sec-design-deep-analysis.md

Deep Analysis of Security Considerations for lua-nginx-module

This document provides a deep analysis of the security considerations for applications utilizing the lua-nginx-module, based on the provided Security Design Review document. It aims to identify potential threats, vulnerabilities, and propose actionable mitigation strategies specific to this module.

1. Objective, Scope, and Methodology

Objective:

The primary objective of this deep analysis is to conduct a thorough security assessment of the lua-nginx-module ecosystem, as described in the Security Design Review document. This analysis will focus on identifying potential security vulnerabilities arising from the module's architecture, components, and data flow, with a particular emphasis on the risks introduced by embedding Lua scripting within Nginx. The goal is to provide actionable and tailored security recommendations and mitigation strategies to development teams using lua-nginx-module, enabling them to build more secure and resilient applications.

Scope:

This analysis is scoped to the lua-nginx-module project as described in the provided Security Design Review document (Version 1.1, October 26, 2023). The scope includes:

• Components: Nginx Master Process, Nginx Worker Process, Lua VM (LuaJIT), Lua C API Bridge, ngx.* Lua Libraries, User Lua Scripts, and their interactions with external services.
• Data Flow: Request processing lifecycle within Nginx and the points where Lua scripts are invoked and can manipulate request/response data.
• Threat Categories: Injection Vulnerabilities, Access Control and Authorization Bypasses, Resource Exhaustion and Denial of Service (DoS), Information Disclosure, Server-Side Request Forgery (SSRF), Security of External Lua Files, and Vulnerabilities in ngx.* Libraries and LuaJIT.
• Mitigation Strategies: Focus on actionable and tailored recommendations specific to lua-nginx-module and its Lua scripting environment.

This analysis does not cover:

• Security aspects of Nginx core itself, unless directly related to the interaction with lua-nginx-module.
• Detailed code review of the lua-nginx-module codebase itself.
• Specific application code built using lua-nginx-module beyond general vulnerability patterns.
• Operational security aspects like server hardening, network security, or physical security.

Methodology:

The methodology for this deep analysis will involve the following steps:

1. Decomposition and Understanding: Thoroughly review the Security Design Review document to understand the architecture, components, data flow, and identified security considerations of lua-nginx-module.
2. Component-Based Threat Analysis: Break down the system into key components (as defined in the scope) and analyze the security implications of each component, focusing on how they interact and contribute to the overall security posture.
3. Threat Category Mapping: Map the identified threats from the Security Design Review document to specific components and data flow points. Utilize the STRIDE model and the provided threat categories as a framework for systematic threat identification.
4. Vulnerability Inference: Based on the threat analysis, infer potential vulnerabilities that could be exploited within the lua-nginx-module context, considering the specific capabilities and limitations of Lua scripting within Nginx.
5. Tailored Recommendation Generation: Develop specific, actionable, and tailored security recommendations and mitigation strategies for each identified threat and vulnerability, focusing on leveraging lua-nginx-module features and best practices.
6. Documentation and Reporting: Document the entire analysis process, findings, recommendations, and mitigation strategies in a clear and structured manner.

2. Security Implications of Key Components

This section breaks down the security implications of each key component of lua-nginx-module as outlined in the Security Design Review.

2.1. Nginx Worker Process & Lua VM (LuaJIT)

• Security Implication: Each Nginx worker process hosts a LuaJIT VM. While this isolation enhances stability by preventing crashes in one worker from affecting others, it also means that vulnerabilities within the Lua VM or user scripts in one worker are contained within that worker's scope. However, if a vulnerability allows for code execution outside the Lua VM sandbox (if any exists, which is not explicitly mentioned in the document and unlikely in this context), it could potentially compromise the entire worker process.
• Threats:
  • Resource Exhaustion (CPU/Memory): Malicious or poorly written Lua scripts can consume excessive CPU or memory within a worker process, impacting its ability to handle requests and potentially leading to DoS.
  • Lua VM Vulnerabilities: Although LuaJIT is generally considered secure, vulnerabilities in the LuaJIT VM itself could be exploited if present.
• Specific Recommendations:
  • Resource Limits: Implement resource limits within Lua scripts where possible. While lua-nginx-module doesn't provide direct Lua-level resource limits, developers should be mindful of resource consumption in their scripts and avoid computationally expensive operations or memory leaks.
  • Worker Process Monitoring: Monitor Nginx worker process resource usage (CPU, memory) to detect anomalies that might indicate resource exhaustion attacks or poorly performing Lua scripts.
  • Regular Updates: Keep lua-nginx-module and the underlying LuaJIT updated to the latest versions to patch any known vulnerabilities in the Lua VM or module itself.

