sec-design-deep-analysis.md

Deep Analysis of lua-nginx-module Security

1. Objective, Scope, and Methodology

Objective: This deep analysis aims to thoroughly examine the security implications of using the lua-nginx-module within an Nginx web server environment. The primary goal is to identify potential vulnerabilities, assess their impact, and propose specific, actionable mitigation strategies tailored to the module's architecture and functionality. The analysis will focus on the interaction between Nginx, the Lua VM, custom Lua scripts, and external services, considering various attack vectors and potential security weaknesses. Key components to be analyzed include:

• Nginx Core Interaction: How the module interacts with Nginx's core request processing pipeline.
• Lua VM Sandboxing: The effectiveness and limitations of the Lua VM's sandboxing capabilities.
• Nginx API (provided by lua-nginx-module): The security of the API exposed to Lua scripts.
• Custom Lua Script Security: Best practices and common pitfalls in writing secure Lua scripts.
• External Service Interaction: Security considerations when Lua scripts interact with external services (databases, APIs, etc.).
• Data Flow: How sensitive data flows through the system and potential points of exposure.
• Deployment and Build Processes: Security aspects of the build and deployment pipeline.

Scope: This analysis covers the lua-nginx-module itself, its interaction with Nginx, and the security of custom Lua scripts written to extend Nginx's functionality. It also includes the build and deployment processes as described in the provided design document. It does not cover the security of backend servers or external services except in the context of how Lua scripts interact with them. It also assumes a Kubernetes deployment environment, as specified in the design.

Methodology:

1. Architecture and Data Flow Inference: Based on the provided C4 diagrams, documentation, and the nature of the lua-nginx-module, we will infer the detailed architecture, data flow, and component interactions.
2. Component-Specific Threat Modeling: Each key component (Nginx core, Lua VM, Nginx API, Lua scripts, external service interactions) will be analyzed for potential threats, considering common attack vectors (e.g., injection, XSS, CSRF, DoS, data breaches).
3. Vulnerability Identification: We will identify specific vulnerabilities that could arise from the use of lua-nginx-module, considering both the module's inherent limitations and potential misconfigurations or insecure coding practices.
4. Mitigation Strategy Development: For each identified vulnerability, we will propose specific, actionable mitigation strategies that can be implemented within the Nginx configuration, Lua scripts, or the deployment environment.
5. Risk Assessment Review: We will revisit the initial risk assessment and refine it based on the findings of the deep analysis.

2. Security Implications of Key Components

2.1 Nginx Core Interaction

• Architecture: The lua-nginx-module embeds a Lua VM within Nginx's worker processes. It hooks into various phases of Nginx's request processing pipeline (access, content, header filter, body filter, log) via directives (e.g., content_by_lua_block, access_by_lua_block). This allows Lua scripts to intercept, modify, or generate requests and responses.
• Threats:
  • Bypassing Nginx Security Features: Poorly written Lua scripts could inadvertently bypass Nginx's built-in security features (e.g., request filtering, rate limiting) if they modify request headers or URIs in unexpected ways.
  • Denial of Service (DoS): Lua scripts that consume excessive resources (CPU, memory) or block for extended periods can degrade Nginx's performance or even cause worker process crashes, leading to a DoS.
  • Information Disclosure: Errors or debugging information within Lua scripts could leak sensitive information about the server or backend systems.
• Mitigation:
  • Careful Directive Placement: Use the appropriate directives for the intended task. For example, use access_by_lua_block for authentication/authorization checks before content_by_lua_block.
  • Resource Limits: Use Nginx's lua_shared_dict for shared memory and implement timeouts (lua_socket_read_timeout, lua_socket_send_timeout) and potentially custom resource limiting logic within Lua scripts. Consider using lua-resty-limit-traffic for more advanced rate limiting.
  • Error Handling: Implement robust error handling in Lua scripts to prevent unhandled exceptions from exposing sensitive information. Use pcall or xpcall to catch errors and return appropriate HTTP error responses. Avoid exposing internal error details to the client.
  • Nginx Configuration Review: Regularly review the Nginx configuration to ensure that security features are correctly configured and not inadvertently bypassed by Lua scripts.

