensitive information.

  • Mitigations (Beyond Previous Section):

    • Regular Web Application Security Audits and Penetration Testing: Proactively identify and remediate web application vulnerabilities.
    • DDoS Mitigation Services: Use DDoS protection services for the frps server's public IP.
    • Secure DNS Management: Implement proper DNSSEC and regularly audit DNS configurations to prevent subdomain takeovers.
    • Security Hardening of frps Server: Follow security best practices for hardening the operating system and infrastructure hosting the frps server.
    • HTTP Security Headers: Configure appropriate HTTP security headers (e.g., HSTS, Content-Security-Policy, X-Frame-Options) on the backend web application.
    • Error Handling and Information Leakage Prevention: Implement secure error handling in the web application to prevent information leakage through error messages.

5.2. SSH Access to Internal Networks - Threat Scenarios

  • Scenario: Providing SSH access to internal servers via frp.

  • Threats:

    • SSH Brute-Force Attacks: The public SSH port on frps becomes a target for SSH brute-force password guessing attacks.
    • SSH Key Compromise: If SSH private keys on internal servers are compromised, attackers can gain unauthorized access.
    • Lateral Movement after SSH Compromise: If an attacker gains SSH access to one internal server, they can potentially move laterally within the private network to compromise other systems.
    • Port Forwarding Abuse: Attackers who gain SSH access might abuse SSH port forwarding to tunnel into the private network and bypass other security controls.
    • Weak SSH Configurations: Weak SSH passwords, default configurations, or outdated SSH software on internal servers increase the risk of compromise.

  • Mitigations (Beyond Previous Section):

    • Strong SSH Key-Based Authentication: Mandatory. Disable password-based SSH authentication.
    • SSH Rate Limiting and Intrusion Detection: Implement rate limiting and intrusion detection systems (IDS) on the frps server to detect and block brute-force SSH attempts.
    • Jump Server (Bastion Host) Architecture: Use frp to access a jump server (bastion host) within the private network. Harden the jump server and use it as the single point of SSH entry, further protecting internal servers.
    • Regular SSH Security Audits: Regularly audit SSH configurations, access logs, and key management practices on internal servers.
    • Principle of Least Privilege for SSH Access: Grant SSH access only to authorized users and limit their privileges on internal servers.
    • Multi-Factor Authentication (MFA) for SSH: Consider implementing MFA for SSH access for enhanced security.

5.3. Exposing Other TCP/UDP Applications - Threat Scenarios

  • Scenario: Exposing databases, game servers, or custom TCP/UDP applications.

  • Threats:

    • Application-Specific Exploits: Vulnerabilities in the exposed application (e.g., database injection attacks, game server exploits) become directly accessible.
    • Protocol-Specific Attacks: Attacks specific to the exposed protocol (e.g., database protocol attacks, game protocol exploits).
    • Service Abuse and Resource Exhaustion: Public accessibility can lead to abuse of the exposed service, potentially causing resource exhaustion or denial of service.
    • Data Breaches: If the exposed application handles sensitive data and is not properly secured, data breaches can occur.
    • Unintended Exposure of Sensitive Services: Misconfiguration can lead to accidentally exposing sensitive internal services that should not be publicly accessible.

  • Mitigations (Beyond Previous Section):

    • Application Security Hardening: Thoroughly secure the exposed TCP/UDP applications. Implement strong authentication, authorization, input validation, and other security controls specific to the application type.
    • Protocol-Specific Security Measures: Implement security measures specific to the exposed protocol (e.g., database access controls, query parameterization, game server anti-cheat mechanisms, protocol firewalls).
    • Rate Limiting and Resource Quotas: Implement rate limiting and resource quotas on the frps server or the backend application to mitigate abuse and resource exhaustion.
    • Data Loss Prevention (DLP): Implement DLP measures if sensitive data is handled by the exposed application.
    • Regular Security Assessments and Penetration Testing: Conduct regular security assessments and penetration testing of the exposed applications and frp configurations.
    • Network Segmentation: Segment the private network to isolate exposed applications from other critical internal systems, limiting the impact of a potential compromise.

6. Data Flow Diagrams (Mermaid) - No Changes

(Data Flow Diagrams from the previous version remain valid and are not repeated here for brevity.)

7. Technology Stack - No Changes

(Technology Stack from the previous version remains valid and is not repeated here for brevity.)