2.2. Lua C API Bridge

• Security Implication: The Lua C API Bridge is the critical interface allowing Lua scripts to interact with Nginx internals. Vulnerabilities in this bridge could be severe, potentially allowing Lua scripts to bypass intended security boundaries and directly manipulate Nginx core functionalities in unintended ways.
• Threats:
  • API Bridge Vulnerabilities: Bugs or vulnerabilities in the C code of the API bridge could be exploited to gain unauthorized access to Nginx internals or cause crashes.
• Specific Recommendations:
  • Trust in Upstream: Rely on the security and stability of the lua-nginx-module project and its maintainers. Regularly update the module to benefit from bug fixes and security patches.
  • Limited Direct Interaction: As developers, we don't directly interact with the C API Bridge. Our security focus should be on the secure usage of the ngx.* libraries built upon this bridge and the security of user Lua scripts.

2.3. ngx.* Lua Libraries

• Security Implication: The ngx.* libraries provide extensive and powerful functionalities to Lua scripts, granting fine-grained control over Nginx request processing, networking, and other aspects. Misuse or vulnerabilities in these libraries can lead to significant security risks. The power of these libraries is a double-edged sword – enabling great flexibility but also requiring careful and secure usage.
• Threats:
  • API Misuse: Developers might misuse ngx.* APIs in ways that introduce vulnerabilities, such as improper input handling when using ngx.req.get_uri_args(), insecure socket operations with ngx.socket.tcp, or uncontrolled subrequests with ngx.http.request.
  • API Vulnerabilities: Although less likely, vulnerabilities within the ngx.* library implementations themselves could exist.
• Specific Recommendations:
  • Thorough Documentation Review: Developers must thoroughly understand the documentation and security implications of each ngx.* API they use. Pay close attention to input validation requirements, potential side effects, and secure usage guidelines.
  • Secure API Usage Training: Provide security training to developers focusing on secure coding practices when using ngx.* libraries, highlighting common pitfalls and vulnerabilities associated with each API category (request/response manipulation, network operations, etc.).
  • Code Reviews: Conduct thorough code reviews of Lua scripts, specifically focusing on the usage of ngx.* APIs to identify potential misuse or insecure patterns.
  • Input Validation for API Inputs: Always validate and sanitize any user-controlled data before using it as input to ngx.* APIs, especially those related to network operations, request manipulation, or data handling. For example, when using ngx.req.get_uri_args(), validate the retrieved arguments before using them in further processing.

