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attack-surface.md

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Attack Surface Analysis for handlebars-lang/handlebars.js

Attack Surface: Server-Side Template Injection (SSTI)

  • Description: Attackers inject malicious Handlebars expressions into templates processed on the server, leading to code execution or data breaches.
  • Handlebars.js Contribution: Handlebars.js renders templates based on provided data. If user-controlled data is directly embedded into templates without sanitization, Handlebars will execute injected expressions. The use of triple curly braces {{{ }}} for unescaped output significantly increases this risk.
  • Example:
    • Scenario: A Node.js application uses Handlebars and dynamically constructs a template string using user input: const templateString = '<h1>Welcome, ' + userInput + '!</h1>'; const template = Handlebars.compile(templateString);.
    • Attack: An attacker inputs {{{process.mainModule.require('child_process').execSync('whoami')}}} as userInput.
    • Handlebars Rendering: Handlebars compiles and executes the injected code on the server when the template is rendered.
  • Impact: Remote Code Execution (RCE), Server-Side Request Forgery (SSRF), data exfiltration, denial of service.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Avoid Dynamic Template Construction with User Input: Do not construct template strings dynamically using user input. Pre-define templates and pass data as context.
    • Strict Input Sanitization: Sanitize and validate all user inputs before using them in template context data.
    • Output Encoding/Escaping: Use double curly braces {{ }} for output encoding by default. Avoid triple curly braces {{{ }}} unless absolutely necessary and with extreme caution.
    • Principle of Least Privilege: Run the application with minimal necessary privileges to limit the impact of RCE.
    • Regular Security Audits: Periodically review templates and code for potential injection vulnerabilities.

Attack Surface: Client-Side Template Injection (CSTI)

  • Description: Attackers inject malicious Handlebars expressions into templates rendered in the user's browser, leading to Cross-Site Scripting (XSS).
  • Handlebars.js Contribution: Handlebars.js can be used for client-side rendering. If templates are compiled client-side using user-controlled data, or if data used in client-side templates is not properly sanitized, XSS vulnerabilities can arise.
  • Example:
    • Scenario: A JavaScript application compiles a Handlebars template directly from user input: const template = Handlebars.compile(userInput);.
    • Attack: An attacker inputs <img src=x onerror=alert('XSS')> as userInput.
    • Handlebars Rendering: Handlebars compiles and renders the malicious HTML, executing the JavaScript alert in the user's browser.
  • Impact: Cross-Site Scripting (XSS), session hijacking, website defacement, redirection to malicious sites, information theft.
  • Risk Severity: High
  • Mitigation Strategies:
    • Avoid Client-Side Compilation with User Input: Do not compile templates client-side using user-provided data. Pre-compile templates server-side or during a build process.
    • Strict Input Sanitization: Sanitize and validate all user inputs used in client-side template context data.
    • Contextual Output Encoding: Ensure data is properly encoded for the HTML context when rendered in templates. Handlebars' default escaping with {{ }} helps, but verify it's sufficient.
    • Content Security Policy (CSP): Implement a strong CSP to mitigate the impact of XSS by controlling resource loading sources.
    • Regular Security Audits: Review client-side template usage and JavaScript code for potential XSS vulnerabilities.

Attack Surface: Insecure Helper Functions

  • Description: Custom Handlebars helper functions, if not implemented securely, can introduce vulnerabilities like RCE, SSRF, or XSS.
  • Handlebars.js Contribution: Handlebars allows developers to extend template functionality with custom helper functions. The security of the application becomes dependent on the security of these custom helpers.
  • Example:
    • Scenario: A helper function executeCommand is created to run shell commands based on template arguments: {{executeCommand command}}. The helper uses child_process.execSync(command) without proper input validation.
    • Attack: An attacker crafts a template using {{executeCommand 'ls -al /'}} or {{executeCommand 'rm -rf /'}} (if permissions allow).
    • Helper Execution: The executeCommand helper executes the attacker-controlled command on the server.
  • Impact: Remote Code Execution (RCE), Server-Side Request Forgery (SSRF), Cross-Site Scripting (XSS), information disclosure, denial of service, depending on the helper's functionality.
  • Risk Severity: High to Critical (depending on helper functionality)
  • Mitigation Strategies:
    • Secure Helper Implementation: Implement helper functions with robust security measures.
      • Input Validation: Thoroughly validate and sanitize all inputs to helper functions. Use allow-lists where possible.
      • Output Encoding: Properly encode outputs from helpers to prevent XSS.
      • Secure API Usage: If helpers interact with external APIs or system resources, ensure secure API usage and follow security best practices.
      • Principle of Least Privilege: Limit the privileges of helper functions to the minimum necessary. Avoid operations like shell command execution in helpers if possible.
    • Code Review for Helpers: Conduct thorough security code reviews of all custom helper functions.
    • Use Well-Vetted Libraries: Prefer using well-established and security-audited libraries for common helper functionalities instead of writing custom helpers from scratch.

Attack Surface: Denial of Service (DoS) via Complex Templates

  • Description: Overly complex or deeply nested Handlebars templates can consume excessive server resources during compilation or rendering, leading to DoS.
  • Handlebars.js Contribution: Handlebars' template parsing and rendering process can be resource-intensive for complex templates, especially when combined with large datasets or computationally expensive helpers.
  • Example:
    • Scenario: An attacker submits a request with a very large and deeply nested Handlebars template or data that causes excessive CPU and memory usage during rendering.
    • Attack: Sending a request with a crafted template containing thousands of nested loops or conditional statements, or providing extremely large datasets to be processed by the template.
    • Handlebars Processing: Handlebars attempts to compile and render the complex template, consuming excessive server resources and potentially crashing the application or server.
  • Impact: Denial of Service (DoS), application unavailability.
  • Risk Severity: High
  • Mitigation Strategies:
    • Template Complexity Limits: Establish and enforce limits on template complexity (e.g., nesting depth, template size) during development.
    • Resource Limits: Implement resource limits (CPU, memory, request timeouts) on the server to prevent a single request from consuming excessive resources.
    • Rate Limiting: Implement rate limiting to restrict the number of requests from a single source, mitigating DoS attempts.
    • Template Caching: Cache compiled templates to reduce compilation overhead for frequently used templates.
    • Performance Testing: Conduct performance testing with realistic and potentially malicious template inputs to identify and address performance bottlenecks.