8. Assumptions and Limitations - Refined

8.1. Assumptions

  • Secure Underlying Infrastructure: Assumes the servers, networks, and operating systems hosting frps and frpc are reasonably secure and hardened according to security best practices.
  • Correct and Secure Configuration by Users: Crucially assumes that users configure frp correctly and securely, following security guidelines and best practices. Misconfiguration is a primary source of potential vulnerabilities.
  • Timely Software Updates: Assumes users will proactively keep frp updated to the latest versions to patch known security vulnerabilities and benefit from security improvements.
  • Trust in frp Codebase Integrity: Assumes a reasonable level of trust in the frp codebase itself, that it is free from intentional backdoors or critical design flaws. However, code audits and security reviews are always recommended for critical infrastructure components.
  • Competent Network and Security Administration: Assumes that users deploying and managing frp have a reasonable level of competence in network and security administration principles.

8.2. Limitations

  • Basic Authentication and Authorization: frp's built-in authentication (auth_token) is relatively basic. It lacks advanced authentication methods (e.g., multi-factor authentication, certificate-based authentication) and fine-grained user-level authorization.
  • Limited Built-in Security Features: frp is primarily a reverse proxy and tunneling tool. It does not include advanced security features like Web Application Firewall (WAF), Intrusion Detection/Prevention Systems (IDS/IPS), or advanced traffic analysis capabilities. These need to be implemented externally if required.
  • Single Point of Failure (frps Server): The frps server is a single point of failure. Its compromise or unavailability can disrupt all proxied services. High availability and redundancy measures for frps might be needed for critical deployments (though not natively supported by frp itself).
  • Performance Bottleneck Potential (frps Server): The frps server can become a performance bottleneck under heavy load or with a large number of concurrent connections. Performance tuning and resource scaling of the frps server might be necessary.
  • Dependency on External Security Measures: frp's security relies heavily on external security measures, such as network firewalls, operating system security hardening, application-level security controls, and user security practices. It is not a standalone security solution.
  • Limited Auditability: While frp provides logging, more comprehensive auditability features (e.g., detailed access logs, security event tracking) might be desired for highly regulated environments.

9. Threat Modeling Focus - Using this Document

This design document is intended to be a foundation for conducting a thorough threat model of frp deployments. Here's how to use it for threat modeling:

  1. Identify Assets: List the key assets involved in an frp deployment. These include:

    • frps server (including configuration, binaries, operating system).
    • frpc clients (including configuration, binaries, operating system).
    • Internal applications being proxied.
    • Sensitive data handled by proxied applications.
    • Control channel communication.
    • Proxy data communication.
    • Authentication credentials (auth_token, dashboard credentials).
    • Configuration files (frps.ini, frpc.ini).
    • Logs.

  2. Identify Threats: For each asset, consider potential threats using frameworks like STRIDE or PASTA. Refer to the "Security Considerations" and "Deployment Scenarios and Security Implications" sections of this document for threat ideas. Examples:

    • Spoofing: Unauthorized frpc client connecting to frps.
    • Tampering: Modification of configuration files, interception and modification of proxy data.
    • Repudiation: Lack of audit trails for security events.
    • Information Disclosure: Exposure of sensitive data through web application vulnerabilities, misconfigured proxies, or insecure logging.
    • Denial of Service: DDoS attacks against frps, resource exhaustion on frps or backend applications.
    • Elevation of Privilege: Gaining unauthorized access to internal networks or systems after compromising frps or frpc.

  3. Vulnerability Analysis: Analyze potential vulnerabilities in frp components and configurations based on the "Security Considerations" section. Consider:

    • Weak authentication (auth_token).
    • Lack of TLS encryption.
    • Overly permissive allow_ports.
    • Insecure dashboard configuration.
    • Vulnerabilities in frp codebase (check for known CVEs).
    • Misconfigurations in firewalls and network access controls.
    • Vulnerabilities in proxied applications.

  4. Risk Assessment: Assess the likelihood and impact of each identified threat and vulnerability. Prioritize risks based on severity.

  5. Mitigation Strategies: Develop and implement mitigation strategies for identified risks. Refer to the "Mitigations" sections within the deployment scenarios and the "Security Considerations" section for mitigation ideas. Examples:

    • Enforce strong auth_token and TLS encryption.
    • Configure allow_ports restrictively.
    • Disable the dashboard or secure it heavily.
    • Implement network firewalls and access controls.
    • Harden frps and frpc servers.
    • Secure proxied applications.
    • Implement logging and monitoring.
    • Establish update and patch management procedures.

  6. Security Testing and Validation: Conduct security testing (penetration testing, vulnerability scanning) to validate the effectiveness of implemented mitigations and identify any remaining vulnerabilities.

  7. Continuous Monitoring and Improvement: Continuously monitor frp deployments for security events, review logs, and update security measures as needed. Regularly revisit the threat model and update it based on new threats, vulnerabilities, and changes in the environment.

By following these steps and utilizing this design document, you can perform a comprehensive threat model for frp deployments and proactively address potential security risks. This will help ensure that frp is used securely and effectively to expose internal services while minimizing the attack surface and protecting sensitive data.