2.4. User Lua Scripts

• Security Implication: User Lua scripts are the primary area of security concern. They represent the custom logic introduced into the Nginx environment and are directly controlled by developers. Vulnerabilities in these scripts are the most common and easily exploitable attack vectors in lua-nginx-module applications.
• Threats:
  • Injection Vulnerabilities (Lua, SQL, Command, HTTP Header, Log): As detailed in the Security Design Review, improper handling of user input in Lua scripts can lead to various injection attacks. Lua injection is particularly critical due to the ability to execute arbitrary Lua code within the Nginx worker process.
  • Access Control and Authorization Bypasses: Flaws in custom authorization logic implemented in Lua scripts can lead to unauthorized access to protected resources.
  • Resource Exhaustion and DoS: Poorly written or malicious Lua scripts can cause CPU exhaustion, memory exhaustion, subrequest flooding, or file descriptor exhaustion.
  • Information Disclosure: Verbose error messages, accidental data exposure in logs or responses, and unintended information leakage through Lua scripts can expose sensitive data.
  • Server-Side Request Forgery (SSRF): Unvalidated URLs used in ngx.http.request can lead to SSRF vulnerabilities.
  • Logic Errors and Business Logic Flaws: Errors in the business logic implemented in Lua scripts can have security implications, such as incorrect authorization decisions, data manipulation flaws, or unintended side effects.
• Specific Recommendations:
  • Input Validation and Sanitization (Crucial): Implement robust input validation and sanitization for all user-controlled data within Lua scripts. Use whitelisting, input encoding, and context-aware escaping. Specifically for lua-nginx-module:
    • Use ngx.escape_uri for escaping URI components.
    • Use parameterized queries or prepared statements when interacting with databases via Lua libraries like lua-resty-mysql or lua-resty-postgres.
    • Avoid using loadstring or load with user-provided input. If dynamic code execution is absolutely necessary, implement extremely strict input validation and sandboxing (which is complex and generally discouraged).
    • Sanitize data before logging using functions like ngx.log to prevent log injection.
    • When setting response headers using ngx.resp.header, carefully validate and sanitize any user-provided data to prevent HTTP header injection.
  • Secure Coding Practices (Lua Specific):
    • Principle of Least Privilege: Grant Lua scripts only the necessary permissions and access to Nginx functionalities and external resources. Avoid overly permissive configurations.
    • Error Handling: Implement proper error handling in Lua scripts to prevent verbose error messages from being exposed to users. Use pcall to handle potential errors gracefully.
    • Secure Session Management (if applicable): If Lua scripts manage sessions, use secure session management practices. Leverage libraries like lua-resty-session and follow best practices for session ID generation, storage, and protection.
    • Resource Management: Be mindful of resource usage in Lua scripts. Avoid infinite loops, computationally expensive operations, and excessive memory allocation. Use ngx.timer for asynchronous operations to prevent blocking worker processes.
    • SSRF Prevention: When using ngx.http.request, strictly validate and sanitize URLs. Implement URL whitelisting based on allowed domains or URL patterns. Consider using a proxy for external requests to add an extra layer of security and control.
  • Regular Security Audits and Code Reviews (Mandatory): Conduct regular security audits and code reviews of all user Lua scripts. Use static analysis tools for Lua if available to identify potential vulnerabilities automatically.
  • Security Testing: Perform penetration testing and vulnerability scanning on applications using lua-nginx-module to identify and validate security vulnerabilities in Lua scripts and configurations.
  • External Lua File Security: If loading Lua scripts from external files, ensure proper file permissions and access control to prevent unauthorized modification. Store Lua files in secure locations and restrict access to authorized personnel only. Consider using file integrity monitoring to detect unauthorized changes.

2.5. External Services (Databases, Caches, APIs)

• Security Implication: Lua scripts often interact with external services. Vulnerabilities can arise from insecure interactions with these services, such as SQL injection, insecure API calls, or exposure of credentials.
• Threats:
  • SQL Injection: As mentioned, unsanitized user input in SQL queries within Lua scripts can lead to SQL injection vulnerabilities.
  • Insecure API Interactions: Lua scripts might interact with external APIs insecurely, such as sending sensitive data in query parameters instead of request bodies, or using insecure HTTP instead of HTTPS.
  • Credential Exposure: Hardcoding or insecurely storing credentials for external services within Lua scripts or Nginx configurations can lead to credential theft.
  • Data Integrity Issues: If data exchanged with external services is not properly validated or sanitized, it can lead to data integrity issues within the application.
• Specific Recommendations:
  • Secure Database Interactions: Use parameterized queries or prepared statements when interacting with databases from Lua scripts to prevent SQL injection. Use Lua libraries that support secure database connections (e.g., lua-resty-mysql, lua-resty-postgres).
  • Secure API Interactions: Always use HTTPS for communication with external APIs. Send sensitive data in request bodies, not in URLs. Validate API responses and handle errors gracefully.
  • Credential Management: Never hardcode credentials in Lua scripts or Nginx configurations. Use secure credential management practices, such as environment variables, secrets management systems, or dedicated configuration management tools to store and retrieve credentials securely.
  • Input Validation and Output Encoding: Validate data received from external services and encode data sent to external services appropriately to prevent injection vulnerabilities or data integrity issues.

3. Actionable and Tailored Mitigation Strategies

This section provides actionable and tailored mitigation strategies applicable to the identified threats, specifically for lua-nginx-module projects.