2.2 Lua VM Sandboxing

• Architecture: The lua-nginx-module uses Lua's built-in sandboxing capabilities, which are not designed for complete security isolation. Lua's standard library provides functions that can access the file system, execute external commands, and interact with the network. The lua-nginx-module disables some of these functions (e.g., os.execute, io.popen), but it's crucial to understand the limitations.
• Threats:
  • Sandbox Escape: A determined attacker could potentially exploit vulnerabilities in the Lua VM or the lua-nginx-module itself to escape the sandbox and gain access to the underlying system. This is a high-severity risk.
  • File System Access: Even with some restrictions, Lua scripts might be able to read or write files on the server, potentially accessing sensitive data or modifying system files.
  • Network Access: Lua scripts can use the ngx.socket API to make network connections, potentially exfiltrating data or communicating with malicious servers.
• Mitigation:
  • Principle of Least Privilege: Run Nginx worker processes as a non-privileged user. This limits the damage an attacker can do if they escape the Lua sandbox.
  • Disable Unnecessary Lua Modules: Explicitly disable any Lua modules that are not required by your scripts. This reduces the attack surface. Use a minimal Lua environment.
  • Restrict ngx.socket: Carefully control the use of ngx.socket. If possible, restrict network access to specific hosts and ports using firewall rules (e.g., Kubernetes network policies). Validate and sanitize any user-provided data used in socket operations.
  • Code Review: Thoroughly review Lua scripts for any attempts to access the file system or network in unauthorized ways.
  • Consider lua_package_path and lua_package_cpath: Carefully configure these to prevent loading untrusted Lua modules. Avoid using relative paths.
  • Regular Updates: Keep Nginx, the lua-nginx-module, and the Lua VM up-to-date with the latest security patches.

2.3 Nginx API (provided by lua-nginx-module)

• Architecture: The lua-nginx-module provides a rich API (e.g., ngx.req, ngx.resp, ngx.var, ngx.redirect, ngx.location.capture) for Lua scripts to interact with Nginx. This API allows scripts to read and modify request and response headers, body content, variables, and perform other actions.
• Threats:
  • Injection Attacks: If user-provided data is used directly in API calls without proper sanitization, it could lead to various injection attacks. For example:
    • HTTP Header Injection: Manipulating ngx.req.set_header or ngx.resp.set_header with unsanitized input could allow attackers to inject malicious headers (e.g., for cache poisoning or XSS).
    • Redirection Attacks: Using unsanitized input in ngx.redirect could lead to open redirect vulnerabilities.
    • Variable Manipulation: Modifying Nginx variables (ngx.var) with unsanitized input could affect Nginx's behavior in unexpected ways.
  • Data Leakage: Carelessly exposing sensitive data through the API (e.g., setting response headers with internal information) could lead to data leakage.
• Mitigation:
  • Strict Input Validation: Always validate and sanitize any user-provided data before using it in any API call. Use whitelisting whenever possible.
  • Output Encoding: Encode any data that is included in responses (e.g., headers, body) to prevent XSS attacks. Use appropriate encoding functions (e.g., ngx.escape_uri, ngx.encode_args).
  • Avoid Direct Variable Modification: Be cautious when modifying Nginx variables (ngx.var). Understand the implications of changing these variables.
  • Use ngx.location.capture Carefully: This function can be used to make subrequests within Nginx. Ensure that the target of the subrequest is trusted and that any user-provided data is properly sanitized.

2.4 Custom Lua Script Security

• Architecture: Custom Lua scripts are the core of the lua-nginx-module's extensibility. They are executed within the Lua VM and have access to the Nginx API.
• Threats: This is the most significant area of concern, as custom scripts are entirely under the developer's control and can introduce a wide range of vulnerabilities. All the threats mentioned above (injection, DoS, data leakage, sandbox escape) are relevant here. Specific examples include:
  • SQL Injection: If Lua scripts interact with databases, they are vulnerable to SQL injection if user-provided data is not properly sanitized.
  • Command Injection: If Lua scripts construct shell commands using user-provided data, they are vulnerable to command injection. (Avoid using os.execute or similar functions.)
  • Cross-Site Scripting (XSS): If Lua scripts generate HTML output without proper encoding, they are vulnerable to XSS.
  • Authentication and Authorization Bypass: Incorrectly implemented authentication or authorization logic in Lua scripts can lead to bypass vulnerabilities.
  • Sensitive Data Exposure: Storing secrets (API keys, passwords) directly in Lua scripts is a major security risk.
• Mitigation:
  • Secure Coding Practices: Follow secure coding practices for Lua. This includes:
    • Input Validation: Rigorously validate and sanitize all input.
    • Output Encoding: Encode all output appropriately.
    • Parameterized Queries: Use parameterized queries or prepared statements when interacting with databases to prevent SQL injection.
    • Avoid Shell Commands: Avoid constructing shell commands. If absolutely necessary, use a well-vetted library and sanitize all input.
    • Secure Authentication and Authorization: Implement robust authentication and authorization mechanisms. Use secure session management.
    • Secrets Management: Never store secrets directly in Lua scripts. Use environment variables, a secrets management system (e.g., HashiCorp Vault), or Kubernetes secrets.
  • Code Review: Mandatory code reviews for all Lua scripts, focusing on security.
  • Static Analysis: Use static analysis tools (e.g., luacheck) to identify potential code quality and security issues.
  • Security Testing: Perform regular security testing (penetration testing, vulnerability scanning) of the Nginx server and Lua scripts.
  • Use Libraries: Leverage well-vetted Lua libraries (e.g., lua-resty-openidc for OpenID Connect, lua-resty-string for string manipulation) instead of writing custom code whenever possible.

