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Threat Model Analysis for handlebars-lang/handlebars.js

Threat: Template Injection (Code Execution)

  • Threat: Template Injection leading to Arbitrary Code Execution.
  • Description: An attacker injects malicious Handlebars syntax (e.g., {{#if}}, {{expression}}, {{{unescaped}}}) into a field that is used to construct the template itself, rather than being treated as data within the template. The attacker crafts the injection to execute arbitrary JavaScript code within the application's context (client-side or server-side). For example, an attacker might inject {{#if "1==1"}}alert('XSS'){{/if}} into a field that is used to build the template dynamically.
  • Impact:
    • Client-Side: Complete client-side application compromise. The attacker can steal cookies, modify the DOM, redirect users, access browser APIs, and exfiltrate data.
    • Server-Side: Potential for Remote Code Execution (RCE) on the server, allowing the attacker to access files, execute commands, and compromise the server.
  • Affected Handlebars.js Component:
    • Handlebars.compile(): This function is the primary target when templates are compiled from user-supplied strings.
    • Handlebars.template(): If the precompiled template function itself is constructed from untrusted input.
    • Any custom helpers that dynamically generate template strings.
  • Risk Severity: Critical.
  • Mitigation Strategies:
    • Never compile templates from untrusted input. This is the paramount mitigation. Use static, pre-compiled templates whenever possible.
    • If dynamic template generation is unavoidable:
      • Strictly validate and sanitize any user-supplied data used in template construction using a whitelist approach.
      • Consider sandboxing the template compilation process (complex and may not be fully effective).
      • Use a Content Security Policy (CSP) to restrict inline script execution.
    • Regularly update Handlebars.js.

Threat: Prototype Pollution

  • Threat: Prototype Pollution leading to Denial of Service or Unexpected Behavior.
  • Description: An attacker crafts input that exploits vulnerabilities in how Handlebars (or custom helpers) handles object properties, leading to the modification of Object.prototype. The attacker might use specially crafted object keys or helper arguments to inject properties or methods that will be inherited by all objects. For example, an attacker might try to override the toString method to cause errors.
  • Impact:
    • Denial of Service (DoS) by overriding common methods.
    • Unexpected application behavior and data corruption.
    • Potential bypass of security checks that rely on specific object properties.
  • Affected Handlebars.js Component:
    • Handlebars.registerHelper(): Custom helpers are a common vector for prototype pollution if they don't handle input securely.
    • Internal Handlebars functions that handle object merging or property access (less likely in recent versions, but still a potential concern).
    • Handlebars.Utils.extend(): If used improperly with untrusted input.
  • Risk Severity: High.
  • Mitigation Strategies:
    • Update Handlebars.js to the latest version. This is crucial, as many prototype pollution vulnerabilities have been patched.
    • Audit custom helpers to ensure they don't allow modification of Object.prototype.
    • Use a linter or static analysis tool to detect potential prototype pollution.
    • Consider using Object.create(null) to create objects without a prototype.
    • Validate and sanitize all user input, even if not directly used in templates.