3.1. Input Validation and Sanitization Strategies:

• Context-Aware Escaping: Use context-aware escaping based on where the user input will be used.
  • For URI components: Use ngx.escape_uri to encode user input before including it in URIs.
  • For HTML output (if generating HTML in Lua): Use a Lua HTML escaping library (e.g., lua-resty-template with proper escaping enabled) to prevent cross-site scripting (XSS) if Lua is used to generate dynamic HTML content (though less common in typical lua-nginx-module use cases).
  • For SQL queries: Utilize parameterized queries or prepared statements provided by Lua database libraries to prevent SQL injection.
• Whitelisting and Regular Expressions: Define strict whitelists for allowed characters, formats, or values for user inputs. Use regular expressions for input validation where appropriate, but be mindful of potential ReDoS (Regular expression Denial of Service) vulnerabilities with complex regex patterns.
• Input Length Limits: Enforce maximum length limits on user inputs to prevent buffer overflows or resource exhaustion attacks.
• Data Type Validation: Validate that user inputs conform to the expected data types (e.g., integer, email, URL).
• Canonicalization: Canonicalize user inputs (e.g., URLs, file paths) to prevent bypasses based on different representations of the same input.

3.2. Secure Coding Practices in Lua for lua-nginx-module:

• Avoid loadstring and load with User Input: Never use loadstring or load to execute user-provided input as Lua code. This is a direct Lua injection vulnerability. If dynamic code execution is absolutely necessary, explore safer alternatives or implement extremely strict sandboxing and validation, which is highly complex and generally not recommended.
• Secure Random Number Generation: Use ngx.random() for generating cryptographically secure random numbers when needed for security-sensitive operations like session ID generation or nonce creation.
• Rate Limiting and Resource Quotas: Implement rate limiting using Nginx's built-in limit_req module or Lua-based rate limiting mechanisms (e.g., using ngx.shared.DICT and ngx.timer) to prevent DoS attacks and abuse. Set resource quotas within Lua scripts to limit CPU usage, memory allocation, and the number of subrequests.
• Secure Logging: Sanitize user input before logging using ngx.log to prevent log injection. Avoid logging sensitive information unnecessarily. Configure appropriate log levels to minimize verbosity in production environments.
• Error Handling with pcall: Use pcall to wrap potentially error-prone Lua code blocks to handle errors gracefully and prevent verbose error messages from being exposed to users. Log errors internally for debugging and monitoring.
• Minimize External Dependencies: Carefully evaluate and minimize the use of external Lua libraries. If using external libraries, ensure they are from trusted sources, regularly updated, and audited for security vulnerabilities.

3.3. SSRF Mitigation Strategies for ngx.http.request:

• URL Whitelisting: Implement strict URL whitelisting for ngx.http.request. Define a list of allowed domains or URL patterns that Lua scripts are permitted to access. Validate the target URL against this whitelist before making the request.
• URL Parsing and Validation: Parse and validate URLs provided as input to ngx.http.request. Ensure that the scheme is http or https and that the hostname is within the allowed whitelist. Prevent usage of internal IP addresses or reserved IP ranges in URLs.
• Proxy for External Requests: Consider routing external HTTP requests made by Lua scripts through a dedicated proxy server. This proxy can enforce additional security policies, such as URL filtering, request inspection, and logging, providing an extra layer of defense against SSRF attacks.
• Disable Redirect Following (if possible and applicable): In some cases, disabling automatic redirect following in ngx.http.request can help mitigate certain SSRF attack vectors.

3.4. Access Control and Authorization in Lua:

• Centralized Authorization Logic: Consolidate authorization logic in reusable Lua functions or modules to ensure consistency and reduce the risk of errors.
• Role-Based Access Control (RBAC): Implement RBAC in Lua scripts to manage user permissions based on roles.
• Input Validation for Authorization Decisions: Validate all inputs used in authorization decisions (e.g., user roles, permissions, resource IDs) to prevent bypasses due to manipulated input.
• Regularly Review Authorization Logic: Periodically review and audit the authorization logic implemented in Lua scripts to identify and correct any flaws or vulnerabilities.
• Testing Authorization Rules: Thoroughly test authorization rules to ensure they function as intended and prevent unauthorized access.

3.5. Security of External Lua Files:

• Secure File Storage: Store external Lua files in secure locations with restricted access. Use appropriate file system permissions to prevent unauthorized modification or access.
• File Integrity Monitoring: Implement file integrity monitoring to detect unauthorized changes to external Lua files.
• Code Signing (Advanced): For highly sensitive applications, consider code signing Lua files to ensure their authenticity and integrity.

By implementing these actionable and tailored mitigation strategies, development teams can significantly enhance the security posture of applications built using lua-nginx-module, mitigating the identified threats and building more resilient and secure systems. Regular security assessments, code reviews, and adherence to secure coding practices are crucial for maintaining a strong security posture over time.