2.5 External Service Interaction

• Architecture: Lua scripts can interact with external services (databases, APIs, etc.) using the ngx.socket API or other Lua libraries.
• Threats:
  • Injection Attacks: Similar to the Nginx API, interactions with external services are vulnerable to injection attacks if user-provided data is not properly sanitized.
  • Data Exfiltration: Malicious Lua scripts could exfiltrate sensitive data to external servers.
  • Authentication and Authorization Issues: Incorrectly handling credentials or access tokens for external services can lead to security breaches.
• Mitigation:
  • Secure Communication: Use HTTPS for all communication with external services.
  • Input Validation and Sanitization: Validate and sanitize all data sent to external services.
  • Secure Credential Management: Store credentials for external services securely (see Secrets Management above).
  • Rate Limiting: Implement rate limiting for requests to external services to prevent abuse.
  • Network Policies: Use Kubernetes network policies to restrict network access from the Nginx pods to only the necessary external services.

2.6 Data Flow

• Data Flow Diagram: (Refer to the C4 diagrams, but consider a more detailed flow within the Lua VM)

  Client Request --> Nginx --> Lua VM --> Lua Script --> [Nginx API, External Services] --> Lua Script --> Nginx --> Client Response
  

• Threats: Sensitive data can be exposed at multiple points in this flow:
  • Client Request: The initial request may contain sensitive data in headers, query parameters, or the request body.
  • Lua Script: Lua scripts may process, store, or log sensitive data.
  • Nginx API: Sensitive data may be passed to or from the Nginx API.
  • External Services: Sensitive data may be exchanged with external services.
  • Nginx Logs: Nginx logs may contain sensitive data.
  • Lua Logs: Custom logging within Lua scripts may expose sensitive data.
• Mitigation:
  • Data Minimization: Only process and store the minimum necessary data.
  • Encryption in Transit: Use HTTPS for all communication.
  • Encryption at Rest: Encrypt sensitive data stored in databases or other persistent storage.
  • Secure Logging: Configure Nginx and Lua logging to avoid logging sensitive data. Use a secure logging solution.
  • Data Masking/Redaction: Mask or redact sensitive data in logs and error messages.

2.7 Deployment and Build Processes

• Architecture: The build process uses a CI pipeline (GitHub Actions), static analysis (luacheck), security linters, Docker image building, and deployment to Kubernetes.
• Threats:
  • Compromised CI/CD Pipeline: An attacker could compromise the CI/CD pipeline to inject malicious code into the Docker image.
  • Vulnerable Base Image: Using a vulnerable base image for the Docker container could expose the application to known vulnerabilities.
  • Insecure Container Registry: Storing Docker images in an insecure container registry could allow attackers to access or modify them.
  • Insecure Kubernetes Configuration: Misconfigured Kubernetes settings (e.g., RBAC, network policies) could expose the application to attack.
• Mitigation:
  • Secure CI/CD Pipeline: Protect the CI/CD pipeline with strong authentication, access controls, and auditing. Use signed commits.
  • Minimal, Secure Base Image: Use a minimal, secure base image (e.g., Alpine Linux) and keep it up-to-date.
  • Secure Container Registry: Use a secure container registry with authentication and access controls.
  • Kubernetes Security Best Practices: Follow Kubernetes security best practices, including:
    • RBAC: Use Role-Based Access Control to restrict access to Kubernetes resources.
    • Network Policies: Use network policies to control network traffic between pods.
    • Pod Security Policies: Use pod security policies to enforce security constraints on pods.
    • Secrets Management: Use Kubernetes secrets to manage sensitive data.
    • Regular Security Audits: Perform regular security audits of the Kubernetes cluster.
  • Dependency Scanning: Scan dependencies (both Lua modules and system packages) for known vulnerabilities.
  • Image Scanning: Scan the Docker image for vulnerabilities before deployment.

3. Risk Assessment Review

The initial risk assessment identified several key risks. This deep analysis confirms and expands upon those risks:

• Potential for vulnerabilities in custom Lua scripts: This remains the highest risk. The analysis highlights numerous specific vulnerabilities that can arise from insecure coding practices.
• Limited sandboxing of the Lua environment: The analysis confirms that the Lua sandbox is not a complete security boundary and can be bypassed.
• Reliance on Nginx's core security: The analysis emphasizes the importance of correctly configuring Nginx's security features and ensuring that Lua scripts do not bypass them.

The deep analysis also identifies additional risks related to:

• External service interactions: The potential for injection attacks and data exfiltration when Lua scripts interact with external services.
• Deployment and build processes: The risks associated with compromised CI/CD pipelines, vulnerable base images, and insecure Kubernetes configurations.

The overall risk level associated with using lua-nginx-module is medium to high, depending on the complexity of the Lua scripts, the sensitivity of the data handled, and the effectiveness of the implemented security controls. The mitigation strategies outlined above are crucial for reducing this risk to an acceptable level. Continuous monitoring, regular security testing, and ongoing code reviews are essential for maintaining a secure deployment.